- Email: [email protected]
Absorption spectra in the near ultraviolet of some α- and β-monosubstituted derivatives of thiophene
SpectrochimicaActa, 1958, Vol. 12, pp. 350 to 354. PergamonPress Ltd., London.
Absorption spectra in the near ultraviolet of some amonosubstituted derivatives of thiophene* R.
ANDRISANO
and G.
and p-
PAPPALARDO
Department of Pharmaceutical and Toxicological Chemistry, University of Parma, Italy (Received 25 January 1958) Abstract-The near ultraviolet absorption spectra in solution of some a-monosubstituted thiophenes have been discussed in relation to the analogous derivatives of furan and thiophene. An electron-acceptor substituent modifies the first excited level of the molecule more in furan than in benzene, thiophenes being intermediate, since the transition energy decreases in the series furan > thiophene > benzene. This order in the series is similar to that expected from the resonance energies. The absorption spectra of some /?-substituted derivatives of thiophene have been described and compared with cc-derivatives and with the analogous compounds of furan. In these compounds too the substituent is more conjugated when in the u-position. THE results reported in this paper, already briefly mentioned in a preliminary note [ 11, concern the spectroscopic properties of some a-monosubstituted derivatives of thiophene (C,H,S-X) in comparison with the related a-monosubstituted furanes (C,H,O-X) and benzenes (C,H,-X). X being an electron-acceptor group, such as -CO-, COOR, CONH,. We have also studied /I-substituted derivatives of thiophene, in order to see Table 1. Near ultraviolet
-
I_ Ama*
i H CHO COCH, COOH COOCH, COOC,H, CONH,
log
E
Amax
log E
I
1mar
loge
(A)
(A) 2.25 3.20 3.03 2.80 2.71 2.96 2.85 2.90 2.85 2.70
soectra (ethanol solutionsl t
C,H,S-X-a
C&H,-X
(4 2545 2800 2790 2685 2780 2725 2800 2720 2800
absorntion
2035 2440 2420
3+7(l) 4.20(2) 4.08(3)
2245
3.97(3)
2290
4.05(3)
2290 2250
4.08(3) 3.95(4j
C,H,O-X-a(5)
&a**
log &
(A)
2msx
log & Amax log &
(A)
(A)
3.77 3.85 3.84
2600 2600
(a) 4.02(b) 3.98(c)
2000 2720 2700
4.00(6) 4.12 4.11
3.76
2460
3.93(d)
2425
4.03
-
-
2680
3.86
2480
3.97(e)
2505
4.13
-
-
2690 2720
3.89 3.87
2490 2480
3.95(f) 3.78(g)
2510 2480
4.13 4.06
2310 2850 2820 ~2640
2270 2255
2lao
3.48 3.57
3Y3
* Contribution from C.N.R. t Numerals in parentheses refer to spectral data previously recorded; letters refer to preparations. (1) DOUB L. and VANDENBELT I. M., J. Amer. Chem. Sot. 1947 69 2714. (2) GILLAM A. E. and STERN E. S., An Inntroduction to Electronic Absorption Spectroscopy (Edited by ARNOLD E.), p. 126 1954. (3) ANDRISANO R. and PAPPALARDO G., Atti accad. Lincei 1953 (8) 15 64. (4) LEY H. and SPECXER H., Ber. dtsch. Chem. Bes. 1939 72 192. (5) ANDRISA~COR. and PAPPALARDO G., &zz. Chim. Ital. 1955 85 1430. (6) GILLAM A. E. Zoc. cit. p. 132. (a) Commerical product subjected to additional purification. (b) KING and NARD, J. Org. Chem. 1948 13 635. (c) HARTOU~H H. D. and KOSAK A. J., J. Amer. Ghem. SOC. 1947 69 3093. (d) HARTOU~H H. D. and CONLEY L. I., ibid. 1947 69 3096. (e) b.p, 82°C.; found: S = 21, 97; C,H,O$ requires S = 22, 52. (f) SELENK W. and OCRS R., Ber. dtsch. Chem. Gee. 1915 48 678. (g) NARNSEN R., Rer. d&h. Chem. Cues. 1884 17 2192. - flex. 350
Absorption spectra in the near ultraviolet of some G(-and p-monosubstituted derivatives of thiophene
whether the sequence found in the furan series [2] (namely conjugation in 8) is also followed in the thiophene series.
