Factors affecting the carbonyl stretching frequencies of dialkyl ketones and alkyl alkanoates in various solvent systems

VlBID~ONAL

SPECTROSCOPY ELSEVIER

Vibrational Spectroscopy 7 (1994) 1-29

Factors affecting the carbonyl stretching frequencies of dialkyl ketones and alkyl alkanoates in various solvent systems R.A. Nyquist Analytical Sciences Laboratory, 1897F Building, The Dow Chemical Company, Midland, Michigan 48667, USA (Received 14th August 1993)

Abstract

The frequency behavior of the carbonyi stretching vibration, v(C---O), is explained in terms of the reaction field, steric effects, inductive effects, and intermolecular hydrogen bonding. Our present study of dialkyl ketones and alkyl alkanoates in a variety of solvents was undertaken to obtain additional information on soluteLsolvent interaction as an aid in the elucidation of molecular structure. Key words: Infrared spectrometry; Alkyl alkanoates; Carbonyl group; Dialkyl ketones; Solvent systems

1. Introduction

also affects the v(C=O) frequency. The contribution of the resonance form

We have previously studied dialkyl ketones [1,2] and alkyl alkanoates [3,4] in C H C I 3 a n d / o r CC14, as 1 and 2% solutions. The v(C---O) mode for dialkyl ketones and alkyl alkanoates decreases in frequency with increasing electron donation (the inductive effect) of the alkyl group (R and R') to the C--O group which increases the contribution of the resonance form A or B [2]. Oe

I R--C--R' • A

Oe

I I

R'--C--O--R"

B Resonance forms A and B weaken the C--O bond, and weakening the C=O bond causes v(C=O) to occur at a lower frequency. In the case of alkyl alkanoates the alkyl group R" of the group OR"

Oe

I

R--C~O--R" increases progressing in the order methyl, ethyl, isopropyl, and ten-butyl for the R" group. Thus, the C---O bond is increasingly weakened progressing in the order methyl, ethyl, isopropyl, and tert-butyl alkanoate. The alkyl groups of the alkyl alcohols used as solvents for solutes such as dialkyl ketones and alkyl alkanoates affect the v(C=O) frequency in two ways. The alkyl alcohols form intermolecular hydrogen bonds with the free pair of electrons on the carbonyl group of compounds such as ketones and esters. The acidity of the OH proton for the alkyl alcohols increases in the order tert-butyl, isopropyl, ethyl, and methyl alcohol, and on this

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R.A. Nyquist / Vi&rationalSpectmsc~

2

basis methyl alcohol should form the strongest intermolecular hydrogen bond with the free pair of electrons on the carbonyl oxygen atom of a ketone or ester. The other factors to consider in the formation of intermolecular hydrogen bonding is the basicity of the carbonyl group and the steric effect of the alkyl groups for both the carbonyl containing compound and the alkyl alcohol. The larger the steric factor of the alkyl groups the further the distance between the alkyl alcohol proton and the free pair of electrons on the carbonyl oxygen atom. The 6O:HO bond distance, the basicity of the carbonyl oxygen atom, and the acidity of the OH proton determines the strength of the 6O:HO intermolecular hydrogen bond and the v(C==O) frequency. The strongest intermolecular hydrogen bond is formed when the 60 :HO bond distance is at a minimum and the acidity of the OH proton and the basicity of the 60 oxygen atom are at a maximum. In this case, where the intermolecular hydrogen bond is the strongest, the v(6O:HO) mode will occur at the lowest frequency possible for a given series of esters or ketones. However, not all of the shift to lower frequency is the result of intermolecular hydrogen bonding. Part of the decrease in the v(C=O :HO) frequency is attributed to intermolecularly hydrogen bonded alkyl alcohol molecules surrounding the dialkyl ketone or alkyl alkanoate molecules which are not intermolecularly hydrogen bonded with an alkyl alcohol.

7 (I 994) I-29

f

zk-0 8 L-o8 i\ ‘R,, 0'

0'

R”

Complex D The order of magnitude of the frequency decrease for the v(M) (ROH) mode for the ketone or ester in alkyl alcohol solution compared to v(C=O) (hexane) for a ketone or ester in hexane solution is comparable to the frequency decrease for the v(C==O) (ether) mode for the ketone or ester in diethyl ether or methyl tertbutyl ether solution compared to the v(C=O) (hexane) mode for the ketone or ester in hexane solution. /l/2

(3

\1/203

>c=o

A

B Both A and B are surrounded by intermolecularly hydrogen bonded (ROH), molecules. The surrounding (ROH), molecules displace dipolar interaction between ketone molecules or between ester molecules such as complexes C and D depicted below. R \ R\ @C-oe 6I3c-oe R’I

R’I Complex C

n

Complex E l/2 @ R”’ \

>C=O:H ‘O-R

”

R oe&-oe

R’,, l/2@

0’ ‘R,

n

Complex F 0 R

R

\ 0

( O:H,

&J-o8 /

3’

Ill

Y

Complex G

RA.

Nyquist

/ V~mtional

@

P (

O:H,

Ii

RL3~c - oe

._H'lnO(

p

R’

R

Complex H Complexes such as E or F shown above are possible for dialkyl ketone or alkyl alkanoate in diethyl ether, and complexes such as G or H shown above are possible for dialkyl ketone or alkyl alkanoate in alkyl alcohol solution where the alkyl alcohol molecules are intermolecularly hydrogen bonded with each other and surrounding the dialkyl ketone or alkyl alkanoate molecules, but are not intermolecularly hydrogen bonded to the C=O group of dialkyl ketone or alkyl alkanoate molecules. Complexes E and G and complexes F and G are similar in nature, and would be expected to have comparable dipolar

S’ctmcopy

7 (1994)

l-29

3

effects upon the Y(C=O) frequency in solution with either a dialkyl ether or an alkyl alcohol. As indicated in the text, the major factor affecting the v(C==O:HO) (ROH) frequency is the intermolecular hydrogen bond (C=O : HO). The minor factor affecting both the v(C=O) (ROH) and v(C==O:HO) (ROH) modes for ketones or esters is their dipole interaction with intermolecularly hydrogen bonded alkyl alcohol solvent molecules such as depicted in complexes F and G. 2. Experimental Infrared spectra were recorded using a Nicolet 710 system. Frequencies reported are those printed out by the computer. Instrument resolution employed in these experiments is 4 cm:’ which is smaller than the half-band widths of the carbonyl stretching absorption bands. The dialkyl ketones were prepared as 1% (w/v) solutions using the different solvents listed in Table 1. The

Table 1 Infrared data for aikyl alkanoates in various solvents at 0.5% (w/v) concentration (carbonyl stretching frequencies, in cm-‘) Solvent

MA

MP

MIB

MTMA

Hexane

1755.44

1752.09

1747.85

1742.79

Diethyl ether Methyl t-butyl ether Carbon tetrachloride Benzene 1,2-Dichlorobenzene Nitrobenxene Acetonitrile Benxonitrile Nitromethane Methylene chloride Chloroform Chloroform-d reti-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol Dimethyl sulfoxide (Neat) Carbon disulfide

1750.99 1750.81 1748.00 1746.10 1743.45 1741.46 1741.75 1741.42 1740.08 1739.44 1737.40 1737.46 1751.44 1751.21 1749.49 1747.56 1737.48 1745.18 1746.85

1747.79 1747.31 1744.58 1742.94 1739.74 1739.91 1738.73 1738.49 1737.10 1735.78 1734.11 1734.23 1747.68 1748.56 1746.59 1743.67 1734.61 1742.97

1743.84 1743.45 1740.59 1738.92 1735.54 1734.08 1734.58 1734.32 1732.92 1731.77 1730.91 1730.83 1743.33 1743.92 1742.46 1739.52 1730.97 1740.79

1738.98 1738.67 1735.84 1734.41 1732.45 1731.40 1731.63 1731.96 1730.83 1730.64 1729.86 1729.66 1738.56 1738.58 1737.52 1735.69 1728.49 1736.60

1743.56

1739.60

1735.11

MTMA

1724.18 1722.13 1722.27 1720.99 1720.05 1720.02

1720.94

EA

EP

EIB

ETMA

1749.76 1745.39 1745.10 1741.77 1740.00 1737.32

1746.72 1742.79 1742.44 1739.05 1737.24 1733.55

1735.54 1735.71

1731.94 1732.66

1743.04 1739.25 1738.90 1735.70 1733.98 1730.28 1729.65 1729.27

1736.44 1732.40 1732.00 1728.32 1726.91 1723.84 1725.29 1722.04

1735.55 1734.00 1733.18 1732.24

1732.69 1730.72 1729.39 1727.71

1729.12 1727.23 1725.74 1724.90

1721.64 1720.03 1718.72 1718.58

1731.47 1746.41 1745.43 1743.72 1741.53 1732.31 1741.50 1740.80

1727.67 1742.83 1742.93 1741.22 1738.35 1729.08 1738.87

1724.85 1739.30 1739.26 1737.70 1734.89 1726.09 1736.73

1718.75 1733.22

1738.22

1734.87

1727.93

1732.76 1730.96 1728.60 1719.87 1730.26

AN ‘[7] 0.0 3.9 (4.4) 8.6 8.2 (13.8) 14.8 18.9 15.5 (18.5) 20.4 23.1 23.1 (3.7) (3.7) (6.3) (7.6) 19.1 (8.7) (7.9)

Abbreviations: AN = acceptor number; MA = methyl acetate; MP = methyl propionate; MIB = methyl isobutyrate; MTMA = methyl trimethylacetate; EA = ethyl acetate; EP = ethyl propionate; EIB = ethyl isobutyrate; ETMA = ethyl trimethylacetate. a Values in parentheses are determined by IR in the present study.

