Steric hindrance revealed by absorption spectra of 2-acyl-3,5-dialkylfurans and of their polynitrophenylhydrazones

Spectrochimica dcta, 1963,vol. 19,pp. 367to 378. Pergamon PressLtd. Printedin Northern Ireland

Steric hindrance revealed by absorptionspectra of 2-acyl-3,5dialkylfurans and of their polynitrophenylhydrazones ALEXANDRU

T. BALABAN, PETRE T. FRANGOPOL and ELENA KEPLINGER* Institute of Atomic Physics, Bucharest, Roumania (Received

1962)

29 January

Abstract-Infrared and ultra-violet absorption spectra of 2-acyl-3,5dialkylfurans (11~f) and of their 2,4-di-(IVu-e), 2,6-di-(Vad) and 2,4,6-trinitrophenylhydrazones (VIu-e) were determined in order to discover why 2-pivaloyl-3-methyl-5-t-butylfuran (IIf) does not form ketonic derivatives, and why 2-isobutyryl-3-methyl-5isopropylfuran (Ire) gives lighter-coloured 2,4-DNP and 2,4,6-TNP and yields neither a 2,6-DNP nor a semicarbazone. Ketones (1Ia-f) present very similar U.V. and I.R. spectra, and likewise the spectra of DNP’s and TNP’s (IV-VI; u-d) are very similar, but the U.V. spectra of 2,4-DNP (IVe) and of 2,4,6-TNP(VIe) differ markedly from the corresponding spectra of derivatives (u-d). It is concluded that the hypochromic and hypsochromic shifts observed in (IVe) and (Vie) are due to steric hindrance.

IT WAS shown [l] that symmetrically-substituted 2,4,6-trialkylpyrylium salts (I) which are easily accessible by diacylation of olefins [2] undergo a ring contraction on treatment with hydrogen peroxide in acid medium yielding 2-acyl-3,Giialkylfurans

(II). R’ I

R’

I

/c\ 'CH2

-HX-H20

H,C

+

R-CO'@OC-R

R-H20

Hz% -HX

2x0 II

I

Designation

a

b

c

d

e

f

R R’

Me Me

Me Et

Me iPr

Et Me

iPr Me

tBu Me

On the other hand, these acylfurans revert to six-membered heterocycles on catalytic hydrogenation in the gaseous phase which affords 2,4,6trialkyltetrahydro-

pyran* [31. When it was attempted to characterize (III) and 2,4-DNP’s (IV)? the unexpected

the acylfurans (II&--j) as semicarbazones observation was made [l] that the di-t-

* Laboratory of Chemical Physics, Bucharest University. t The shorthand notations DNP and TNP will be used for dinitrophenylhydrszone and trinitrophenylhydrazone respectively. [I] A. T. B~ABAN and C. D. NENITZESCU, Chmn. Ber. 83, 799 (1960). [2] A. T. BALABAN and C. D. NENITZESCU, Studii 8i Cercetiiri Chim. Acad. R.P.R. 9, 251 (1961); Rev. chim. Acud. rt5p. populaire Roumaine 6, 269 (1961). [3] N. I. SW=, I. F. BEL’SEI, A. T. BALABAN and C. D. NE~TZESCU, Ime&. Akad. Nauk, Otdel. khim. Nauk 491 (1962). 367

368

A. T. BALABAN, P. T. FRANGOPOLand E. KEPLINGER

butyl-substituted acylfuran IIf gives no ketonic derivatives while the diisopropylsubstituted acylfuran IIe does not yield a semicarbazone but gives a 2,4-DNP (IVe) which however is formed more slowly, and is lighter coloured (both in crystalline state and in solution) than the other 2,4-DNP’s (IVaA). It was concluded that steric hindrance in derivatives e and f was responsible for these phenomena. The present spectroscopic investigation was undertaken in order to get a clearer insight into these facts. New functional derivatives were prepared: 2,6-DNP’s (V) (the sterically hindered ketones IIe and IIf did not afford these derivatives) and 2,4,6-TNP’s (VI) (no such derivative of ketone IIf could be prepared, but ketone IIe yielded slowly a 2,4,6-TNP (Vie) which was lighter coloured than the other 2,4,6TNP’s). The I.R. and U.V. spectra of ketones (IIa-f), I.R. spectra of 2,4-DNP’s (IVu-e) and U.V. spectra of neutral and basic solutions of 2,4-DNP’s, 2,6-DNP’s and 2,4,6-TNP’s were studied. EXPERIMENTAL The preparation and physical constants of ketones (IIa-f), semicarbazones (1IIa-d) and 2,4-DNP’s (IVa-e) were described previously [l]. New functional derivatives are described below. Derivatives

of 3,5-dimethyl-2-acetylfuran

(IIa)

