Chain length effects in paraffins—III

Tetrabd~n,

1966, SUPPI

8, Part I, pp. 93-99.

Pcrgamon

Rar

Ltd.

Printed in Great Britain

CHAIN LENGTH EFFECTS IN PARAFFINS-III ALKYL CHAIN LENGTH EFFECTS ON THE NITRATION OF HYDROQUINONE METHYL C1-CIL ALKYL DIETHERS* F. KORTE, W. KLEIN and E. D. SCHMID Organ&h-Chemisches Institut der Universitat BUM and Physikalisch-Chemisches Institut der Universitlt Freiburg i. Br. (Received 8 June 1966) AhstractSystematic studies were performed in order to ascertain the influence of alkyl groups of varying chain length on the isomer ratio produced during the nitration of asymmetrically substituted hydroquinone diethers. Parameters depending on chain length. branching and substitution which are to a first approximation representative of the postulated etfect of the alkyl chains, can be calculated from the respective quantities of 2- and 3-substituted nitro derivatives isolated. The experimental results are explained by our “model of electron-distribution in pamBIns”. The hydroquinone oxygen possibly enhances the measured etTect. Taking other experimental works into account, the mode1 was reformulated. RECEN~Y we described a model’*’ explaining various reactions of the paraffins by an increase of electron density towards the chain ends. The most important argument for the formulation of this model was the isomer ratio resulting from nitration of hydroquinone methyl-alkyl diethers. Since the results reported by Sir Robert Robinsonetal.3 substantially support that earlier model we have repeated these studies using TLC and UV spectroscopy. We succeeded in reproducing his results in all essential parts and furthermore to supplement them by additional examples. Sir Robert Robinson et al. performed the nitration in acetic acid, at temperatures varying according to the chain length. The m.p. of the resulting mixtures were compared with the corresponding m.p. curves of the pure isomers. It was assumed that the reaction mixture contained only the isomers I and II. We were able to show, however, that under these conditions the formation of small quantities of impurities, perhaps by ether cleavage and oxidation of the resulting phenols, could not be ignored.

NO, ISOMER I

ISOMER II

*Dedicated to Sir Robert Robinson on his eightieth birthday.

1F. Korte and H. Hover, Tetruhedron 21,1u17 (1965). a H. Hover. H. Mergard and F. Korte, Licbigs Ann. 685,89 (1965). 3 Si R. Robinson and J. C. Smith, J. Chem. Sot. 392 (1926); J. Allan, A. E. Oxford, R. Robinson and J. C. Smith, Ibid. 401 (1926); see also R. Robinson, J. It&m Chem. Sot. 38, 445 (l%l); L. J. Goldsworthy, J. Chem. Sot. 1148 (1936). 93