11.02

7.00

8.81

1716.66 1716.22 1715.91 1715.61 1715.34 1714.93 1714.82 1714.61 1714.42 1714.45 1714.36 1714.23 1714.09 1713.88 1713.59 1713.33 1712.95 1712.42 1712.13 1711.39

1720.20

MPK

9.86

1717.35 1716.28 1715.82 1715.45 1715.07 1714.74 1714.33 1714.19 1713.92 1714.00 1713.91 1713.67 1713.41 1713.16 1712.86 1712.34 1711.94 1711.47 1711.05 1710.37

1720.23

MBK

and/or

12.01

1716.32 1715.28 1714.39 1713.89 1713.44 1712.56 1711.91 1711.62 1711.28 1711.23 1711.17 1710.90 1710.67 1710.14 1709.77 1709.50 1709.22 1708.99 1708.64 1708.05

1720.06

IBMK

8.88

1716.91 1716.08 1715.49 1715.11 1714.81 1714.60 1714.37 1714.23 1714.10 1714.02 1713.98 1713.84 1713.69 1713.51 1713.26 1712.82 1712.45 1712.04 1711.63 1710.83

1719.71

DEK

10.26

1715.79 1714.78 1714.23 1713.96 1713.69 1713.46 1713.14 1712.91 1712.69 1712.61 1712.49 1712.41 1712.27 1712.15 1711.96 1711.71 1711.34 1710.51 1709.94 1707.72

1717.98

MIK

10.51

1716.01 1715.00 1714.72 1714.05 1713.64 1713.44 1713.11 1713.07 1712.79 1712.64 1712.56 1712.27 1712.12 1712.07 1711.79 1711.41 1710.98 1710.49 1709.41 1707.28

1717.79

MIK2 a

10.62

1712.59 1711.57 1710.78 1710.46 1710.18 1709.90 1709.57 1709.33 1709.04 1708.91 1708.79 1708.62 1708.46 1708.17 1707.87 1707.41 1706.82 1706.23 1705.72 1705.03

1715.65

SBMK

8.52

1716.43 1714.00 1713.41 1712.78 1712.54 1711.97 1711.76 1711.65 1711.42 1711.26 1711.12 1711.00 1710.86 1710.73 1710.58 1710.42 1710.12 1710.03 1709.31 1708.73 1707.91

EIK2 a

CHCl, solutions (cat-bony1 stretching frequencies, cm-‘)

1709.52 1707.32 1706.43 1705.79 1705.47 1705.25 1705.05 1704.94 1704.78 1704.74 1704.72 1704.67 1704.57 1704.45 1704.33 1704.16 1704.06 1703.86 1703.51 1702.85 1701.93 7.59

10.90

TBMK

1715.80 1713.67 1711.50 1710.80 1709.67 1709.14 1708.83 1708.61 1708.29 1708.00 1707.91 1707.73 1707.68 1707.27 1707.08 1706.76 1706.64 1706.27 1705.88 1705.47 1704.90

DIK2’

: i: t v 3 ! z 3 h

1681.72 1681.70 1681.66 1681.56 1681.54 1681.44 1681.43 1681.30 1681.28 1681.21 1681.12

5.20

2;

1682.17 1681.94

7.89

? $

1683.05 1682.46

1681.07 1680.96 1680.81 1680.70

1685.90 1684.71

DTBK2’

1709.19 1708.13 1706.53 1705.96 1705.54 1705.33 1705.11 1704;93 1704.81 1704.72 1704.64 1704.63 1704.54 1704.44 1704.27 1704.07 1703.93 1703.71 1703.32 1702.57 1701.40

TBMK2 a

Abbreviations: DMK = dimethyl ketone; MEK = methyl ethyl ketone; MPK = methyl propyl ketone; MBK = methyl butyl ketone; IBMK = isobutyl methyl ketone; DEK = diethyl ketone; MIK = methyl isopropyl ketone; SBMK = see-butyl methyl ketone; EIK = ethyl isopropyl ketone; DIK = diisopropyl ketone; TBMK = tert-butyl methyl ketone; DTBK = di-teti-butyl ketone. a MIK2, EIK2, DIK2, TBMK2, and DTBK2 were run at 2% concentration in Ccl, and/or CHCI, solutions [5].

A (cm-‘)

1721.35

1718.64 1717.41 1716.59 1716.18 1715.81 1715.41 1715.10 1714.67 1714.31 1714.14 1714.15 1713.96 1713.57 1713.47 1712.97 1712.67 1712.16 1711.53 1711.20 1710.33

1717.48

1716.06 1715.89 1715.07 1714.93 1714.21 1713.87 1713.67 1713.40 1713.21 1713.14 1712.95 1712.75 1712.57 1712.27 1712.09 1711.87 1711.65 1711.26 1710.88 1710.48

MEK

10.74 19.40 26.53 32.60 37.57 41.93 45.73 49.06 52.00 54.62 57.22 60.07 63.28 66.73 70.65 75.06 80.06 85.05 92.33 100.00

DMK

00.00

Mel% CHCI, /cCI,

Table 2 Infrared data for diaIky1 ketones in 1% Ccl,

1722.35 1719.57 1719.30 1717.69 1715.83 1714.43 1712.79 1713.33 1712.66 1712.20 1712.03 1710.60 1710.75

Hezane Diethyl ether Methyl t-butyl ether Carbon tetrachloride Benzene 1,2-Dichlorobenzene Nitrobenzene Acetonitrile Benzonitrile Nitromethane Methylene chloride Chloroform Chloroform-d rertButy1 alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol Dimethyl sulfozide (Neat) Carbon distdfide

1717.00 1716.20 1709.22 1714.62 1716.28

DMK

Solvent

1727.20 1723.00 1722.78 1721.30 1718.60 1716.69 1714.30 1713.95 1713.90 1712.40 1712.60 1710.20 1710.29 1722.00 1720.50 1719.40 1718.60 1709.70 1716.91 1720.10

MEK

1722.62 1719.97 1719.26 1709.91 1716.72 1718.89

1725.73 1722.02 1721.82 1726.20 1717.82 1715.99 1714.33 1714.03 1713.60 1712.50 1712.94 1711.39 1711.36

MPK 1725.83 1722.08 1721.85 1729.21 1717.86 1715.88 1714.10 1713.85 1713.34 1712.32 1712.46 1710.40 1710.37 1722.00 1720.50 1719.40 1718.60 1709.83 1717.14 1719.08 1725.16 1721.77 1721.59 1728.01 1717.40 1714.61 1712.35 1712.01 1712.06 1710.31 1710.44 1708.01 1708.03 1721.95 1720.10 1719.45 1718.15 1707.82 1715.98 1718.66

IBMK 1725.05 1721.43 1721.14 1719.63 1717.40 1715.47 1713.83 1713.65 1713.24 1712.37 1712.47 1710.70 1710.81 1722.49 1720.53 1717.78 1717.00 1710.13 1715.94 1718.32

DEK

stretching frequency, cm-‘)

MBK

Table 3 Infrared data for dialkyl ketones in dilute solution &bony1

1719.32 1718.30 1714.64 1706.53 1715.34 1716.54

1723.35 1719.85 1719.64 1717.98 1715.63 1713.91 1712.04 1711.26 1711.96 1709.61 1710.43 1707.72 1707.64

MIK 1721.78 1717.88 1717.50 1715.65 1713.28 1710.71 1708.90 1708.66 1709.86 1707.38 1707.62 1705.03 1705.01 1717.72 1716.47 1716.22 1713.47 1704.64 1712.50 1714.38

SBMK 1721.40 1718.20 1717.93 1716.50 1714.00 1712.11 1710.60 1710.60 1710.20 1709.50 1709.60 1708.20 1708.25 1718.80 1716.20 1715.40 1715.00 1707.40 1713.54 1715.20

EIK

1720.30 1717.80 1717.54 1716.00 1713.50 1709.41 1707.60 1707.80 1706.80 1706.30 1706.40 1705.30 1705.37 1718.00 1717.00 1716.00 1715.00 1704.60 1713.97 1714.50

DIK

TBMK -1714.70 1711.58 1711.22 1709.52 1707.84 1705.84 1704.37 1704.47 1704.36 1703.29 1703.12 1701.93 1701.97 1712.24 1710.41 1708.59 1707.65 1701.49 1707.92 1708.46

1690.29 1687.73 1687.65 1685.92 1684.51 1683.25 1681.66 1681.56 1682.21 1680.57 1680.49 1680.81 1680.91 1687.01 1686.99 1686.37 1684.83 1680.07 1685.72 1684.94

DTBK

0.0 3.9 4.4 8.6 8.2 12.7 14.8 18.9 15.5 16.0 28.4 23.1 23.1 3.7 3.9 6.3 7.6 19.3 7.2 8.5

AN

[61

F

R.A. Nyquist / VZwatkmal Spctroscopy 7 (1994) l-29

6

mol% CHCIJCCl, solutions used in Table 2 were prepared from stock solutions of 1% (w/v> Ccl, solution and 1% (w/v) CHCl, solution with the exceptions of (MlK2), (EIK21, (DIK2), (TBMK2), and (DTBK2) which were run at 2% (w/v) solutions (see ref. 5). For example, 2 in this case for (DTBK2) means 2% (w/v) for di-tertbutyl ketone. All solutions were run in 0.1 mm cells for IR examination.