Oxime, prepared in dil. ethanol in the presence of sodium acetate buffer, m.p. 82” (from dil. ethanol), colourless. Found: C, 62.57; H, 7.24; N, 9.38. C,H,rNO, requires C, 62.72; H, 7.24; N, 9.15 per cent. Phenylhydrazone, m.p. 108” (from ethanol), yellow. Found: C, 73.43; H, 7.15; N, 12.45. C,,H,,N,O requires C, 73.65; H, 7.06; N, 12.27 per cent. p-Nitrophenylhydrazone, m.p. 223” (from ethanol-benzene), red. Found: C, 61.37; H, 5.61; N, 15.62. C,,H,,N,O, requires C, 61.53; H, 5.53; N, 15.38 per cent. 2,6-Dinitrophenylhydrazone (Vu), m.p. 143” (from ethanol), brick-red. Found: C, 52.80; H, 4.53; N, 17.71. C,,H,,N,OS requires C, 52.83; H, 4.43; N, 17.60 per cent. 2,4,6-Trinitrophenylhydrazone (Via), m.p. 224” (from ethanol-benzene), dark-red. Found: C, 46.13; H, 3.71; N, 19.03. C,,H,,N,O, requires C, 46.28; H, 3.60; N, 19.28 per cent. Derivatives

of 3-ethyl-5-methyl-2-acetylfuran

(IIb)

p-Nitrophenylhydrazone, m.p. 184-185” (from ethanol), red. Found: C, 62.61; H, 6.03; N, 14.85. Cn,H1,N303 requires C, 62.70; H, 5.96; N, 14.63 per cent. 2,4,6-Trinitrophenylhydrazone (VIb), m.p. 204” (from ethanol-benzene), dark-red. Found: C, 47.87; H, 4.07; N, 18.60. C,,H,,N,O, requires C, 47.75; H, 4.01; N, 18.56 per cent. Derivatives of 3-methyl-5-ethyl-2-propionylfuran

(IId)

2,6-Dinitrophenylhydrazone (Vd), m.p. 149” (from ethanol), brick-red. Found: C, 55.37; H, 5.42; N, 16.30. C,,H1,N,OB requires C, 55.48; H, 5.24; N, 16.17 per cent. 2,4,6-Trinitrophenylhydrazone (VId), m.p. 159” (from ethanol-benzene), dark-red. Found: requires C, 49.10; H, 4.38; N, 17.90 per cent. C, 49.15; H, 4.39; N, 17.83. C,,H,,N,O, Derivative

of 3-methyl-5-isopropyl-2Gobutyrylfuran

(Ire)

2,4,6-Trinitrophenylhydrazone (Vie), m.p. 123” (from ethanol), red, was formed more slowly, was lighter coloured and more soluble than other 2,4,6-TNP’s. Found: C, 51.52; H, 5.05; N, 16.55. C,,H,,N,O, requires C, 51.55; H, 5.04; N, 16.70 per cent. The behaviour of ketone IIe towards 2,4-dinitrophenylhydrazine and picrylhydrazine was closely similar. No semicarbazone (IIIe) and 2,6-DNP (Ve) of this ketone could be obtained. No functional derivative of the stronger hindered 3-methyl-5-t-butyl-2-pivaloylfuran (IIf) could be prepared.