94

F. Koim,

W. KUIIN mm E. D. Scmm

The quantitative separation of the isomer mixtures (synthesized under strictly constant reaction conditions) by TLC and comparison of the W absorption of the zones containing I and II proved, on the other hand, to be sufficiently accurate for our purposes. Nitrohydroquinone alkyl diethers show W absorption at 357 rnp, 264 rnp and at 219 rnp. Whereas the less characteristic max at 219 rnp has a very high r-value (about 12300), the max at 264 rnp, with an E-value of about 2500, appeared as a shoulder and showed, therefore, not a sutliciently reproducible extinction. Consequently, the max at 357 rnp was chosen as a basis for our measurements. The hydroquinone methyl-alkyl diethers were prepared according to the conventional procedure of the Williamson synthesis3v4 and tested for uniformity by TLC in various solvent systems (some by gas chromatography). They were identified by their UV, IR and NMR spectra. Hydroquinone alkyl diethers show UV absorption at 287 rnp and 266 mp. The IR spectra are typical forp-disubstituted aromatic compounds in the region of 1590 to 1610 cm-’ and show a strong C-O-C band at 1240 cm-’ and the aromatic -C=Cbands between 1500 and 1510 cm-‘. The NMR spectra show the aliphatic protons as well as four identical aromatic protons at r= 3.30 for all ethers investigated. The nitrations were performed under uniform conditions, regardless of alkyl substituents. Suspensions of the ethers in acetic acid were allowed to react with an excess of nitric acid (two equis). Since the nitration of the ethers with long chain aliphatic groups did not occur at lower temperatures while at higher temperatures larger quantities of by-products were formed, all the nitrations were initiated at 25” and then allowed to continue at 0”. In order to account for possible errors during the next steps-dilution with water and extraction with chloroform-due to different distribution coefficients of the two isomers in aqueous acid and chloroform-the nitration reaction was included in the determination of the absolute error of the results. For identification, the isomers formed were compared by TLC with the corresponding pure substances, which were prepared according to :3,4 Due to the o&o-directing influence of the methoxy group and the me&directing inlIuence of the ethylcarbonate group the nitration of p-methoxyphenyl-ethylcarbonate gave exclusively 3-nitro4methoxy-phenylethylcarbonate which on saponification yielded 3-nitro4methoxyphenol. Ether cleavage occurs predominantly at the methoxy group in ortho position to the nitro group of nitrohydroquinonemethyldiether. TLC pure 2-nitro4methoxyphenol is obtained upon recrystallization. O_alkylation gives the pure isomers I and II. On a TLC isomer I has a somewhat greater &value than isomer II. There is no marked difference in the IR spectra of the two isomers. All the nitrohydroquinonediethers investigated show the absorption pattern of 1,2+trisubstituted aromatic compounds in the range of 1580 to 1630 cm-‘, Y,,._Nol at 1350 cm-‘, and 6ar,,m.-_No2 at 1530 cm-’ In order to measure the ratio of isomer I to II, the two yellow zones on the TLC which contained the two respective isomers were quantitatively eluted and idilI of the eluates directly measured at 357 rnp with the Gary 14 Spectrophotometer. The ir/irr_ value was then multiplied by the quotient QU/EIto give the proportionalqua&ies ofthe isomers resulting from the nitration., ’ L. Claisen, 0. Eister, Liebigs Ann. 401, 29 ( 1913).

95

Chain length effects in padins-III

In order to determine the accuracy of the measurements (the reproducibility of the data approaches 1%) and to eliminate systematic errors as well as those depending on chain length, the puie isomers of R = Et, n-Bu and n-heptyl, respectively, were accurately weighed in together. These model mixtures were then subjected to the nitration conditions described above-in this case, however, with only one mole of nitric acid. They were worked up as before and measured by WV. T’he isomer ratios of the starting compositions were found again within 1.% deviation. In order to measure the influence of possible small temperature fluctuations in the reaction mixture during nitration, the ether with R=n-butyl was nitrated at 5 and 10”; the isomer ratios were equal to the ratio obtained at 0’. During nitration of the methyl ethyl diether, samples were withdrawn after the addition of 10 %, 30 % and 80 % of the normal amount of HNOJ, and analysed as described. The ratio of I : II remained constant. To eliminate indeterminate

TABLE 1. DEPENDENCYISI ON R

(Isomer ratio) R

--

idin

m/a

I:11

244/246

._I_._

1:113

--

1. C2Hs

l-55

1.54

I-63

2.

n-C3H,

1.66

243/244

1.66

1*80

3.

n-C,H9

1*71

1

1.71

1l86

4.

n-CSH,,

1.74

2411242

1.74

5.

n-C,HIS

1.74

241/242

1*74

6.

n-C1oH2l

la76

1

7.

n-C16H33

la81

1

l-74

1

2.36

2421243

2.22

2421243

2xP6

1

2.66

2-22

1

2.22

C 8.

K’r

-A AZ

14. .rrr

143

2311227

1-w

2161213

1*30

2121208

0*83

m/m

2.36

L 15.