3. Results and discussion Table 3 lists IR data for dimethyl ketone (DMK), methyl ethyl ketone (MEK), methyl propyl ketone (MPK), methyl butyl ketone (MBK), isobutyl methyl ketone (IBMK), diethyl ketone (DEK), methyl isopropyl ketone (MIK), set-butyl methyl ketone (SBMK), ethyl isopropyl

Fig. 1. Plots of v(C=O) (solvent or neat) vs. v(O)

ketone (EIK), diisopropyl ketone (DIK), tert-butyl methyl ketone (TBMK), and di-tert-butyl ketone (DTBK) in 19 different solvents at 1% solutions, and for each ketone in the neat liquid phase. Fig. 1 shows plots of Y(GO) (solvent or neat) vs. v(<=<=<=<=<=<=<=<=<=<=--o) (hexane) minus v(C=O) (solvent or neat). Each of the 12 straight line plots are parallel to one another, and the relationship between the GO frequency and the difference in frequency always produces a straight line plot [51. The highest AC=01 frequency in the series in hexane solution is exhibited by MEK (1727.20 cm-‘) and the lowest v(C==O) frequency in this series in hexane solution is exhibited by DTBK (1690.29 cm- ‘1. Excluding inter-molecularly hydrogen bonded v(C==O: HOR) (in ROH) frequencies, the highest v(C.4) frequency in this series in dimethyl sulfoxide solution is exhibited by DEK

(hexane) minus v(M)

(solvent or neat) for dialkyl ketones.

R.A. mquist / Viirational Specrroscpy 7 (1994) I-29

study (the solvent acceptor numbers (AN) for a variety of solvents have been established using NMR and (C,H,),F’=O [61). Several factors determine the type and amount of solute-solvent interaction. These are: dipolar interaction, intermolecular hydrogen bonding, relative basicity of solute or solvent, relative acidity of solute or solvent, molecular geometry of the solute, molecular geometry of the solvent, steric factors of the solute, and steric factors of the solvent. These factors determine the extent of solute-solvent interaction. Therefore, the published AN values are not a precise constant applicable for accurately predicting vibrational frequencies of functional groups such as ~(00) for dialkyl ketones present in a particular solvent. Fig. 2 shows plots of the v(C=O) frequencies of the dialkyl ketones in hexane solution vs. the summation of the sigma star values (CU *) for the two alkyl groups (increasing negative u* values indicate increasing electron donation to the carbony1 group). With the exception of dimethyl

(1710.13 cm-‘), and the lowest v(C==O)frequency in this series in dimethyl sulfoxide solution is exhibited by DTEK (1680.07 cm-‘). Table 4 compares the frequency difference between ~(0) (hexane) for each dialkyl ketone and v(C=O) (solvent) for each dialkyl ketone in each of the other solvents, excluding the frequency differences between v(C=O) (hexane) and v(C=O : HOR) (in ROH) for the intermolecularly C=O : HOR for the four alcohols used as solvents. A study of Table 2, excluding hexane, shows that the frequency separation between ~((50) (hexane) and v(w) (diethyl ether) is different for each of the 12 dialkyl ketones. Moreover, the frequency separation between v(CLO) (hexane) and Y(C=O) (solvent) is different for each of the dialkyl ketones in each of the other 17 solvents. These data indicate that the solvent acceptor number (AN) concept for a particular solvent is not plausible since the frequency separation between ~(0) (hexane) and v(C=O) (solvent) is not a constant for any of the solvents used in this

Table 4 IR data for the frequency difference between ~(0) of the other solvents (in cm-‘)

7

for each diethyl ketone and v(c-0)

(solvent) for each dialkyl ketone in each

Solvent

DMK

MEK

MF’K

MBK

IBMK

DEK

MIK

SBMK

EIK

DIK

TBMK

DTBK

Hexane Diethyl ether Methyl r-butyl ether Carbon tetrachloride Benzene 1,2-Dichlorobenzene Nitrobenzene Acetonitrile Benzonitrile Nitromethane Methylene chloride Chloroform Chloroform-d rert-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol Dimethyl sulfoxide (Neat) Carbon disulfide

0.00 2.78 3.05 4.66 6.52 7.92 9.56 9.02 9.69 10.15 10.32 11.75 11.60

0.00 4.20 4.42 5.90 8.60 10.51 12.90 13.25 13.30 14.80 14.60 17.00 16.91 5.20 6.70 7.80 8.60 17.50 10.29 7.10

0.00 3.71 3.91 5.53 7.91 9.74 11.40 11.70 12.13 13.23 12.79 14.34 14.37

0.00 3.75 3.98 5.62 7.97 9.95 11.73 11.98 12.49 13.51 13.37 15.43 15.46 3.83 5.33 6.43 7.23 16.00 8.69 6.75

0.00 3.39 3.57 5.15 7.76 10.55 12.81 13.15 13.10 14.85 14.72 17.15 17.13 3.21 5.06 5.71 7.01 17.34 9.18 6.50

0.00 3.62 3.91 5.42 7.65 9.58 11.22 11.40 11.81 12.68 12.58 14.35 14.24 2.60 4.50 7.30 8.10 14.92 9.11 6.73

0.00 3.50 3.71 5.37 7.72 9.44 11.31 12.09 11.39 13.74 12.92 15.63 15.71

0.00 3.90 4.28 6.13 8.50 11.07 12.88 13.12 11.92 14.40 14.16 16.75 16.77 4.06 5.31 5.56 8.31 17.14 9.28 7.40

0.00 3.20 3.47 4.90 7.40 9.29 10.80 10.80 11.20 11.90 11.80 13.20 13.15 2.60 5.20 6.00 6.40 14.00 7.86 6.20

0.00 2.50 2.76 4.30 6.80 10.89 12.70 12.50 13.50 14.00 13.90 15.03 14.93 2.30 3.30 4.30 5.30 15.70 6.33 5.80

0.00 3.12 3.48 5.18 6.86 8.86 10.33 10.23 10.34 11.41 11.58 12.77 12.73 2.46 4.29 6.11 7.05 13.21 6.78 6.24

0.00 2.56 2.64 4.37 5.78 7.04 8.63 8.73 8.08 9.72 9.80 9.48 9.38 3.30 3.30 3.90 5.50 10.22 4.57 5.35

5.40 6.20 13.13 7.73 6.07

2.63 6.09 15.82 9.01 6.84

4.03 5.05 8.71 16.82 8.01 6.81

AN ’ [6] 0.0

8.6 (1::;) 14.8 18.9 15.5 (16.0) 20.4 23.1 23.1 (3.7) (3.9) (6.3) (7.6) 19.3 (7.2) (8.5)

The frequency differences between v(W) and v(C==O: HOR) for each dialkyl ketone in each of the four alcohols are excluded (see Table 2). a IR AN values in parentheses were calculated from the average of four ethyl alkanoates studies (see Table 10).

8

R.A. Nyquist / V~mbonal Spectroscopy 7 (1994) l-29

ketone, the ~(0) frequencies for the dialkyl ketones decrease as the electron contribution from the alkyl group(s) to the carbonyl group increases. With an increase in contribution of electrons to the carbonyl group there is an increase in the contribution of the resonance form

R-C-R’ CB which weakens the GO bond, and consequently the u(C=O) mode vibrates at a lower frequency. Fig. 3 shows plots of v(C=O) for dialkyl ketones in hexane solution vs. the summation of the steric factor CEs of the two alkyl groups. The larger the negative Es value the larger the steric factor of the alkyl group. The Es value for the alkyl groups are: methyl (01, ethyl ( - 0.071, propyl C-0.361, butyl (- 0.391, isobutyl C-0.931, isopropyl ( - 0.471, set-butyl (- 1.101, and tert-butyl

(- 1.54). With the exception of the dimethyl ketone, the V(M) frequencies for dialkyl ketones decrease as the negative value for CEs becomes larger. The extent of dipolar interaction between dialkyl ketones is dependent upon the steric factors of the alkyl groups and the relative + and charges on the carbon and oxygen atoms (@C-O 8 1, respectively. With a larger contribution from resonance form @C-O 8 , a larger dipolar interaction between the dialkyl ketones

R\ R\ ec-oe ec-oe

I R’I

R’I

In

is expected in the neat liquid phase. We suggest that the difference between the ~(00) frequency for a dialkyl ketone in dilute solution in hexane and the v(C=O) frequency for the same dialkyl ketone in the neat liquid phase is a mealo

-3.4Q --

0.45

--

Fig. 2. Plots of &GO) (hexane) vs. Y&T * [7_lfor dialkyl ketones.

