A POSSIBLE PHYSICAL DISTINCTION BETWEEN ISOMERIC U- AND p-ALDOXIMES

By R. J. W. LE F&VRE* and R. K. PIERENS*

[Man'u9cript received June 5, 19611

Five pairs of geometrically isomeric aldoximes show consistent differences when examined as solutes in the near-infrared (0 .7 to 2.6 p) region. Whereas a-oximes exhibit two absorption peaks, a t 2.010-2.030 p and a t 2.065-2.085 p, @-oximes exhibit only one, a t 2 046-2.060 p.

I. INTRODUCTION No rapid, simple, and non-destructive method for distinguishing the u-

from the p-formst of geometrically isomeric aldoximes (I) and (11) appears to exist. The original chemical test proposed by

R-C-H R -C -H Hantzsch (1891)-vie. the easy regeneration of the I I 1 I one aldoxime from its acetate, and the formation N-OH HO-N of a cyanide from that of the other-has none of

a-isomer p-isomer these qualities. Optical properties, according to (1) (11) Auwers and Ottens (1924), are not trustworthy

guides, nor evidently are dissociation constants and degrees of hydrolysis of oxime salts, since Brady and Goldstein (1926) showed that the latter led to conclusions opposite to those then accepted. Later, Patwardhan and Deshapande (1944) examined the conductivities of u- and p-oximes in liquid sulphur dioxide and found the p-isomers to be the stronger acids conformably with relationships as (I) and (11), and the principles stated by Ostwald (1892) for acids of the ethylene series. Measurements of dipole moments, as undertaken by Sutton and Taylor (1931, 1933), may be applicable in certain cases, but not generally ; the same is true of special reactions such as the easy iso-oxazole ring closure noted by Brady and Bishop (1925) with only one of the forms of 2-chloro-5-nitrobenzaldoxime.

Apart from the argument used by Hendricks et al. (1936)-that because salicylaldoxime acetate shows no OH absorption, the molecule possesses the p-configuration-infrared spectra do not seem to have been utilized in this problem until Palm and Werbin (1953, 1954) recorded data, over the ordinary rock-salt range, for five pairs of isomeric oximes. The most pronounced difference occurred with bands appearing at o. 3250 and c. 3115 cm-l in the a- and 6-oximes respectively. Although these are lower than the frequencies expected for " free " hydroxyl groups in oximes (Bellamy 1958, quotes 3500-3650 em-l) and most

* School of Chemistry, University of Sydney. t Prefixes a- and P- are here used according to the convention suggested by Brady and Bishop

(1925).

DISTINCTIOK BETWEEX ISOMERIC ALDOXIMES 513

probably arise from " associated " hydroxyl (Califano and Liittke (1956) give YO= as 3650 cm-l in gaseous ecetoneoxime, as 3185 om-l in the crystalline oxime, and as 3218 cm-l in the liquid material), we thought it, of interest to examine some isomeric pairs in the neer-infrared region in the hope that clearer distinctions of diagnostic value might be revealed.

11. EXPERIMENTAL (a) Solutes and Solvents

The oximes were obtained by methods described by Patterson and McMillan (1908), Patterson, McMillan, and Montgomerie (1912), Brady (1914), Brady and Goldstein (1927), Hauser and Hoffenberg (1955), or Vogel (1956) ; the samples used had melting points ("C) as follow :

a-Furfuraldoxime p-Furfuraldoxime o-Nitrobenzaldoxime Piperonaldoxime Anisaldoxime Acetaldoxime o-Chlorobenzaldoxime Cyclohexanoneoxime Thiophen-2-aldoxime

Commercial carbon tetrachloride was dried over calcium chloride, fiItered, fractionated, and stored over calcium chloride. Benzene was dried over sodium wire, fractionated, partially frozen, the solid separated, remelted, and kept over sodium. Other solvents were of A.R. grade and were used without further purification.

(b) Apparatus All spectra have been recorded on Perkin-Elmer double-beam instruments,

the No. 21 spectrometer (with rock-salt prism) for Nujol mulls over the 2 -5-15 p infrared (i.r.) range, and the " Spectracord " model 4000 for solutions over the 0.7-2.7 p near-infrared (n.i.r.) range ; cell path-lengths in the latter cases were 5.0 cm with chloroform as solvent and 10.0 cm with carbon tetrachloride or carbon disulphide.

(c) Infrared Spectra The i.r. spectra obtained confirmed in essential particulars the details

reported by Palm and Werbin (1953) ; slight differences can be attributed to particle sizes, crystal forms, etc.