-

16. -C-C-C-

0.85

2.29

96

F. KORT~, W. KLE~ AND E. D.

&3ihUD

errors, all the ethers were nitrated twice and four times chromatographically separated and measured by UV. The results presented in Table 1 demonstrate that the electron donor function of the alkoxy group in l-position changes in dependence of the alkyl chain length. Steric effects would be expected to favour substitution in me&-position. In this paper solvatation effects will not be discussed. If-as a working hypothesis-possible polarizability effects are neglected it must be concluded from these results, that with increasing length of the alkyl chain the electron density in the o-position of the ring increases compared with the density at the mposition. Branchings act like several convergent chains and enhance the effect observed at the chain ends or at the position of possible electron flow; thus methyl groups have a stronger influence than methylene groups. Electronegative substituents in the alkyl chain (Table 1, Nr. 13-16) show the expected influence on the isomer ratio. The above effect could be measured for n-alkyl chains between C1 and C16. Four effects might be considered to explain these results: the field effect, u- and v-inductive effects, and resonance. Since the a-inductive and the field effects depend on the polarity of substituents, they can be ignored for the almost unpolar alkyl chains. On this assumption and modifying our earlier model the following interpretation of the results is suggested: The electron density distribution in the alkyl chain is such that the total electron functions possess components of p,-symmetry (where z is the direction perpendicular to the aromatic ring). We assume that the p,-function of the alkyl electrons augments with increasing chain length. The greater the p,-character, however, the greater are also the resonance and inductive exchange effects operating between the alkyl electrons and the n-electrons of the aromatic ring. In other words: we trace the observed enhancement of the electron donor effect of the alkyl chain back to an increase of the p,character of the alkyl electrons with increasing chain length. According to this model the observed donor effect of the alkyl group would depend on the resonance and on the inductive donor effects of the p,-components of the two free electron pairs at the oxygen atom. In this context the results of recent work’ are of special interest: they show that the polarity of the C-H-bond in the ground state is C+-H-. These experimental results are in good agreement with the calculations of Coulson.6 Formerly the odelocalization in long chain paraffins has been suggested on the basis of physical experiments (see e.g. Dewar’). To our knowledge, this is the first time where it has been established, that an alkyl chain of one to sixteen carbon atoms gradually influences a chemical reaction. EXPERIMENTAL IR spectra: Perkin Elmer Spectrograph 221; UV spectra : Cary-14 Spectrophotometer. NMR spectra: “Kis 90” Spectrograph. The hydroquinonemethyl-alkylethers were prepared according to the lit.‘e4 using analytical grade chemicals. Nirrurion. Hydroquinone methyl-alkyl ether (0.05 mole) were suspended in AcOH (15 ml). At 25” one drop of a soln of 9.6 g (0.1 mole) 68% HNOs in AcOH (15 ml) was added. After cooling to 0” the remaining HNO, was added over a period of 2 h. The mixture was then kept for 3 h at 25”, 5 E. D. S&mid, V. Hoffmann, R. Joeckle and F. Langenbucher, 6 C. A. Co&on, Trans. Faraday Sot. 38,433 (1942). 7 M. J. S. Dewar, Gem. Engng. News 43,86 (1%5).

Spekrrochim. Actu in press.

Chain length effects in parafbs-III

97

100 ml ice water were added and the separated oil was extracted twice with chf (30 ml). The chf phase was extracted with 1% KHCOs aq (10 ml) and twice with water (20 ml), dried over Na$SO,, and the solvent evaporated. The isomers were isolated in nearly quantitative yield. The pure nitrohydroquinone methyl-alkyl diethers were prepared according to Ref. 3 and 4. TABLE 2. PHYSICAL DATA OF THE HYDROCHINONE