RA Nyqukt / Vibrational Spechoscopy 7 (1994) l-29

sure of the dipolar interaction between dialkyl ketone molecules in the neat liquid phase, since there is essentially no dipolar interaction between dialkyl ketones in dilute solution in hexane and no dipolar interaction between the solute and hexane. Fig. 4 shows plots of the u(C=O) frequencies for the dialkyl ketones in dilute solution in hexane vs. the frequency difference between a dialkyl ketone in dilute solution in hexane and the same dialkyl ketone in the neat liquid phase. With the exception of dimethyl ketone, the plots show that as the v(M) (hexane) mode decreases in frequency, the frequency separation between v(C=O) (hexane) and v(C=O) (neat liquid) decreases. Fig. 5 shows a plot of the frequency difference between v(C=O) for dialkyl ketones in dilute solution in hexane and v(G=O) for dialkyl ke-

9

tones in the neat liquid phase vs. the summation of the sigma star (Ca*) values of the two alkyl groups. With the exception of dimethyl ketone, the frequency difference v(C==O) (hexane) v(C=O) (neat liquid), cm-‘, increases as CC* increases. As mentioned above, the higher Ca* values cause more contribution from the resonance form

which induces more of a dipolar interaction between dialkyl ketone carbonyl groups. Thus, one would expect the largest dipolar interaction to occur between DTBK molecules since this molecule has the largest Ca * value. As discussed

11.lwthylTelt-butyl 12. DcCm4wtyl

10

R.A. Nyqukt / Viirationai Spectroscopy 7 (1994) l-29

previously, the term u(C==O) (hexane) - v(C=O) (neat liquid), cm-‘, is a measure of the dipolar interaction between

k-08

I I R’

n

molecules and the dipolar interaction should be the largest for DTBK, but the value for DTBK is the lowest (4.57 cm-r) and the value for DEK is the highest (10.29 cm-‘) in the dialkyl ketone series. Thus, behavior of v(M) is the opposite of what is expected for an increase in the CC* value. This difference we attribute to the large steric factor of the tert-butyl group (Es = - 1.54) compared to the steric factor of the ethyl group (Es = - 0.07) which prevents the ROH proton from getting as close to the CL0 group in the core of DTBK compared to the core of DEK.

Fig. 6 shows a plot of the summation of the steric values for the two alkyl groups CEs for dialkyl ketones vs. v(C=O) (hexane) - ~(0) (neat liquid), cm-‘. This plot shows that v(O) (hexane) - Y(C==O)(neat liquid), cm-‘, decreases as the CEs value increases, and this is the order expected for the CC* vs. Y(C=O) (hexane) v(C==O)(neat liquid), cm-‘. The steric and inductive effects of the two all@ groups are simultaneously affective, and it is apparent that the steric factor dominates the dipolar interaction in that the increasing distance (d) between

3C-0 R’I

2k

- oe

8

R’I

led-1 molecules offsets the effect of the increasing Ca*

0 .

R-C-RI R

--

1.

Rl

ai

2. ai 3. ai 4. ai ~~~ 7. CH3 8. Q13

a5 “-C3ti, n-c4n, W-C@9 C$I, iSO-C31i7

9. %H, ;y. kugc3n7

=-C/b ISO-C3t17 ko-0-4H7

. 3 12. et-c4n9

m-Cl% tat-C4H9

UC-O,

Fig. 4. Plots of 1430)

cm-1 (hexme)

(hexane) vs. v(C=O) (hexane) minus v(CO) (neat liquid) for the dialkyl ketones.

1709.55 1707.17 1705.45 1704.64

MIK

1719.32 1718.30 1714.64

MIKHO

1717.0 1716.2

rertButy1 alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

1711.82 1710.31 1709.04 1708.03

rerr-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

DMKHO

DMK

Solvent

1718.8 1716.2 1715.4 1715.0

EIK

1722.0 1720.5 1719.4 1718.6

MEK

1709.2 1707.7 1706.1 1704.8

EIKHO

1711.23 1709.80 1708.50 1707.50

MEKHO

1718 1717 1716 1715

DIK

1723.10 1719.64

MPK

1705.6 1704.2 1703.1 1701.1

DIKHO

1712.27 1710.49 1708.56 1706.84

MPKHO

1717.72 1716.47 1716.22 1713.47

SBMK

1722.62 1719.97 1719.26

MBK

1706.70 1704.92 1703.27 1702.03

SBMKHO

1711.81 1710.13 1708.63 1707.52

MBKHO

1712.24 1710.41 1708.59 1707.65

TBMK

1722.49 1720.53 1717.78 1717.00

DEK

Table 5 Infrared data for v(C=O) (ROH) and v(C==O:HO) (ROH) for diaikyl ketones in solution with aIky1 alcohol (in cm-‘)

1702.24 1701.12 1699.82 1698.41

TBMKHO

1711.80 1710.34 1710.13 1707.66

DEKHO

1687.01 1686.99 1686.37 1684.83

DTBK

1721.95 1720.10 1719.45 1718.15

IBMK

1678.48 1677.88 1676.83 1675.15

DTBKHO

1709.46 1708.03 1706.82 1705.89

IBMKHO

R.A. Njquist / Viratiod

12

contribution to the dipolar interaction between dialkyl ketones in the neat liquid phase. Figs. 7-9 are plots of ~(60) (hexane), cm-‘, vs. (Cc * + XEs)/(Ca * XCEs), ~(60) (neat liquid), cm-‘, vs. (Ca * + CEs)/(Ca *XCEs), and ~(00) (hexane) - ~(60) (neat liquid), cm-‘, for dialkyl ketones, respectively. With the exception of dimethyl ketone, Y(C=O) (hexane), v(C=O) (neat liquid), and v(C=0) (hexane) - v(C=O) (neat liquid), cm-i, all increase in frequency as the (Ca * + CEs)/(Ca * XCEs) value decreases. The correlations combining both u * and Es show that both the inductive and steric factors contribute to the extent of dipolar interaction between dialkyl ketone molecules. Fig. 10 shows a plot of v(C==O) (Ccl,) vs. v(C==O)(CHCl,) for methyl alkyl ketones in CCI, solution and in CHCl, solution. The plot shows that the 140) frequencies decrease as the branching is increased on the a-carbon atom of

Spectroscopy 7 (1994) I-29

the alkyl group. The plot shows that the ~(60) frequency shift for each methyl alkyl ketone in going from Ccl, solution to CHCl, solution is different since the individual points on the plot do not fall on the linear plot. In other words the solute-solvent interaction is different for each compound in Ccl, solution and in CHCI, solution. In CHCI, solution the solute-solvent interaction could be R @HCl,C-Cl0

&-0

8 Q H-CC12Cle),

I

R’

and in Ccl, solution the solute-solvent tion could be

(eCl,CCle

“\ @ c-o

8 @ Cl-CCl,Cle

R’,

(1) l

0.00

T 0.05

--

0.10

--

-0.15--

-0.20--

E2!2 -

R

(1) cII3 (2) cII3 (3) cH3 (4lcH3 (5) cII3 (6) C2q

-

Rl

zz, C3H7 c4H9 h-C4% CZHS

Fig. 5. Plots of v(C=O) (hexane) minus 1460)

(neat liquid) vs. ZU * [I for

ketones.

interac-

), .

RA.Nyq~t/~~rationalSpcctroscow 7(1994)1-29 This plot then also suggests that inductive and steric factors of the alkyl groups of the dialkyl ketones affects solute-solvent interaction. Figs. 11 and 12 are plots of u((==o) (Ccl,), cm- ‘, vs. (Ca * + CEs)/(Co *XCEs) and v(C=C) (CHCl,), cm- ‘, vs. (Ca * + CEs)/(Co * + XEs) for methyl alkyl ketones, respectively. With the exception of DMK, a relatively smooth correlation is noted for methyl alkyl ketones in Ccl, solution. In CHCl, solution, both DMK and MEK do not correlate with the other members of this series. Fig. 13 shows plots of u(C=O) for half of the dialkyl ketones studied in CI-ICl,/CCl, solutions and Fig. 14 shows plots of the other half of the dialkyl ketones studied in CI-ICl,/CCl, solutions. All 12 plots show that v(C=O) for each of the dialkyl ketones decreases in frequency as the mol% CHC13/CCl, increases. All of the plots show that the rate of the v(C==O) frequency de-

13

crease is the highest between 0 and N 15 mol% CHCl,/CCl,. This relatively rapid rate of decrease, we suggest, results from a shift in the equilibrium between solvent molecules and solute-solvent molecules such as A below. @CICCl,CI 8 -

R \ e c-o

~clccI,Cle A

@CICCl,CI 8 e H-CC&I “\ ~CICCl,CI 8 @C-O

8 -

R”

8 -

8 e H-CCl,CI

8 -

R” B

elH-Cl&Cl

“\ 8 "/C-O 8 e H-CC12Cle

R’ +(eCICCl,H@),

C

0.0

-02 -0.4

-0.6

I (5) “3

-22--

(w+5 CI) cH3 (8) ai3 (9) C2H5 y; lsg’C3H7

a.. --

3 (12) et-c4I.4

(K-O

Fig. 6. Plot of v(c-0)

(hexane) minus vG0)

(h-

)-vc-o(ncatUqoid~),cm-l

(neat liquid) vs. EEs [8] for dialkyl ketones.