rl-, U
o-
(Y)
(I’I)
conjugation
in a >
X
Experimental The derivatives of thiophene were prepared by known methods (see footnotes of Tables 1 and 2) and purified by crystallization or distillation dn vaczbo. Table 2. Near ultraviolet absorption spectra (ethanol solutions) * C,H,S-X-_B
C,H,O-X-_B(
1)
X= 2max K
El COCH, COOH COOCH, COOC,H, CONH,
2310 2396 2390 2405 2400 2410
log E 3.77 3*80(a) 3.79(b) 3.95(c) 3.95(c) 3.89(a)
1max A
log E
2000
4.00
2320 2380 2380
3.36 3.40 3.38
-* See footnote of Table 1. (1) ANDRISANO R. and PMPALARDO G., &zz,Chim. It&. 1953 83 340. (a) CAMPAIQNEE. and LE SEUR W. M., J. Amer. Chewa.Sot. 1948 70 1555. (b) RINEERS I., Rec. Truw. Chim. 1936 (4) 55 991. (c) RINEERS I., Rec. Trm., Chim. 1934 (4) 53 643.
Spectroscopic measurements (see Tables 1 and 2; Figs. 1 and 2) were made with a Unicam Spectrophotometer SP 500 in the range 2100-3000 A, in ethanolic solutions 1 : lOO*OOO,reading every 20 8, and for the important regions every 10 f% (Beer’s law is applicable throughout).
Fig. 1.
351
R. ANDRISANV
and G. PAPPALARDO
40 -..._
ggEcH’&
2
3.5
; 30
3.5
2
n
h,
A
Fig. 2.
Discussion We may first examine the first transition of benzene, thiophene and furan, which falls at 2500, 2310 and 2000 d respectively. As the data in Table 1 show, the transition energy in every case decreases in the series: furan > thiophene
> benzene
With acetyl derivatives, for example, we have the values given in Table 3. The series derived from the spectroscopic data is parallel to that which was obtained from the resonance energy, and this may imply that the two phenomena are related. Table 3. Change of the transition energy (AE, kcal/mol) the compounds Ar-X
H
x= Ar=
due to substituent X in
COCH,
E
E
(kcel/mol)
(kcal/mol)
AE (kcal/mol)
\ C,H,O-a C4H3S--a C&&r-
143 124 112
(n = 2OOOA) (I = 2310A) (n = 2550A)
106 101 102
(,I = 2700A) (1 = 2820A) (A = 2790A)
37 23 10
If this hypothesis is correct, it might be concluded that the substituent modifies the first excited level of the molecule much more in the case of furan than in that of benzene, thiophene being intermediate. If the spectral changes due to the same substituent in the CC-or /?-position are compared, there is an obvious analogy with the furan series [2], i.e. the changes of the spectroscopic characteristics with u-substituents are more marked than those with the /I-substituents (see Figs. 3 and 4). 352
Absorption spect~e in the neaS ultraviolet of some a- and ~-~n~ub~t~tut~
derivthtivesof tbiuphene
Mloreover in the ease of the er-substituents a second band arises in this region of the speot~m~ while the ~-substitution causes only a small bathochromic shift of the original thiophene or furan band: this may arise because of greater conjugation in the DCthan in the B-form.
RX example, the values obtained with carbomethoxy group as s~bstitue~t are given in Table 4. From the values reported in Table 4, we may infer that: (1) The decrease of transition energy in the @lerivatives follows the same order furan > thiophene, This is another experimental fact which may be correlated with the greater aromatic charac~r of the thiophe~e nucleus compared with that of furan, (2) The greatest change in the traus~t~a~energy occurs in every case with the 5
353
R.
ANDRISANO
and
G. PAPPALARDO
Table 4. change of the transition energy (A& kcaljmol) due to subetituent X, in the Ar-X compounds, when it occupiesthe a- or ,%position COOCH,
AE Position of substituent C,H,O-
143
(1 = 2000 A)
C&W--
124
(a. = 2310_&)
E (kcaljmol)
(kcal/mol)
(n = 2505 8) (A = 2380A) (I = 2680 11) 118-6 (A = 2405il)
29 23 17.5 5.5
114 120 106.5
j
!
a-subatituent. Theae results provide further experimental evidence for the conclusions of SCHOMAKER snd PAULIN@ [3] and are consistent with the observation that the a-substituent induces s conjugation through all the perimeter of the ring (Ia), while for the ~-substituent the conjugation is limited (II&).
[1]ANDRISANOR~~~PAPPAURDO G.,Bol.Sot.chim. id. Bologna 1956 [Z] ANDRISANO R. and PAPPALARDO G., ffuzz. Chain.Ital. 1953 83 340, [3] SCHOMAICERand PAULINC+,J.AWW.C~~WLSOC.1939 61 1769 1881.