Steric hindrance revealed by absorption spectra

369

Infrared absorption spectra were recorded with an automatic Jena URlO spectrophotomer in the range 400-5000 cm- l, Ketones (IL-f) were studied in liquid film and in carbon tetrachloride solution (cell thickness O-015 and 0.03 mm), and 2,4-DNP’s (IVu-e) in CCI, solution and in KBr pellets. Ultraviolet and visible adsorption spectra were studied with C@4 and Jena VSU-1 spectrophotometers at room temperature. Ketones (IIa-f) were studied in absolute ethanol, 2,4-DNP’s (IV+e) in pharmaceutical chloroform, 2,6-DNP’s and 2,4,6-TNP’s in 1,2-dichloroethane solution. The alkaline solutions were prepare< by mixing 1 ml lob4 molar solutions of DNP’s or TNP’s in dichloroethane with 5 ml 10e2 molar solutions of sodium hydroxide in 95% ethanol; as reference solvent a mixture of 1 ml dichloroethane with 5 ml of the same ethanolic hydroxide solution was used. The deep colour developed on alkalinization of the hydrazone solutions was quite persistent, so that no extrapolat,ion to initial mixing moment was necessary. All wavelengths are given in rnp. The lower limit of the spectra is 220 m,u for ethanolic solutions and 240 rnp for chloroform or dichloroethane solutions. RESULTS I. Infrared absorption spectra

A. Ketones (II&f). All I.R. spectra of 2-acylfurans (IIa-f) are very similar. In the 1650-1850 cm-l range, the C=O stretching band appears at 1670-1675 cm-l in ketones II+e and at 1664 cm-l in IIj, as a very strong band; a medium-intensity band appears at 1795-1805 cm-l with constant position and intensity in all six band appears in ketones (II&e) at 1722-1729 cm-1 ketones ; a medium-intensity and at 1706 cm-l in (IIf) ; and lastly a medium or strong band appears at 17681774 cm-l in ketones (IIa-aY), which is split in ketone (Ire) into 1752 (medium) and 1780 (weak), and displaced at 1750 cm-l in ketone (II!). In the 1400-1650 cm-l range, three very strong ring stretching bands appear at 1602-1608 (+), 1535-1540 (Ye), and 1403-1410 cm-l (Y& in ketones (II+e) and at 1599, 1529 and 1400 cm-l in ketone (IIf). The strong or very strong band appearing at 1258-1268 cm-l in ketones (II&e) and at 1288 cm-l in (IIf) is probably due to the y14vibration mode. The ring-breathing vibration (Ye) appears at 999-1008 cm-1 as a medium or strong band in ketones (IIa-e) and as a shoulder at 982 cm-l in (IIf). These assignments are in agreement with literature data for furans [PlO]. Band notations are after LORD and MILLER [111. [4] E. R. ALEXANDER and S. BALDWIN, J. Am. Chem. Sot. 73, 356 (1961); H. A. J. CROSS, S. G. E. STEVENS and T. H. E. WATTS, J. Appl. Chem. 7, 562 (1957); W. DAASCH, Chem. & Ind. (London) 1113 (1958). [5] H. A. J. CROSS and T. H. E. WATTS, Chem. & Ind. (London) 1161 (1958). [6] T. KUBOTA, Tetrahedron 4, 68 (1958). [7] A. R. KATRITZI(Y, Quart. Revs. (London) 13, 353 (1959). [8]A. R. KATRITZKY and J. M. LAGOWSKI, J. Chem. Sot. 657 (1959). [9] L. H. BRIGGS and L. D. COLEBROOK, J. Chm. Sot. 2458 (1960). [lo] P. MIRONE, Atti accad. naz. Lincei, Rend., Classe sci. jh., mat. e nut. 16, 483 (1954). [ 1l] R. C. LORD Jr. and F. A. MILLER, J. Chem. Phya. 10, 328 (1942). 24