‘Jo-C4% c2H5 iso-C31i, SZC-C~H,,. iSO-C3H, tso-C3I1, tert-c4ll,, ten-C411,,

14

R.A. Nyquist / IJihaional

As the mol% CHClJCCl, increases, the equilibrium shifts from A to B to C when the solute molecules exist only in CI-ICl, solution in the case of C. Table 5 lists IR data for v(C=O) (ROH) and v(C=O :HO) (ROH) for dialkyl ketones in solution with alkyl alcohol. Table 6 lists IR data for v(C==O) (hexane) minus v(C==O: HO) (ROH) for dialkyl ketones. Table 7 lists IR data for v(C=O) (hexane) minus v(C=O) (ROH) for dialkyl ketones. Table 8 lists IR data for v(C=O) (ROH) minus v(C=O:HO) (ROH) for dialkyl ketones. Table 9 lists the percentage of the shift of the v(C=O : HO) (ROH) frequencies attributed to intermolecular hydrogen bonding for dialkyl ketones in solution with alkyl alcohol. The data in Table 5 show that the v(GO) (ROH) frequencies always occur at higher frequencies than the u(C=O: HO) (ROH) frequencies for each of the dialkyl ketones. For example

Spectroscopy

7 (1994) l-29

v(C=O) (CH,OH) for DMK occurs at 1716.2 cm-’ and v(GO : HO) (CH,OH) for DMK occurs at 1708.3 cm-‘. In addition, the v(Ca) (ROH) and v(C==O:HO) (ROH) frequencies for each of the dialkyl ketones decrease in the order: u(C=O) (teti-C,H,OH), v(CS) (iso-C,H,OH), v(O) (C,H,OH), v(C=O) (CH,OH), v(cr-0 : HO) (tert-C,H,OH), v(C!=O : HO) (iso-C,H,OH), v(GO : HO) (C,H,OH), and v(C=O : HO) (CH,OH). For example, the frequencies for MEK in the four different alcohols decrease in the order: v(C=O) (tert-C,H,OH), 1722.0 cm-‘; v(C=O) (iso-C,H,OH), 1720.5 cm-‘; v(C=O) (C,H,OH), 1719.4 cm-‘; v(C=O) (CH,OH), 1718.6 cm-‘; v(C=O : HO) (fert-C,H,OH), 1711.23 cm-‘; v(C=O : HO) (iso-C,H,OH), 1709.80 cm- ‘; u(C=O : HO) (C,H,OH), 1708.50 cm-‘; and u(GO : HO) (CH,OH), 1707.50 cm-‘. We also note in Table 5 that both u(C=O) (ROH) and u(GO :HO) (ROH) for the dialkyl ketones .

Fig. 7. Plot of v(C=O) (hexane) vs. (Ca * + CEs)/(Ec * XCEs) for the dialkylketones.

(1)

R.A. Nyquist / Viiratitntai

Spectroscopy7 (1994) l-29

decrease in frequency as the branching is increased on the a-carbon atom(s). The data in Table 6 show that the frequency difference between AGO) (hexane) and &GO) (ROH) for the dialkyl ketones increases in the order of the alcoholic solution: reti-C,H,OH, iso-C,H,OH, C,H,OH, and CH,OH. The data in Table 7 show that the frequency difference between v(C=O) (hexane) and v(C==O)(ROH) for the dialkyl ketones also increases in the order of the alcoholic solution: tert-C,H,OH, isoC,H,OH, C,H,OH, and C&OH. The data in Table 8 show that the frequency differences between v(CX3) (ROH) and v(GO:HO) (ROH) for each of the dialkyl ketones do not shift in a consistent manner in the order of the alcoholic solution: teti-C,H,OH, iso-C,H,OH, C,H,OH, and CH,OH. It is well known that the intermolecular hydrogen bonding between a carbonyl group and

15

a hydroxyl group (C=O: H-O) lowers the carbony1 stretching frequency from v(C%O) to ~00 : HO), viz. v(C=O) (ROH) to v(C==O: HO) (ROH). We have noted above that both v(C=O) (ROH) and v(cFO: HO) (ROH) for the dialkyl ketones shii in frequency with change in ROH (tert-C,H,OH, iso-C,H,OH, C,H,OH, and CH,OH). Since dialkyl ketone molecules exist in alcoholic solution in a non-hydrogen bonded state (R&=0) and an inter-molecularly hydrogen bonded state (R&=0 : HOR’) where both species are surrounded in solution with intermolecularly hydrogen bonded (ROH), molecules, not all of the difference in the frequency between u(C=O) (hexane) and v(C=O: HO) (ROH) can be attributed to intermolecular hydrogen bonding between the carbonyl groups and the OH group (R,GO : HOR’). We suggest that the percentage of the shift in the v(C=O : HO) (ROH) frequency attributed to intermolecular hydrogen bonding

l (1)

-16 %a4

t 1688

16ea

1690

1692

SW

169S

ssa

1700

UC-o,

Fig. 8. Plot of v(M)

1m2

cm-1hat

(neat liquid) vs. (Ca * + Es)/@

,704

lrn

17*

,710

17,2

liquid)

* )(Es)

for dialkyl ketones.

I,,.

,,,s

171:

R.A. IVyquist/ V&rational Spctmscopy 7 (1994) l-29

16

for dialkyl ketones in solution with an alkyl alcohol is obtained by the following equation: u(c-0: HO) (ROH) = iv(~) (ROH) - y(c = 0 : HO) (ROH)] /[v(C=O) (hexane) - v(GO:HO) (ROH)]

Table 9 lists the calculated percentage of the frequency shift attributed to intermolecular hydrogen bonding between the alcoholic OH proton and the free pair of electrons on the carbonyl oxygen atom for the dialkyl ketone. The percentage of the v((==o : HO) frequency shift attributed to intermolecular hydrogen bonding between (ROH:O=CR,) decreases in the order tertC,H,OH : O=CR,, iso-C,H,OH : O=CR,, C,H,OH: O=CR,, and CH,OH: O=CR,. The lowest percentage calculated is that for MIK in dilute solution with methyl alcohol (49.0%), and the highest percentage calculated is that for DIK

in dilute solution with tert-butyl alcohol (84.4%). Presumably these results are determined by the relative acidity of the alcoholic proton, the relative basicity of the carbonyl oxygen atom which is dependent upon the inductive contribution of the two alkyl groups of the dialkyl ketone together with the steric effect of the alkyl groups in both the dialkyl ketone and the alkyl alcohol. 3.1. Alkyl alkanoates Table 1 lists IR data for methyl acetate (MA), methyl propionate (MP), methyl isobutyrate (MIB), methyl trimethylacetate (MTMA), ethyl acetate (EA), ethyl propionate (EP), ethyl isobutyrate (EIB), and ethyl trimethyl acetate (ETMA) at 0.5 wt.% in various solvents. Table 10 lists IR data for the v(C==O) frequency for alkyl alka-

“T

4 --

-s -l (8)

-s --

-.I

--

TLP*LEs

cT.dad

a---

(3) ul3

-a--

=3a7 c4m9

(41-3 (5) cH3

-10--

‘so-W9 =2m5 ‘so-C3H,

(6) v5 (7) ui3 (81 CH3 (9) C2H5

.I, --12--

y;

--=4Hg ‘so-C3H7

bp7

.Izl-(12) teA4n9

‘s”-c3n7 teat-c4n9 ten-c4n9

-u --15--16 4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

90

9.5

10.0

(YC-o(heune)-vc-o(neptLiquid),cm-1

Fig. 9. Plot of v(c-0) (hexane) minus v(C=O)(neat liquid) vs. (Xu * + XES)/(CU* )(jXs) for dialkylketones.

t 90.5

17

R.A. Nyquist / Vlmtional Spectroscopy 7 (1994) I-29

.

cm , 1x3

,710

l (1)

Rl

-- R (lmi3 w-3 (3) -3 (0~3 <9ar3 (6) a3

-3 cr4 c3*7 C4%

0-3 ON -3

=-cd+ M-=4%

171,

m

“-Cr% bo-C3H7

,112

1713

1714

,715

UC-o,

Fig. 10. Plot of v(M)

Table 6 Infrared data for ~00)

(3)

(hexane) minus v(GO

(Ccl,)

vs. v(W)

,716

cd

,717

171*

1719

mo

,729

1722

wa,)

(CHCls) for methyl alkyl ketones.