(d) Near-infrared Spectra Table 1 lists the wavelengths (A, in microns) at which absorption maxima

were observed. The usual intensity indicating annotations, weak, medium, strong, etc. are added, but these apply only within each spectrum and not between, spectra which, due to the low solubilities of p-forms, sometimes had to be taken a t very different concentrations.

TABLE 1

NEAR-INFRARED ABSORPTION MAXIMA RECORDED WITH SOLUTIONS OF ALDOXIMES

c+Benzaldoxime in CCl,

(1% w/v)

h Intensity of

(3- Benzaldoxime in CC1,

(sat. soln.)

a-Benzaldoxime in CS,

(1% w/v)

a-Furfuraldoxime in CS,

(1 % w/v)

P-Benzaldoxime in CS,

(sat. soln.)

1 . 1 3 , ~ ~

P-Furfuraldoxime in CS,

(1% w/v)

DISTIECTIO?; BETWEEN ISOMERIC -4LDOXIMES 515

TABLE 1 (Continued)

h Intensity of

a-m-Nitrobenzald- oxime

in CHCI,

( 1 % wlv)

a-p-Chlorobenz- aldoxime

in cs, (1 % w/v)

P-m-Nitrobenzald- oxime

in CHCI,

(1% w/v)

(3-p-Chlorobenz- aldoxime

in CS2 (sat. soln.)

a-p-Nitrobenzald- oxime

in CHCl,

(1 % w/v)

0 . 9 7 , ~ ~ 1 . 1 2 , ~ ~ 1 . 3 4 , ~ ~ 1.42,m

c. 1 . 5 5 , ~ ~ 1.65,m.sh 1.67,m.sh 1 . 8 4 , ~ ~

a-o-Chlorobenz- aldoxime

in CS, (1 % w/v)

(3-p-Nitrobenzald- oxime

in CHC1, (sat. s o h )

a-o-Nitrobenz- aldoxime

in CHC1, (1 % w/v)

0 . 9 7 , ~ ~ 1 . 1 2 . ~ ~

R. J. W. LE FEVRE AND R. K. PIERENS

TABLE 1 (Continued)

a-Anisaldoxime in CS, (1% w/v)

A Intensity of

cc-Piperonaldoxime in CC1, (sat, soln.)

Acetaldoxime I i Cyclohexanone Oxime

in CHC1, (1% w/v) in CS, (1% w/v)

Thiophen-2-aldoxime in CS, (sat. soln.)

111. DISCUSSIOK (a) Assignments

Because of the high general absorption of chloroform above c. 2 -2 p recordings for solutions in this solvent were not continued to longer wavelengths. Between 0 . 9 and 2 .2 p, five regions of common absorption can be discerned in Table 1, namely, (i) 0.97-0.98 p (or 10300-10200 cm-I), (ii) 1.41-1.43 p (or

DISTINCTION BETWEEN ISOMERIC ALDOXIMES 517

7090-6990 cm-l), (iii) 1.61-1 a71 p (or 6210-5850 cm-I), (iv) 2 -01-2 a08 p (or 4975-4810 em-l), (v) 2.12-2 a19 p (or 4720-4565 om-1).

Of these, (iii) should contain the first overtone of the 0-H fundamental ; Mecke and Oswald (1951) quote 2vCH as 5988 om-1 (1.67 p) in benzene or 5921 cm-l(1.689 p) in chloroform, and 3vCH in the same two liquids as 8787 cm-I (1 el38 p) and 8703 em-l (1 el49 p) respectively ; the absorptions detected with many of the oximes at 1.1-1.2 p may therefore be attributed to 3vCH. Indeed, since chloroform itself absorbs at O.965,, 1 .412,2.010,2.054,2 -112, and 2 el87 p there is no a priori justification for connecting features in regions (i) to (v) with other than CH groups ; against this, however, are the facts that 2vOH should also fall into (ii) and 3vOH into (i). The first hydroxyl overtone at c. 7000 cm-l (o. 1.42 p) is well known in relation to hydrogen bonding through the work of Wulf, Hendricks, Hilbert, Liddel, and others since 1933 (cf. Summary by Pauling 1945 ; Hendricks et al. 1936) and of course if oximes are H-bonded associated solutes (e.g. as formulated by Fabian, Legrand, and Poirier 1956) perhaps neither 2voE nor 3voH should be expected in the spectra. In any case disentanglement from 2vCH would be difficult, e.g. Lippert and Mecke consider 4vCH for the aldehydic 0-H in acetaldehyde, furfuraldehyde, and benzaldehyde to be 10300-10500 em-l, and 3voH for several alcohols to be within the same limits.