370

A. T. BALABAN, P. T. FRANGOPOL, and E. KEPLINGER

Bands due to substituents appear at 1467-1471 and 1381-1385 cm-1 in ketones (I&x--e) (antisymmetrical and symmetrical methyl bending vibrations) ; the former band is split into two strong bands at 1468 and 1485 cm-l in (IIf). In the region of UC-H stretching vibrations, four bands appear at 2870-2890 m, 2930-2940 ms, 2965-2985 vs, and 3097-3112 mw cm-l the last one being due to the nucleus-bound hydrogen atom. The strong band appearing at 1166-l 171 cm-l in (IIad), at 1183 cm-l in (IIe) and at 1217 cm-l in (IIf) is tentatively attributed to the @C-H in plane deformation vibration. No assignment for the out-of-plane yC-H mode and for skeletal modes below 1000 cm-l is attempted. Other non-assigned common bands of ketones (11~J@) appear at 450-460 w, 580588 mw, 600-610 mw, 627-635 m, 699-716 w, 759-762 m, 816-820 ms, 880-890 vw, 939-949 vs (splitted in Ire), 968-980 ms, 1035-1042 m, 1060-1072 mw, 1140-1150 ms, 1225-1240 m and 1360-1370 s. Weak overtone or combination bands appear around 1880, 1920, 2035, 2240, 2370, 2650, 2740, 3190, 3320 (overtone of the C=O stretching band) and 4090 w. B. 2,4-Dinitrophenylhydrazones (IVa-e). Very few abnormal features are apparent in the I.R. spectra of 2,4-DNP’s (IVa-e). The bands listed for 2,4-DNP’s [ 121 are visible in the spectra: the N-H stretching vibration appears at 3310 cm-l in IVad and at 3294 cm-l in IVe; the bands at 1620 vs, 1595 vs, 1520 s, 1333 vs, 1312 vs, 1225 m, 1139 s, 1099 s, 1064 m, 923 m, 837 s and 743 s were all assigned by JONES et al. [12] to vibrations of the dinitrophenylhydrazone moiety. Furthermore, bands due to the furan moiety are also apparent: 3110 w, 2978 m, 2930 m, 2880 w (C-H stretchingbands), 2370 VW (combinationband), 1603s(superposedonthe 1595banddue to phenyl ring stretching, but clearly splitted in IVe), 1533 m, 1425 s, 1270 s (furanic 1470 m, 1380 m (methyl ring stretching bands: v15, vs, vg and v14 respectively), bending bands) 1000 w, 1035 w (ring breathing bands) and 978 w. Other bands common to all five 2,4-DNP’s are visible at 680 m, 645 m, 530 w, 510 m and 460 w. II.

Visible and ultraviolet spectra in neutral solutions

A. Ketones (II@. As seen from Table 1, all spectra are very similar, presenting two bands at 230-233 mp (E 1750-1950) and at 284-285 m,u (E 14,500-16,500). These bandsare also encountered in furfural(230 rnp, E 3500; 275m,u, E13,750) and 2-acetylfuran (225m,u, E 3000; 275mp, E 13,900) [13]. The former band is related to the socalled K [14] or E [ 15, 161 or C [ 17, 181 band appearing at 242 m,u in acetophenone, while the latter is related to the so called B [l4-181 band at 278 m,u in acetophenone, though the intensity ratio of these two bands in acetophenone and in ketones IIa-f is reversed. A very weak band related to the so called R-band [14] appearing at [12] L. A. JONES, J. C. HOLMES and R. B. SELIGMAN, Anal. Chem. 29, 191 (1956). [13] R. F. RAFFAUF, J. Am. Chem. Sot. 72, 753 (1950). [14] A. E. GILLAM and E. S. STERN, An introduction. to Electronic Absorption Spectroscopy in Organic Chemistry. Edward Arnold, London (1954). [15] W. A. SWEENEY and W. M. SCHUBERT,J. Am. Chem. Sot. 76,4625 (1954); W. M. SCHUBERT, J. ROBINS and J. L. HAUN, Ibid. 79, 910 (1957). [16] K. BOWDEN and E. A. BRAUDE, J. Chem. Sot. 1068 (1952). [17] W. F. FORBES and W. A. MUELLER, Can. J. Chem. 33,1145 (1955), and subsequent papers. [18] G. D. HEDDEN and W. G. BROWN, J. Am. Chem. Sot. 75, 3744 (1963).

Steric hindrance revealed by absorption spectra

371

320 rnp in the spectrum of acetophenone (E 54) was not determined in the present study. Since the band notation employed by FORBES and MUELLER [ 171 is conflicting with that proposed by GRAMMATICAKIS[ 191, we denote these bands as E- and B-band ; for DNP’s and TNP’s the related notation of TIMMONS [20] will be followed. B. 2,4-Dinitrophenylhydrazones (1Vu-e). The spectra of 2,4-DNP’s (1Vu-d) are very similar, presenting three bands: E, at 264-266 rnp (E 12,000-13,000), E, at 304-306 rnp (E 9750-9850) and E, at 407-409 rnp (E 25,000-26,000), but the spectrum Table 1. Ultraviolet

spectra of ketones (IIu-f)

in absolute ethanol

B band

E band Ketone

IIa IIb IIC IId IIe

IIf

t max

EmaX

I max

233 233 233 229 229 232.5

1748 1960 1940 1930 1800 1750

284 285 285 284 285 284.5

Table 2. Spectra of 2,4-DNP’s

2,4-DNP

IVa IVb IVC IVd IVe

E, band ~~ ji mr&x Em&X 264 266 265 266.5 265

12,500 12,175 12,850 13,250 15,860

(IVa-e)