: HO) (ROH) for dialkyl ketones (in cm-‘)

Solvent

DMK

MEK

MPK

MBK

IBMK

DEK

MIK

SBMK

EIK

DIK

TBM

DTBK

teti-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

10.53 12.04 13.31 14.32

16.0 17.4 18.7 19.7

13.46 15.24 17.17 18.89

14.02 15.70 17.20 18.31

15.70 17.13 18.34 19.27

13.25 14.71 14.92 17.39

15.50 17.88 19.60 20.41

15.08 16.86 18.51 19.75

12.2 13.7 15.3 16.6

14.7 16.1 17.2 19.2

12.46 13.58 14.88 16.29

11.81 12.41 13.46 15.14

Table 7 Infrared data for ufC=OI (hexane) minus vG0)

(ROH) for dialkyl ketones (in cm-‘)

Solvent

DMK

MEK

MPK

MBK

IBMK

DEK

MIK

SBMK

EIK

DIK

TBMK

DTBK

terf-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

5.35 6.15

5.2 6.7 7.8 8.6

3.11 5.76 6.47

3.83 5.33 6.43 7.23

3.21 5.06 5.71 7.01

2.56 4.52 7.27 8.05

4.03 5.05 8.71

4.06 5.31 5.56 8.31

2.6 5.2 6.0 6.4

2.3 3.3 4.3 5.3

2.46 4.29 6.11 7.05

3.28 3.30 3.92 5.46

B.A. hryquist /

18

c31ctl3 <4xH,
V&ational

Spectroscopy

7 (1994) I-29

c3H7 c4x7 ~O-Crtls b=3H7 --=I% --=4Hg

t I,10

,711

,712

17,s

m4

UC-o.

,715

me

,717

17,s

VW

,720

tnc

17:

cm-1CCCl4l

Fig. 11. Plot of u(GO) (Ccl,) vs. (Z o * + ZEs)/(Zo

* XZEs) for dialkyl ketones.

Table 8 Infrared data for v(c=O) (ROH) minus v(c=O : OH) (ROH) for dialkyl ketones (in cm-‘) Solvent

DMK

MEK

MPK

MBK

IBMK

DEK

MIK

SBMK

EIK

DIK

TBMK

DTBK

tert-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

7.96 8.17

10.80 10.70 10.9 11.1

12.61 11.08 -

12.49 11.34 11.74

12.49 12.07 12.63

10.69 10.19 7.65 9.34

12.15 12.85 10.08

11.02 11.55 12.95 11.44

9.6 8.5 9.3 10.2

12.4 12.8 12.9 13.9

10.00 9.29 8.77 9.24

8.53 9.11 9.54 9.68

Table 9 Percentage of the shift of the v(C=O : HO) (ROH) frequency attributed to intermolecular hydrogen bonding for dialkyl ketones in solution with alkyl alcohol Solvent

DMK

MEK

MPK

MBK

IBMK

DEK

MIK

SBMK

EIK

DIK

TBMK

DTBK

alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

67.5 61.5 59.8 57.1

82.7 58.3 56.3

79.6 64.5 -

72.9 65.9 64.1

80.7 69.3 65.8 65.5

68.0 51.3 46.3

73.1 68.5 65.6 49.0

78.7 62.0 70.0 57.9

84.4 79.5 60.8 61.4

80.3 68.4 75.0 72.4

72.2 73.4 58.9 56.7

70.9 63.9

tert-Butyl

R.A. Nyquist/ ViimtionalSpcctroscw 7 (1994) I-29

the a-carbon atom of the C-C=0 group (acetate, propionate, isobutyrate, and trimethylacetate). The increasing inductive contribution of electrons to the carbonyl group progressing in the series acetate to trimethyl acetate contributes to the increasing resonance from

noates in hexane solution minus the v(c;--O) frequency for alkyl alkanoates in a solvent or neat (v(C==O) (hexane), cm-r, minus Y(C=O) (solvent or neat), cm-‘). Fig. 15 shows plots of the v(C=O) frequency for alkyl alkanoate in a solvent or neat vs. the frequency difference between v(C==O) (hexane) minus v(C=O) (solvent or neat). The data for v(C==O: HO) (ROH) for the alkyl alcohol .solvents are not included in Fig. 15, but are plotted separately in Fig. 16. The plots in Figs. 15 and 16 are linear and parallel to each other (see Fig. 1 and the explanation for dialkyl ketones). The plots in Fig. 15 show that as the v(C=O) (solvent or neat), cm-r, mode decreases in frequency, the frequency difference between v(C==O) (hexane) and Y(C==O) (solvent or neat), cm-‘, increases. Fig. 15 also shows that v(C=O) (hexane) for the methyl and ethyl esters decrease in frequency in the order of increased branching on

(1) (2) (3) (4) Q (6)

19

0e

Oe

R-C-0-CH, @

or

R-C-0-C,H, @

which weakens the C==Obond. Weakening of the (r-0 bond causes v(C=O) to occur at lower frequencies as observed in this study. The ethyl esters occur at lower frequency than the corresponding methyl ester, since the contribution from the mesomeric effect upon esters increases in the order of the methyl, ethyl, isopropyl, and tertbutyl analogs [3]. Table 1 shows that MTMA exhibits two Y(GO) frequencies in seven of the nineteen solvents used

ai cH3 cH3 ai ai3 cff3

oaf3 @I cH3

-16

1

I

1701

1702

1703

1704

1705

17tm

K-O,

Fig. 12. Plot of v(W)

cm-l

1707

170s

t7w

1710

(CHC13)

(CHCl,) 1s. (Zu * + EEs)/(Ea

* + Cl%) for methylalkylketones.

1711

1712

Hexane Diethyl ether Methyl t-butyl ether Carbon tetrachloride Benzene 1,2-Dichlorobenzene Nitrobenzene Acetonitrile Benzortitrile Nitromethane Methylene chloride Chloroform Chloroform-d terf- Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol Dimethyl s&oxide (Neat) Carbon disulfide

Solvent

4.45 4.63 7.44 9.34 11.99 13.98 13.69 14.02 15.36 16.06 18.04 17.98 4.00 4.23 5.95 7.88 17.96 10.26 8.59

0.00

0.00 4.30 4.78 7.51 9.15 12.35 12.18 13.36 13.60 14.99 16.31 17.98 17.86 4.41 3.53 5.50 8.42 17.48 9.12 8.53

MP 0.00 4.01 4.40 7.26 8.93 12.31 13.77 13.27 13.53 14.93 16.08 16.94 17.02 4.52 3.92 5.39 8.33 16.88 7.06 8.25

MIB 0.00 3.81 4.03 6.95 8.38 10.34 11.39 11.16 10.83 11.96 12.15 12.93 13.13 4.23 4.21 5.27 7.10 14.30 6.19 7.68 21.85

-

18.61 20.66 20.57 21.80 22.74 22.77

-

MTMA 4.37 4.66 7.99 9.76 12.44 14.22 14.05 14.21 15.76 16.58 17.52 18.29 3.35 4.33 6.04 8.23 17.45 8.26 8.96

0.00

EA 0.00 3.93 4.28 7.67 9.48 13.17 14.78 14.06 14.03 16.00 17.33 19.01 19.05 3.89 3.79 5.50 8.37 17.64 7.85 8.50

EP 0.00 3.79 4.14 7.34 9.06 12.76 13.39 13.77 13.92 15.81 17.30 18.14 18.19 3.74 3.78 5.34 8.15 16.95 6.31 8.17

EIB 0.00 4.04 4.44 8.12 9.53 12.60 11.15 14.40 14.80 16.41 17.72 17.86 17.69 3.22 3.68 5.48 7.84 16.57 6.18 8.51

ETMA 0.00 4.14 4.46 7.29 8.95 11.75 12.83 13.80 14.22 15.39 16.36 17.70 17.70 4.29 3.97 5.51 7.93 17.60 8.16 8.26

M/4 0.00 4.03 4.38 7.78 9.40 12.74 13.39 14.05 14.24 16.00 17.23 18.13 18.31 3.55 3.90 5.59 8.15 17.15 7.15 8.54

E/4 0.00 4.09 4.42 7.54 9.18 12.25 13.11 13.93 14.23 15.69 16.79 17.92 18.00 3.92 3.93 5.55 8.04 17.38 7.66 8.40

[M +El/Z

(solvent or neat) for alkyl alkanoates in various solvents (in cm-‘)

MTMA

(hexane) minus v(GO)

MA

Table 10 Infrared data for v(GO)