354
14 100.
Absorption spectra in the near ultraviolet of some amonosubstituted derivatives of thiophene* R.
ANDRISANO
and G.
and p-
PAPPALARDO
Department of Pharmaceutical and Toxicological Chemistry, University of Parma, Italy (Received 25 January 1958) Abstract-The near ultraviolet absorption spectra in solution of some a-monosubstituted thiophenes have been discussed in relation to the analogous derivatives of furan and thiophene. An electron-acceptor substituent modifies the first excited level of the molecule more in furan than in benzene, thiophenes being intermediate, since the transition energy decreases in the series furan > thiophene > benzene. This order in the series is similar to that expected from the resonance energies. The absorption spectra of some /?-substituted derivatives of thiophene have been described and compared with cc-derivatives and with the analogous compounds of furan. In these compounds too the substituent is more conjugated when in the u-position. THE results reported in this paper, already briefly mentioned in a preliminary note [ 11, concern the spectroscopic properties of some a-monosubstituted derivatives of thiophene (C,H,S-X) in comparison with the related a-monosubstituted furanes (C,H,O-X) and benzenes (C,H,-X). X being an electron-acceptor group, such as -CO-, COOR, CONH,. We have also studied /I-substituted derivatives of thiophene, in order to see Table 1. Near ultraviolet
-
I_ Ama*
i H CHO COCH, COOH COOCH, COOC,H, CONH,
log
E
Amax
log E
I
1mar
loge
(A)
(A) 2.25 3.20 3.03 2.80 2.71 2.96 2.85 2.90 2.85 2.70
soectra (ethanol solutionsl t
C,H,S-X-a
C&H,-X
(4 2545 2800 2790 2685 2780 2725 2800 2720 2800
absorntion
2035 2440 2420
3+7(l) 4.20(2) 4.08(3)
2245
3.97(3)
2290
4.05(3)
2290 2250
4.08(3) 3.95(4j
C,H,O-X-a(5)
&a**
log &
(A)
2msx
log & Amax log &
(A)
(A)
3.77 3.85 3.84
2600 2600
(a) 4.02(b) 3.98(c)
2000 2720 2700
4.00(6) 4.12 4.11
3.76
2460
3.93(d)
2425
4.03
-
-
2680
3.86
2480
3.97(e)
2505
4.13
-
-
2690 2720
3.89 3.87
2490 2480
3.95(f) 3.78(g)
2510 2480
4.13 4.06
2310 2850 2820 ~2640
2270 2255
2lao
3.48 3.57
3Y3
* Contribution from C.N.R. t Numerals in parentheses refer to spectral data previously recorded; letters refer to preparations. (1) DOUB L. and VANDENBELT I. M., J. Amer. Chem. Sot. 1947 69 2714. (2) GILLAM A. E. and STERN E. S., An Inntroduction to Electronic Absorption Spectroscopy (Edited by ARNOLD E.), p. 126 1954. (3) ANDRISANO R. and PAPPALARDO G., Atti accad. Lincei 1953 (8) 15 64. (4) LEY H. and SPECXER H., Ber. dtsch. Chem. Bes. 1939 72 192. (5) ANDRISA~COR. and PAPPALARDO G., &zz. Chim. Ital. 1955 85 1430. (6) GILLAM A. E. Zoc. cit. p. 132. (a) Commerical product subjected to additional purification. (b) KING and NARD, J. Org. Chem. 1948 13 635. (c) HARTOU~H H. D. and KOSAK A. J., J. Amer. Ghem. SOC. 1947 69 3093. (d) HARTOU~H H. D. and CONLEY L. I., ibid. 1947 69 3096. (e) b.p, 82°C.; found: S = 21, 97; C,H,O$ requires S = 22, 52. (f) SELENK W. and OCRS R., Ber. dtsch. Chem. Gee. 1915 48 678. (g) NARNSEN R., Rer. d&h. Chem. Cues. 1884 17 2192. - flex. 350
Absorption spectra in the near ultraviolet of some G(-and p-monosubstituted derivatives of thiophene
whether the sequence found in the furan series [2] (namely conjugation in 8) is also followed in the thiophene series.