Emsx 16,360 16,650 15,750 14,750 15,800 15,500 in chloroform

E, band 1 max 305.5 305 304 306 (303)

E, band ElUax

9750 9775 9875 9850 (6000)

I max 409 408 408 407 389

ElMX 25,000 25,000 25,500 25,900 23,600

of the lighter coloured compound IVe is sensibly different (cf. Fig. 1) : the first band E, appears at the same wavelength, 265 rnp, with a slightly higher intensity; the second band E, also maintains its position but, the intensity is so much reduced that the band appears as a shoulder; its extinction coefficient in Table 2 was calculated by substraction of extrapolated adjacent bands. Most interesting, because it affects the colour, is the hypsochromic shift of the principal band E, by about 20 mp, accompanied by a small hypochromic effect. The number and position of bands in the non-hindered 2,4-DNP’s are in agreement with literature data for furanic 2,4DNP’s, which are reported to present three bands at about 260, 300 and 390 rnp [12, 211. Since, as seen from Tables 1 and 2, the non-hindered ketones and 2,4-DNP’s (a-d) give practically identical spectra, no further distinction between derivatives of these non-hindered ketones (aA) will be made in the following discussion and in the illustrations. [19] P. GRAMMATICAKIS,Bull. sot. chim. France, 1372 (1954) and subsequent papers in the series. [20] C. J. TIMMONS, J. Chem. Sot. 2613 (1957). [21] F. H. STADTMAN, J. Am. Chem. Hoc. 10, 3583 (1948).

372

A. T. BALABAN, P. T. FRANGOPOL and E. KEPLINGER

200

300

400

so0

’

60

My, Fig. 1. U.V.

spectra of 2,4-dinitrophenylhydrazones (IV) in chloroform; line: non-hindered (aA); broken line: hindered (e).

full

6 15,000

-

10,000

-

5,000

-

200

cc c-------%

300

400

500

660

h,mp

--. 700

Fig. 2. U.V. spectra of non-hindered 2,6-dinitrophenylhydrazones (Vad); fullline: neutral solution in dichloroethane; broken line: basic medium.

C. 2,6-Dinitrophenylhydrazones (Vad). Since the 2,6-DNP Ve could not be prepared, the spectra of all available 2,6-DNP’s (Vu-d) are very similar, and no spectral evidence of steric hindrance may be found. As seen in Fig. 2, there appears only one clearly evidenced band E, at 313-314 m,u (8 16000) with a shoulder E, at 280 rnp (E ca. 4000), while the broad band at 400-500 cm-l with low extinction coefficient (5000-8000) allows only a poor resolution into separate maxima: the principal maximum E, is at 420-425 m,u (E SOOO),and there is evidence of a more remote B band at higher wavelengths (ca. 490 rnp, E ca. 5000).

Steric hindrance

revealed

by absorption

373

spectra

20,000 & is,000 -

10,000 -

5,000 -

I;00

3bo

’

61 0

500 A, mp

Fig. 3. U.V.

spectra full line:

of 2,4,6-trinitrophenylhydrazones non-hindered (aA); broken line:

(VI) in dichlorocthane; hindered (e).

30.000 -

25,000

-

20#000& 15.000 -

io.000 -

5,000 -

I

200

r

360

400

500

Fig. 4. U.V. spectra of 2,4-dinitrophenylhydrazones non-hindered (a-d); broken line:

600

7 h,mp

(IV) in basic medium; hindered (e).

full line:

D. 2,4,6- Trinitrophen ylhydrazones (Via-e) . The spectra of picrylhydrazones Via-e are closely related to those of 2,4-DNP’s, as evident from Fig. 3. In the nonhindered compounds (Via-d), band E, appears at 275-276 rnp (E ca. 15,000), band E, at 295-297 m,u (E ca. 14,500) and band E, at 433-435 rnp (E ca. 27,000). Thus bands E, and E, come close together, and band E, appears at longer wavelengths than in In the hindered picrylhydrazone Vie, band E, (268 rnp, E 13,000) 2,4-DNP’s (IVud). undergoes small hypsochromio and hypochromic effects, the intensity of band E, (ca. 303 rnp, E ca. 3000) is enormously decreased (this strong hypochromic effect

A. T. BALABAN,P. T. FRANGOPOL and E. KEPLINGER

374

makes it very difficult to indicate the position of the peak by interpolation, so that the value 303 m,u for band E, is not very meaningful), while band E, (405 rnp, E 19,000) undergoes a hypsochromic shift of ca. 20 rnp. Thus the steric hindrance has closely similar effects on the spectra of 2,4-DNP’s and 2,4,6-TNP’s. III.