0.00 3.9 8.6 8.2 14.8 18.9 15.5 20.4 23.1 23.1 19.1 -

22.72 24.39 26.27 27.37 -

-

-

-

AN [61 [M + El/2

8.7 7.9

3.7 3.7 6.3 7.6 -

-

(29.1) ’ 33.5 37.1 41.3 -

18.5 -

13.8 -

4.4 -

-

-

-

AN [61 [Est AN]

RA. Nyquist/ Vihtioml

in this study. This is the only ester in this study that shows two Y(GO) bands in solution separately with acetonitrile, benzonitrile, nitromethane, methylene chloride, chloroform, and dimethyl sulfoxide. [One of the two bands of the esters in solution with alkyl alcohols is attributed to intermolecularly hydrogen bonded CC=0 : HOR) as discussed elsewhere in this paper.] One of the IR bands occurs in the region 1728.491731.96 cm-’ and the other in the region 1720.02-1724.18 cm-‘. The cause of the two v((F-0) modes for MTMA in CHCl,/CCl, solutions has been attributed to Fermi resonance [4]. Fig. 16 shows plots of v(C=O: HO) (ROH), cm-‘, for alkyl alkanoates vs. v(C=O) (hexane) minus v(C=O : HO) (ROH), cm- ‘. These plots show that as v(GO:HO) (ROH) decreases in frequency the frequency separation between v(C=O) (hexane), cm-l, and ~00 : HO) (ROH), cm-‘, increases. In addition the intermolecularly

Fig. 13. Plots of Y(M)

Spectmscopy7 (1994) 1-B

21

hydrogen bonded v(C=O:HO) (ROH) frequencies decrease in the same order as those exhibited by v(C=O) (ROH) for these same alkyl alkanoates in alkyl alcohol solution. The plots also show that the u(GO : HO) (ROH) mode for each alkyl alkanoate in alkyl alcohol solution decreases in frequency in the solvent order: tert-butyl alcohol, isopropyl alcohol, ethyl alcohol, and methyl alcohol. We attribute the difference in frequency between v(C=O) (ROH) and v(C=O : HO) (ROH) for each allryl alkanoate to intermolecular hydrogen bonding between the free pair of electrons on the carbonyl group and the OH proton of the alkyl alcohol used as the solvent. The strength of the intermolecular hydrogen bonding within each alkyl alkanoates series increases as the contribution from the resonance forms Oe R-C-O-R’ 8

Oe and

for dialkylketonesvs. mot% CHCl,/CCl,.

R-C=g-R’

22

R.A. Nyquist / Vi&rationalSpectroscopy 7 (1994) I-29

increases. Steric factors of the alkyl groups of both the ester and alkyl alcohol determine the intermolecular distance between the C=O group and the OH proton, and also determines the strength of the intermolecular hydrogen bond. The weakest C==O: HO intermolecular hydrogen bond is formed in the case of methyl acetate in dilute solution in teti-butyl alcohol, and the strongest intermolecular hydrogen bond is formed in the case of ethyl trimethylacetate in dilute solution in methyl alcohol. Fig. 17 shows plots 0f ~(60) (~0~1, cm-l, vs. ~((50 : HO) (ROH), cm-‘, for the alkyl alkanoate in dilute solution with alkyl alcohols. These plots show that ~(00) (ROH) decreases in frequency as v(C==O:HO) (ROH) decreases in frequency.

ao--

scqoewe ia c.Hcds

Table 11 lists IR data for &==O : HO) (ROH) for alkyl alkanoates in solution with alkyl alcohol. Table 12 lists IR data for v(C=O) (hexane) minus ~(60: HO) (ROH) for alkyl alkanoates. Table 10 also lists IR data for ~(60) (hexane) minus v(W) (ROH) for alkyl alkanoates. Table 13 lists IR data for ~(00) (ROH) minus ~(60 :HO) (ROH) for alkyl alkanoates. Table 14 lists the percentage of the shift of the ~(60 : HO) (ROH) frequencies attributed to intermolecular hydrogen bonding for alkyl alkanoates in solution with alkyl alcohol. Comparison of the V(M) (ROH), cm-‘, IR data for alkyl alkanoates in alkyl alcohol given in Table 1 with the ~(00: HO) (ROH), cm-‘, data given in Table 11 shows that ~(00) (ROH) always occurs at higher frequency than

solution

1, 2.3, s, 5.6 TO-.

60--

I M&36 50-CnC13/cC14 l--

D_

R

Rl

symbol

(1) C-s,

b.C3H,

0.

(2) CR3 co CH3 (4) bmy7 (5)CH3 (6) _-C4H9

'5o-C3H7 _-C4H9 bC3H7 Q=t-C4Hs tabc4li9

n 0

0 A

A

30--

M--

Fig. 14. Plots of u(CbO) vs. mol% CHCl,/CCl,

for dialkyl ketones.

R.A. NLquirt/ V&at&al

v(C=O : HO) (ROH), and the reason for the lower frequency exhibited by v(C=O:HO) (ROH) is that the alkyl alcohol proton is inter-molecularly hydrogen bonded to the free pair of electrons on the alkyl alkanoate carbonyl group (C=SI : HOR). The data in Table 12 shows that the frequency difference between v(C=O) (hexane) and v(GO :HO) (ROH) for each set of alkyl alkanoates increases in the solvent order: rert-butyl alcohol, isopropyl alcohol, ethyl alcohol, and methyl alcohol. Table 13 shows that the frequency difference between v(C=O) (ROH) and v(C=O :HO) (ROH) for alkyl alkanoates generally increases in the solvent order: tert-butyl alcohol thru ethyl alcohol, and then the frequency difference decreases in going from ethyl alcohol solution to methyl alcohol solution. We suggest that the percentage of the shift in the v(C=O: HO) (ROH) frequency attributed to intermolecular hydrogen bonding for alkyl alka-

Spectroscopy 7 (1994) I-29

23

noates in solution with alkyl alcohol is obtained by the following equation: % frequency shift attributed to the R-C-OR’ 8 iiOR intermolecular hydrogen bond v(C=O) (ROH)-v(C=O:HO) = Y(C=O) (hexane)-

v(CkO:HO)

(ROH) (ROH) x loo

Table 14 lists the calculated percentages of the frequency shift attributed to intermolecular hydrogen bonding between the C=O group and the ROH proton. The percentages vary from 64.53 for EA in solution with methyl alcohol to 85.68 for ETMA in solution with tert-butyl alcohol. Presumably these results are determined by the relative acidity of the alcoholic proton, the relative basicity of the carbonyl oxygen atom which is dependent upon the inductive contribution of the

Fig. 15. Plots of v(GO) (solvent or neat) vs. AGO) (hexane) minus v(M)

(solvent or neat) for akyl alkanoates.

R.A. Nyqd

24

Fig. 16. Plots of v(CO

/

Viirakmal

Spechtawopy 7 (1994) I-29

: HO) (ROH) vs. v(C=O) (hexane) minus v(GO : HO) (ROH) for alkyl alkanoates.

alkyl group of the acid and the alkyl group of the alcohol comprising the alkyl alkanoate together with the steric effect of each alkyl alkanoate and each alkyl alcohol. Fig. 18 shows plot of v(C=O) (hexane) minus v(C=O : HO) (ROH), cm-l, vs. ~(60) (ROH) minus ~(60 : HO) (ROH), cm-‘. These plots are complex indicating that the basicity of the GO group, and the steric factors of the molecular structure of the alkyl alkanoates cause the irregular behavior of the plots progressing in the series of solvents.

Table 10 lists IR data for ~(60) (hexane) minus v(G=O) (solvent or neat), solvent acceptor numbers (AN), and postulated (AN) values. Inspection of Table 10, excluding hexane, shows that the values for ~(60) (hexane) minus v(C=O) (solvent or neat), cm-‘, for each solvent or neat phase vary for each compound in the same solvent. The columns M/4 and E/4 are the averages of ~(60) (hexane) minus ~((30) (solvent or neat), cm-‘. The column CM + El/2 list the averages for the eight esters. These calculated

Table 11 Infrared data for v(C=O : H) (ROH) for alkyl alkanoates in 0.5% (w/v) alkyl alcohol solution (in cm-‘) Sohrent

MA

MP

EA

MIB

EP

EIB

MTMA

ETMA

AN

tertButy1 alcohol Isoprop$ alcohol Ethyl alcohol Methyl alcohol

1734.12 1732.46 1731.42 1730.51

1729.86 1728.44 1726.46 1725.34

1728.66 1727.41 1726.18 1726.56

1724.17 1721.67 1719.32 1718.16

1723.28 1721.54 1719.15 1717.64

1720.53 1718.65 1716.14 1714.47

1717.76 1716.47 1714.86 1713.89

1713.96 1712.37 1710.28 1708.60

29.1 a 33.5 37.1 41.3

B121.

161

R.A. Nyqukt / Virational Spectroscopy 7’@94) l-29

25

! 8

i

I--

I--

I --

, __

,--

a+ vza

,715

1725

wPO(ROH).

Fig. 17. Plots of v(m)

1740

1735

1730

iM

tm

cm-1

: HO) (ROH) for alkyl alkauoates.