(b ) Distinctions between a- and p-Oximes At the outset it was hoped that differences might occur at wavelengths

corresponding to slightly less than twice the near-fundamental OH frequencies recorded by Palm and Werbin or known for OH in OH0 (viz. 2720 and 2820 em-l, cf. Bellamy 1958) because these seemed most likely to be affected by configura- tional changes about the C=N bond. As to the first point, Table 1 shows that between 1.50 and 1.62 p (6667 and 6173 cm-l) the a-oximes of benzaldehyde, and its p-chloro- and p-nitro-derivatives show absorptions absent with the @-isomers ; however, the a- and 6-oximes of m-nitrobenzaldehyde have bands at 1.57, and 1.56, p respectively, while both oximes of furfuraldehyde are transparent between 1 -43 and 1 -62, p.

As to the second point, no consistent differences between the isomers can be discerned in the 2vCH Or 3vCH regions.

Perusal of Table 1, however, shows that, of the five isomeric pairs now examined, the a-oximes have two absorption maxima between 2.0 and 2.1 p end @-oximes only one. Table 2 lists the wavelengths observed. This suggests that, provided an aldoxime is sufficiently soluble in either carbon tetrachloride or carbon disulphide or chloroform, a simple physical indication of configuration may be available.

It was to test this possibility that the spectra of the remaining oximes in Table 1 were recorded. Their behaviours at 2.0-2.1 p are listed in Table 3. All but two thus appear as a-forms. The conclusions regarding piperonaldoxima o-nitro- and o-chlorobenzaldoximes, and anisaldoxime are in agreement with a reversal of earlier allotted configurations (described in the pre-1910 literature, e.g. Beilstein's Handbuch, Vol. VII, VIII, and I X as " anti" forms). Only one form of pyridine-2-aldoxime is on record (Beilstein, Erste ergiinzungswerk,

p. 288) ; we infer that this is also an a-isomer. Thiophen-2-aldoxime, m.p. 133 O C (shown in Beilstein, XVII, p. 285 as " syn "), is evidently a p-form (as is usual for the higher melting member of an aldoxime pair). The single known variety of acetaldoxime likewise, from Table 3, should be a /3-form-an assignment compatible with Kahovec and Kohlrausch's (1942) conclusion that it " sicher nicht in der trams-Form (sylz-Stellung) allein vorlieg ", and with certain chemical studies by Brady and Truszkowski (1924) and Brady and Elein (1926).

A,,. for : Oxime 1 solvenb 1 ,

Benzaldoxime . . CC1, 2.020 2.081 2.052 2.012 2.077 2.048

CHC1, 2.014 2.076 2.049

m-Nitrobenzaldoxime CHC1, 2.016 2.074 p-Nitrobenzaldoxime . . 1 CHCl, 1 2.017 2.074

TABLE 3 ABSORPTION MAXIMA O F SOME SINGLE ISOMERS

2.052 2.048

p-Chlorobenzaldoxime CS, 2.020 2.081 X'urf~~rUIoxim~ . . CS, 1 2.028 2.084

Piperonaldoxime . . o-Nitrobenzaldoxime o-Chlorobenzaldoxime Anisaldoxirne . . Acetaldoxime . . Pyridine-2-aldoxime Thiophen-2-aldoxime

2.052 2.058

CCl, CHCl, CS* (3% CHC1, CHC1, cs,

(c) Origilzs of Absorptiolzs at 2.0-2 1 [J. One may reasonably expect configuration-affected absorptions to be

associated with the bonds C=N, 0 -H, or 0 -H, the fundamental frequencies of which are c. 1670, 2800, and 3650 cm-l respectively ; yet no combination or overtone of these can be convincingly devised to explain features at 4800-5000 om-I (2.1 to 2 -0 p).

, More credibly the observed absorptions may be due to a VOH + SOH combina- tion tone. Several workers have thought the OH deformation made in alcohols to occur near 1340 cm-l because such absorptions " are sensitive to changes in

DISTINCTION BETWEEN ISOMERIC ALDOXMES (319

the hydrogen-bonding pattern " (cf. Bellamy 1958, p. 96). Palm and Werbin (1954) found that both a- and P-aldoximes as solutes in benzene show a common absorption at 1303-1311 cm-l, but that only a-forms exhibit a band a t c. 1265 cm-l ; they note that in different types of compounds evidence of OH bending vibrations has been produced between 1020 and 1420 em-l. Since v o ~ is 3650 om-I, combinations such as :

3650 +I330 =498O cm-l (2 -008 p.) 3650 +I300 =4950 cm-1 (2 -020 p.) 3650 $1158 =GO8 om-1 (2.080 p.)

are sufficient to encompass present observations.

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