Vkible and ultraviolet spectra in alkaline solutions

A. 2,4-Dinitrophenylhydrazones (IVa-e). 2,4-DNP’s of furyl-ketones were reported [12] to give persistently-coloured basic solutions presenting a principal maximum at ca. 470 my (E 26,000) and a secondary maximum at 270 rnp. Indeed (cf. Fig. 4), non-hindered 2,4-DNP’s (1Va-d) present two maxima at 485 rnp (E 23,000, E,--band)

200

’

300

400

sbo

6bO Amp

Fig. 5. U.V. spectra of 2,4,6-trinitrophenylhydrazones (VI) in basic medium;

full line: non-hindered (ad);

broken line: hindered (e).

and 277 rnp (E 11,000, Es--band) but in addition two shoulders are evident, one corresponding to the intermediate band in neutral furanic 2,4-DNP’s at ca. 328 rnp (E 5000, El--band), and the other which is responsible for the non-symmetrical aspect In the of the principal absorption band appears at ca. 590 m,u (E 7000, NO,--band). hindered compound IVe hypsochromic effects are observed: the principal band appears at 460 mp (E 21,000, E,--band) and the E,--band appears at 268 rnp (E 13,300); the intermediate El--band is completely hidden, and the NO,--band is observed as a shoulder around 570 rnp. B. 2,6-Dinitrophenylhydrazones. As seen from Fig. 2, 2,6-DNP’s also present four bands in basic medium: the E,--band at 286 rnp (E 14,800), the El--band at 330 m,u (E 12,000) the E,--band, much less intense than in 2,4-DNP’s or 2,4,6-TNP’s at 440 m,u (E 5600) and the NO,--band, clearly outlined and more intense than in 2,4-DNP’s or 2,4,6-TNP’s, at 630 rnp (E 6800). These long wavelength bands are responsible for the green colour of 2,6-DNP’s in alkaline medium. C. 2,4,6-Trinitrophenylhydrazones (Via-e). In basic medium the non-hindered

picrylhydrazones (VIacd) present two maxima at 274 m,u (E 14,000, E,--band) and 485 rnp (E 27,000, E,--band) and a shoulder at ca. 312 rnp (E 8000, El--band).