rl-, U
o-
(Y)
(I’I)
conjugation
in a >
X
Experimental The derivatives of thiophene were prepared by known methods (see footnotes of Tables 1 and 2) and purified by crystallization or distillation dn vaczbo. Table 2. Near ultraviolet absorption spectra (ethanol solutions) * C,H,S-X-_B
C,H,O-X-_B(
1)
X= 2max K
El COCH, COOH COOCH, COOC,H, CONH,
2310 2396 2390 2405 2400 2410
log E 3.77 3*80(a) 3.79(b) 3.95(c) 3.95(c) 3.89(a)
1max A
log E
2000
4.00
2320 2380 2380
3.36 3.40 3.38
-* See footnote of Table 1. (1) ANDRISANO R. and PMPALARDO G., &zz,Chim. It&. 1953 83 340. (a) CAMPAIQNEE. and LE SEUR W. M., J. Amer. Chewa.Sot. 1948 70 1555. (b) RINEERS I., Rec. Truw. Chim. 1936 (4) 55 991. (c) RINEERS I., Rec. Trm., Chim. 1934 (4) 53 643.
Spectroscopic measurements (see Tables 1 and 2; Figs. 1 and 2) were made with a Unicam Spectrophotometer SP 500 in the range 2100-3000 A, in ethanolic solutions 1 : lOO*OOO,reading every 20 8, and for the important regions every 10 f% (Beer’s law is applicable throughout).
Fig. 1.
351
R. ANDRISANV
and G. PAPPALARDO
40 -..._
ggEcH’&
2
3.5
; 30
3.5
2
n
h,
A
Fig. 2.
Discussion We may first examine the first transition of benzene, thiophene and furan, which falls at 2500, 2310 and 2000 d respectively. As the data in Table 1 show, the transition energy in every case decreases in the series: furan > thiophene
> benzene
With acetyl derivatives, for example, we have the values given in Table 3. The series derived from the spectroscopic data is parallel to that which was obtained from the resonance energy, and this may imply that the two phenomena are related. Table 3. Change of the transition energy (AE, kcal/mol) the compounds Ar-X
H
x= Ar=
due to substituent X in
COCH,
E
E
(kcel/mol)
(kcal/mol)
AE (kcal/mol)
\ C,H,O-a C4H3S--a C&&r-
143 124 112
(n = 2OOOA) (I = 2310A) (n = 2550A)
106 101 102
(,I = 2700A) (1 = 2820A) (A = 2790A)
37 23 10
If this hypothesis is correct, it might be concluded that the substituent modifies the first excited level of the molecule much more in the case of furan than in that of benzene, thiophene being intermediate. If the spectral changes due to the same substituent in the CC-or /?-position are compared, there is an obvious analogy with the furan series [2], i.e. the changes of the spectroscopic characteristics with u-substituents are more marked than those with the /I-substituents (see Figs. 3 and 4). 352
Absorption spect~e in the neaS ultraviolet of some a- and ~-~n~ub~t~tut~
derivthtivesof tbiuphene
Mloreover in the ease of the er-substituents a second band arises in this region of the speot~m~ while the ~-substitution causes only a small bathochromic shift of the original thiophene or furan band: this may arise because of greater conjugation in the DCthan in the B-form.
RX example, the values obtained with carbomethoxy group as s~bstitue~t are given in Table 4. From the values reported in Table 4, we may infer that: (1) The decrease of transition energy in the @lerivatives follows the same order furan > thiophene, This is another experimental fact which may be correlated with the greater aromatic charac~r of the thiophe~e nucleus compared with that of furan, (2) The greatest change in the traus~t~a~energy occurs in every case with the 5
353
R.
ANDRISANO
and
G. PAPPALARDO
Table 4. change of the transition energy (A& kcaljmol) due to subetituent X, in the Ar-X compounds, when it occupiesthe a- or ,%position COOCH,
AE Position of substituent C,H,O-
143
(1 = 2000 A)
C&W--
124
(a. = 2310_&)
E (kcaljmol)
(kcal/mol)
(n = 2505 8) (A = 2380A) (I = 2680 11) 118-6 (A = 2405il)
29 23 17.5 5.5
114 120 106.5
j
!
a-subatituent. Theae results provide further experimental evidence for the conclusions of SCHOMAKER snd PAULIN@ [3] and are consistent with the observation that the a-substituent induces s conjugation through all the perimeter of the ring (Ia), while for the ~-substituent the conjugation is limited (II&).
[1]ANDRISANOR~~~PAPPAURDO G.,Bol.Sot.chim. id. Bologna 1956 [Z] ANDRISANO R. and PAPPALARDO G., ffuzz. Chain.Ital. 1953 83 340, [3] SCHOMAICERand PAULINC+,J.AWW.C~~WLSOC.1939 61 1769 1881.
354
14 100.