METHYL-ALKYL

Alkyl group

DIETHERS

l’hba. in ChfJ

vcoccm-1

Me

56

288

3.54

1510

1590-1610

1235

Et

37

288 226

3.49 4.01

1505

1590-1610

1230

n-Pr

27-29

287 225

3.43 3.96

1510

1590-1610

1230

n-Bu

27

287 225

3.42 3.99

1590-1610

1230

29

287 226

340 3,97

1510

1590-1610

1210-1230

nC7H15

43

287 226

340 3.98

1505

1590-1610

1230

n-G

52

287 226

3.36 3.93

1510

1590-1610

1210-1230

68

287 225

3.42 3.97

1510

1590-1610

1210-1230

~-W-II

I

0%

n-G6H33

7

1500

-c-c-c-c

O-01175

287 226

3.36 3.89

1510

1590-1610

1230

F: --cm-c

0*01/68-70

287 226

3.40 3.91

1510

1590-1610

1230

Y -c-c-c

0.2173-75

287 226

3.59 3.98

1510

1590-1610

1230

o-1168

287 226

3.39 3.89

1510

1590-1610

1225

-X-L-C

0*1/70-72

287 226

3.51 4.09

1510

1590-1610

1230

-C-C-C-Cl

O-01/94

3;:

:4:

1510

1590-1610

1230

z

:s:

1510

1590-1610

1230

C -c!’

\

C C

C -LC-C-Cl

@05/75-77

-C-C-C-

34

287 225

3.37 4.03

1510

1590-1610

1230

-C-C-C-cN

36

287 226

3.38 4.00

1510

1590-1610

1240

98

F. KORTE,W. m

AHDE. D. !kmm

TABLE 3. UV AND IR smcnu

Alkyl group

Isomer

Me

357

:I :I

Et n-Pr n-Bu

n-CJ-h n-G

Amaxrnp

oH21

-c&L

--L-c&c

E

2465

::7’

OF THEISOMERS m.-N srn-1

“arom.-NO2

1350

1530

1354 1353

1529 1528

357 357

2430

1353 1353

1529 1528

I II

357 357

2430 2430

1350 1352

1528 1529

:I :I :I :I

357 357

%i

1353 1353

1529 1525

357 357

2420 2410

1355 1353

1530 1528

357 357

2420 2420

1353 1353

1529 1529

357 357

%

1355 1355

1529 1530

:I

357 357

2410 2410

1355 1355

1530 1530

I

357 357

1355 1355

1528 1528

II

357 357

1355 1355

1528 1530

L

357 357

1355 1350

1529 1528

:I

357 357

2430 2430

1355 1355

1530 1530

:I

357 357

2275 2310

1353 1355

1529 1530

:I

357 357

zi

1353 1355

1529 1529

:I

357 357

1353 1355

1529 1530

:I

357 357

1350 1355

1530 1530

C-C -C’ \ C --c

C C

-C-L -C-C-C-Cl -C-c-C-C-Cl

&LC-C* -C-N

tXk1

Chain length effects in pataffins--nI

99

Separurion Md LIP anulysis. The nitration mixture (80 mg) was placed on a 50 x 20 cm thin-layer plate, prepared with 25 g of silica gel PF 254 or H (Merck) and 5 times developed in ligroin (60/95) + 5 % MeGH (for R > C,+ 2 % diisopropyl ether). The two yellow zones were quantitatively transferred to a sintered glass filter, eluted 5 times with 10 ml MeGH, the eluates diluted to 100 ml. Two ml of this soln were diluted to 10 ml and measured with the Gary-14 Spectrophotometer against MeGH soln of equal silica gel concentration. Acknow/edgemenrs-We thank Dr. H. D. Seharf, Shell Grundlagenforsehung Birlinghoven, for discussions and Dr. H. Weitkamp, Shell Grundlagenforsehung Birlinghoven, for taking the NMR spectra. This work was supported by a grant from the Deutsche Forschungsgemeinschaft,

GmbH, Schloss GmbH, Schloss Bad Godesberg.