Steric hindrance revealed by absorption spectra

375

A long-wavelength hidden band around 600 rnp may hardly be guessed. The presence of a strong band just outside the lower limit of the spectrum (ea. 235 m,u, E ca. 30,000) is also apparent. In the hindered compound (Vie) (cf. Fig. 5), the principal E,--band appears at 483 rnp (E 27,000), as in the non-hindered 2,4,6TNP’s, but the &,--band becomes a shoulder around 270 m,u (F cts. 13,000) ; the El--band is visible as a small shoulder at ca. 310 rnp (E ca.7000). DISCUSSION On the basis of the non-reaeti~Tity of ketones II$ and (in part) IIe, and of the small but systematic deviations in the infrared spectra of ketone IIf and of 2,4-DNP IVe, it may be concluded that in the fundamental state of ketone IIf and 2,4-DNP IVe a small steric hindrance exists ; it may also be assumed that similar or larger steric hindrance is present in the fundamental state of the 2,4,6-TNP Vie which could be prepared, as well as in 2,6-DNP Ve and in all derivatives III-VIf which could not be prepared. Molecular models after Stuart-Briegleb support this conclusion in that they show small sterieally conditioned deviations from coplanarity both in ketone IIf and in derivatives IVe and Vie. In electronic absorption spectra, the steric effects influence both the fundamental and the excited state, so that conclusions must be drawn with care. In the case of hindered acetophenones, e.g. pivalophenone or 2;6-~methyla~etophenone [22-261 the hypsochromic or hypochromic effects were accounted for by BRAUDE and coworkers [22, 263 and FORBES and MUELLER [17] on the assumption of deviations from planarity in the fundamental and/or excited state. A better explanation was provided by HEILBRONNERand GERDIL [27] and HEILBRONNER[28] who considered the bond orders in the fundamental and excited stateand developed a general theory covering all kinds of shifts due to steric effects. In the furan series there are few examples of steric hindrance [29] ; molecular models show that 3-alkyl-2-acylfurans (the 5-alkyl group plays a small part) are less hindered than 2-alkylacetophenones. Polynitrophenylhydrazines and -hydrazones generally present two bands. Most of the spectra-structure correlations were made on the principal (E,) band in 2,4DNP’s which appears at ea.350 rnp in 2,4-DNP’s of aliphatic ketones, at ea.375 rnp in 2,4-DNP’s of aromatic or mono-olefinic ketones, and at over 390 rnp in 2,4-DNP’s [22] E. A. BRAUDE, Determination of Chemical Structures by Physical Methods (Edited by E. A. BRAUDEand F. C. NACROD)p. 131.Academic Press, New York (1955); E. A. BRAUDE and E. S. WAIGRT, I’rogress ila Stereochemistry (Edited by W. KLYNE) Vol. 1, p. 126, Butterwortbs, London (1954); E. S. WAICHT and R. L. ERSKINE, Steric Effects in Con~~~e~Sy8~~ (Edited by G. W. GRAY) p. 73. Butte~o~bs, London (1958); E. A. BRA~DE and R. L. ERSKINE,J. Chem.Sot. 4673 (1956). [23] M. T. 0’S~~uarr~~ss~ and W. H. RODEBUSH,J. Am. Chem.Sot. 62, 2906 (1940). [24] L. H. SCHWARTZMANN and B. B. CORSON,J. Am. Chem. Xoc. 76, 781 (1954). [25] J. N. MVRRELL,J. Chem. Sot. 3779 (1956). [26] E. A. BRAUDE and F. SONDFIEIMER, J. Chem. Sot. 3754 (1955). [27] E. HI+XLBRONNER and R. GERDIL, He&. Ghim. Acta 39, 1996(1956). [ZS] E. ~EILBRON~, ~~n-~enze~o~ Aromatic Cafe (Edited by D. GIN~B~RC+)p. 171. Interscience,New York (1959). [29] P. RA~~IART-LUCAS, J. HOCH and J. KLEIN, Compt. rend. $332, 336 (1951); R. PALLAUD and I?. DELAVEAU,Ibid. 287, 1254(1953).

376

A. T. BALABAN, P. T. FRAN~OPOL and E. KEPLINGER

of compounds with several double bonds conjugated with the carbonyl group [12, 20, 301. Geometrical isomerism and conformation also influence the spectra of 2,4DNP’s [31-331. It may be safely assumed that the principal (E,) band is polarized along the axis noted x in formula VII, i.e. in the excited state the rr-electron density on the polynitrophenyl groups is larger than in the fundamental state. This assumption also explains the bathochromic shift of band E, on alkalinization, i.e. on

Jx

passing from VIII to IX, because the conjugation along axis x increases in this conversion. Salts containing anions (IX, R = Ph) were recently isolated [34} and their spectra are in a~eement with the spectra of basic solutions of compound (VIII, R = Ph). On the other hand, the intermediate &-band, which was said to be characteristic for 2,4-DNP’s of furanic ketones [12] also appears in some 2,4-DNP’s of dienones and styryl-ketones [20, 351 and we assume that it is due to a transition polarized along axis y in formula VII. Indeed, it was argued [36] that the oxygen bridge in furan is of little importance for the electronic configuration of the furan molecule which behaves very much like a conjugated diene ; geometrically, however, the oxygen bridge is important since it imposes a &-configuration of the dienic system. We suppose the E,-band in DNP’s or TNP’s to be due to an electronic transition involving this cis-dienic system. These assignments for bands E, and E, in hydrazones IV-VI are in line with those made in the case of I,l-diphenyl-2, pol~it~phenylhy~azines [37] and a comparison between their spectra may be seen in Table 3. A close resemblance is observed between corresponding spectra. These assignments for bands E, and E, explain why the intensity of band E, increases, and of band El decreases, in the sequence 2,6-di-,2,4-di-, and 2,4,6-trinitrophenylderivatives. [SO] E. A.BRA~~~~~E.R.H.Jo~s,