(ROH) vs. ~(0

Table 12 Infrared data for the frequency difference between &GO) alkyl alcohol solutions (in cm-‘)

050

(hexane) and v(CkO : H) (ROH) for alkyl alkanoates in 0.5% (w/v)

Solvent

MA

MP

EA

MIB

EP

EIB

MTMA

ETMA

M+E/2

tert-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

21.32 22.98 24.02 24.93

22.23 23.65 25.56 26.75

21.10 22.35 23.58 23.20

23.68 26.18 28.53 29.69

23.44 25.18 27.57 29.08

22.51 24.39 26.90 28.57

25.03 26.32 2784 28.90

22.48 24.07 26.16 27.84

22.72 24.39 26.27 27.37

Table 13 Infrared data for the frequency difference between v(GO) alkyl alcohol solutions (in cm-‘)

(ROH) and v(c-0

: HO) (ROH) for alkyl alkanoates in 0.5% (w/v)

Solvent

MA

MP

EA

MIB

EP

EIB

MTMA

ETMA

M+E/2

tertButy1 alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

17.32 18.75 17.98 17.05

17.82 20.12 20.16 18.33

17.55 17.99 17.54 14.97

19.16 22.25 23.14 21.36

19.55 21.39 22.07 20.71

18.77 20.61 21.56 28.42

20.80 22.11 22.57 21.80

19.26 20.39 20.68 28.00

18.78 20.45 20.71 19.33

26

R.A. Nyquist / k’iirational Spectroscopy 7 (1994) I-29

Table 14 Percentage of the shift of the ~(0 0.5% (w/v) akyl solution

: HO) (ROH) frequency attributed to intermolecular hydrogen bonding for akyI alkanoates in

Solvent

MA

MP

EA

MIB

EP

EIB

tert-Butyl alcohol Isopropyl alcohol Ethyl alcohol Methyl alcohol

81.24 81.59 74.85 68.39

80.26 85.07 78.87 68.52

83.18 80.49 74.39 64.53

80.91 84.98 81.11 71.94

83.40 84.95 80.05 71.21

83.39 84.50 80.15 71.47

ETMA 83.10 84.00 81.07 75.43

85.68 84.71 79.05 71.84

neat alkyl alkanoates 03.7). The two numbers given for the alkyl alcohols are for non-hydrogen bonded and intermolecularly hydrogen bonded solute molecules, respectively. Frequency values for v(M) (hexane) minus u((==o) (solvent or neat) for these alkyl alkanoates (Table 10) or for the dialkyl ketones (see Table 4) are within a

average frequency numbers are similar to the published AN values; Therefore, we have estimated AN values for methyl tert-butyl ether (4.41,

1,2-dichlorobenzene (13.81, nitromethane (l&5), tert-butyl alcohol (3.7 and 22.71, isopropyl alcohol (3.7 and 24.41, ethyl alcohol (6.3 and 26.31, methyl alcohol (7.6 and 27.41, carbon disulfide (7.91, and

235 23.0 225

215 21.0 w-0

(ROH)

K)5

minus vc-ot HO cm-1

IROHI,

20.0 125 ma 125 12.0 r/.? 17.0

36 I-’ 1

16.6

,/-

16.0 15.!

;:

,’

i I’ f5S 14.:

I’

I’

I’

ii

I

21.5

Fig. 18. Plots of v(W)

I

22.0

I

225

I

23.0

I

I

I

215

25.0

2i.5

26.0

1

I

26.5

27.0

23.5

24.0

(UC-o

(hexme) minus K-O: HO(ROH)).cm1

I

275

I

28.0

I

28.5

I

29.0

I

225

I

20.0

(hexane) minus v(C=O : HO) (ROH) vs. ~(60) (ROH) minus v(C=O : HO) (ROH) for alkyl alkanoates.

RA.

Nyquist

/ VZwational

range of numbers for each solute in each solvent, and this indicates that the solute-solvent interaction is different for each compound. Fig. 19 shows plots of v(C=O) for the alkyl alkanoates in various solvents vs. the published AN or IR calculated AN values. These plots show that AN for IR derived AN values are not a precise measure of solute-solvent interactions due to steric factors of the alkyl groups and the basicity of the carbonyl group. Fig. 20 shows plots of v(C=O:HO) (ROH), cm-‘, vs. AN for alkyl alkanoates in alkyl alcohols. With the exception of ethyl acetate in methyl alcohol, the v(m : HO) (ROH) frequencies decrease in essentially a linear manner progressing in the series tert-butyl alcohol thru methyl alcohol. The v(GO) (ROH) frequencies for the methyl esters and the ethyl esters decrease in the order: acetate, propionate, isobutyrate, and trimethylacetate.

Spectroscopy

7 (1994)

I-29

27

Fig. 21 shows a plot of the average of v(C=O) (hexane) minus v(C=O) (solvent or neat), cm-‘, and the average of v(c--O) (hexane) minus v(GO : HO) (ROH), cm-‘, vs. AN numbers derived from NMR data [6] or from postulated AN values determined from the present IR study. We suggest that all AN numbers are of value only for predicting a relative order of magnitude shift for the group frequency of a compound in dilute solution in a particular solvent. 3.2. The effect of solute concentration in solution Table 2 compares the v(C=O) (Ccl,) frequency for MIK (0.5% in CCL,) at 1717.98 cm-i with the &GO) (Ccl,) frequency for MIK (2% in Ccl,) at 1717.79 cm-‘, and also compares the ‘v(C=O) (CCL,) frequency for TBMK (0.5% in Ccl,) at 1709.52 cm-’ with the v(C=O) (Ccl,) frequency for TBMK (2% in Ccl,) at 1709.19

20 --

‘6 --

16 --

14 --

AN 12 --

IO --

2 --

6 --

4 __

2 -_

,724

1726

17211 1730

,732

1734

17s

1m

17-40

1744

1746

Vu)

1750

1752

K-aan-1

Fig. 19. Plots of v(C=O) for the alkyl alkanoates vs. AN for the sohrent or neat alkyl alkanoate.

1754

t vid

RA. li$pist / Viirational Spectroscopy 7 (1994) I-29

28

‘4)tnMC!tt#AkOhOl

B)tnUW-

C)tabopmpyIAkohc

B)

D)l~~bSbutylAkoho

t

1708

,712

,714

,716

Fig. 20. Plots of ~(60

,711

17m

vc=o:

HO(ROH),

1722

1724

,728

1720

17x!

I730

cm-l

: HO) (ROH) vs. AN for alkyl alkanoates in alkyl alcohols.

cm-‘. Table 2 also compares the v(C=O) (CHCl,) frequency for MIK (0.5% in CHCl,) at 1707.72 cm-’ with the v(C=O) (CHCI,) frequency for MIK (2% in CHCl,) at 1707.28 cm-i, and the v(C=O) (CHCl,) frequency for TEIMK (0.5% in CHCl,) at 1701.93 cm-’ with the v(C=O)(CHCI,) frequency for TBMK (2% in CHCl,) at 1701.40 cm-‘. These comparisons show that the v(C=O> mode for these ketones occurs at lower frequency at 2% concentration then it occurs at 0.5% concentration in solution with either Ccl, or CHCl,.

This v(C=O) (Ccl, or CHCl,) frequency difference due to change in solute concentration, we attribute to an increase in the dipolar interaction between ketone molecules R \ @C-oe I

I R’

“\ ec-oe R’I

In

as the concentration of solute molecules increases. This conclusion is consistent with the

R.A. Nyquist / Vibrational Spectroscopy7 (1994) I-29

29

0-ANV¶hWVhNMit A-ANvahwsvialttforAlkylAlkamaW(vc-01

o=ANvduesvialRforAlkyt-(vc-~Ho)

Fig. 21. Plot of the average of v(CXZI) (hexane) minus v(C=O) (solvent or neat) and the average of v(c-0) v(C=O : HO) (ROH) vs. AN numbers derived from NMR data [6] or postulated from IR.

conclusion based on a previous study of the effect of concentration on the ~(60) frequency of ketones in CCI, and CHCl, solutions [l].

4. References [l] R.A. Nyquist, C.L. Putzig and L. Yurga, Appt. Spectrosc., 43 (1989) 983.

(hexane) minus

[2] R.A. Nyquist, T.M. Kirchner and H.A. Fouchea, Appl. Spectrosc., 43 (1989) 1053. [3] R.A. Nyquist, Vib. Spectrosc., 2 (1991) 221. [4] R.A. Nyquist, Appl. Spectrosc., 45 (1991) 92. [S] R.A. Nyquist, Appl. Spectrosc., 43 (1989) 1208. 161 V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York, 1978. [7] R.W. Taft, Jr., in M.S. Newman (Ed.), Steric Effects in Organic Chemistry, Wiley, New York, 1956, p. 501. [8] R.W. Taft, Jr., in M.S. Newman (Ed.), Steric Effects in Organic Chemistry, Wiley, New York, 1956, p. 598.