J.Chem.Soc.498 (1945); J. D‘Ro~ERTs~~C.GREE~, J. Am. Chem. Sot. 68, 214 (1946);H. H. SZMANT and H. J. PLANINSEII,Z&IL 73, 4042 (1950);G. D.JoHNsoN,I~~~.'?~,~?~O(~Q~~);J.P.PHILLIPS, J.Org.Chem. 27,1443( 1962). [31] F.RAMIREZ and A.F. KIRBY,J. Am.Chwn.Soc. 76, 1037(1954). [32] E. A.BRAUDE and C.S.TIMMONS,J. ChemSoc. 3766 (1955). [331 R.MECKE andK.N~~~~,Chern. Ber.93,210(1960). [34] J.A.WEIL~~~ G. A.JANUSONIS,J.OT~. Chem. 27, 1248 (1962). [36] s. YAROSLAVSKY,~.oTg.che~.~, 480 (1960). f36] J. R. MORRIS and F. L. PILAR, Chews. & Ird (lodo?%) 469 (1960). [37] A. T. BALABAN, P. T. FRANGOPOL, M. MARC~LESCU and J. BALLY, Tetrahedron13, 258 (1961).

-CR=N (present work)

0

a4

310 4.11 305

250 4.16 265

Neutral

VIII Basic IX

314

280s

4.404 4851 4.364 59osl 3.84

4.20 330 4.08

305s 4.20

242 4.36

395 4.11

3.60 286 4.17

4.11

4.40

4.11

4094

260s

230

338

490 4.13

E,

NC,

E0

Ez

630 3.83

610 3.64

4.17 274 4.151

275

244 4.18

4.434 485 4.431

4.161 312 3.904

4354

296

4.04

3.87

431 4.18

322

257s

320 3.90

-%

-El

.

(4 ) caused by steric hindrance; s denotes a

3.78 440 3.75

425

440 3.83

4.23

3.57

Eo

222s

NO,

2,4,6-Trinitrophenyl

394

4

2,6-Dinitrophenyl

arrows symbolize increase ( f ) or decrease

3*99J. 328s 3.70&

4.15

4.08

VIII

Basic IX

4.11t 277 t 4.04

267

231s

Neutral

E1

Es

Medium

* Absorption maxima (m,u) and log E are given; shoulder.

It-

-R’

Ph,N (Ref. 1371)

R,N in VIII and IX

2,4_Dinitrophenyl

Table 3. Comparison between the spectra of hydrazine derivatives*

3

%

Y

3 S 8 2

. 3

g

Z Y 8 E

i

& 2

;

F S.

A. T. BALABAN, P. T. FRANGOPOL and

378

E. KEPLINOER

As to the effect of steric hindrance on the ele&ronic absorption spectra, it is manifest that no such effect is apparent in the spectra of ketones I&-f; this result means that the deviations from planarity, or the differences in bond orders, are small and equal in the excited and fundamental states. On the other hand, in 2,4-DNP We and 2,4,6-TNP Vie, the effects of steric hindrance, indicated by arrows in Table 3, on bands E, and E, (the assignment of, and effects on, band E, are not discussed in the present paper) may be interpreted if it is assumed that the molecule of the hindered hydrazones is distorted by rotation around the N-N bond. This distortion affects the energy of the latter state more than of the former, owing to the increased conjugation requirements in the excited state corresponding to band E,: therefore the hypsochromic shift of band E,. The same distortion causes a decrease in the dipole moment variation associated with band E,: therefore the hypochromic effect on band E,. Another factor which might account for these variations is the N-H * - - 0 hydrogen bond. There are I.R. and U.V. spectral arguments [38].for the existence of a hydrogen bond with the nitro groups in o-nitro-substituted phenylhydrazines (cf. VIII). However, the existence of such a bond with the oxygen hetero-atom in furan (cf. VII which has a syn-fury1 configuration) though possible, was not demonstrated. Moreover, the anti-fury1 configuration is less sterically hindered. Acknowledgement-We thank Dr. J. A. WEIL for sending us his manuscript prior to publication and Mrs. C. LUPU for recording the I.R. spectra. [38] E. D. BERGMANN,

7& 9 (1957).

R. IKAN and H. WEILER-FEILCHENFELD, Bull. Research Council Israel