THE INFRARED FREQUENCIES AND INTENSITIES OF THE HYDROXYL GROUP IN 2,6-DI-tert-BUTYL-4-SUBSTITUTED PHENOLS

Abstract

The infrared frequencies, intensities, and apparent half band widths of the 0-H stretching band of a series of 2,6-di-terl-buty1-4-s~1bstituted phe~lols have bcen measured in carbon tetrachloride solution and compared with the corresponding 4-substituted phenols. The freq u e~~c i e s of the former are 36 cm-I higher than the latter and the intensities are also slightly higher. Both the frequency and intensity follow a Harnnlett p u relation. I t is shown that the 0-H bond lies in the plane of the benzene ring, the increase in frequency and intensity being ascribed to a steric repulsion of the phenolic hydrogen by the adjacent tert-butyl g r o~~p . INTRODUCTIONPrevious work on the frequencies and intensities of the hydroxyl group of substituted phenols has dealt mainly with meta-and para-monosubstitution (1, 2, 3, 4), although some work has been done on the effects of orthosubstituents (5) and the frequencies of a few 2,6-di-tert-butyl phenols have been measured, but not with high precision (6, 7).The present work was devoted mainly to the changes in frequency, intensity, and half band width produced by various 4-substituents on the 0-H band in 2,6-di-tert-butyl phenols. The corresponding monosubstituted phenols were also examined. E X P E R I M E N T A LA Becliman I.R.4 spectrophotometer with lithium fluoride optics was used to measure the 0-H bands, the frequencies being calibrated by the standard absorption lines of \\later vapor. The effective slit width was about 4 cm-I. Band areas ivere obtained by graphical integration. Solution concentrations in the range 0.01-0.005 molar were employed and the cell thickness was 1.0 mm. At this concentration even the sterically non-hindered phenols show no evidence of hydrogen bonding in carbon tetrachloride.The preparation of most of the compounds employed has been described previously (8)* but the preparation of 2,6-di-tert-butyl--J--n~ethox).phenol deserves some discussion a s the present data in the literature are sonle\vhat misleading. First attempts to prepare this compound by the butylation of p-methoxyphenol in petroleum ether with isobutylene and sulphuric acid a t 50' C (9) gave, after recrystallization from n-hexane, a GOY0 yield of a product, n1.p. 103.5-104.5" C. Analysis: Found: C, 76.5; H , 10.1; CI-130, 13.4. Calculated for ClSH2402: C, 76.22; H , 10.24; C H 3 0 , 13.13. The melting point agrees with a previously reported value (6), but both the frequency and intensity of the 0-H band were much lower than expected. The required phenol has also been reported by iVIiiller and Ley (7), who prepared it from 4-rnethoxy-2,4,6-tri-tert-butyl-2,5-cyclohexadiene-1-one. Their product melted a t 106-107" and they also obtained an identical compound by the butylation of P-methoxyphenol in 84% yield, but with recrystallization from methanol.An examination of the spectrum of the raw product obtained by butylating p-~neth-oxyphenol showed two bands, the larger corresponcling t o the product already separated (m.p. 103.5-104.5) and the smaller having the frequenc...

The Infrared Frequencies and Intensities of the Hydroxyl Band in Ortho-Substituted Phenols

“…The low frequencies observed must llleail that an o-t-allcyl substituent has an effect on the h~7droxyl group even when the latter is pointing away from it. This effect might be due to a more rigid geometry ill the trans isomer coinpared to phenol (5). An alternative explanation that appears to be more consistent with the results is that t-alliyl groups exert van der Waals attraction on the lone pairs of electro~ls on the oxygen.…”

“…Two parallel straight lines were obtained whose slopes (p) were both -13.7 cm-'. The intensities of the two groups of substituted phenols relative to phenol and to 2,6-di-t-butyl phenol respectively can be represented on a Hammett plot by a single straight line whose slope equals 0.37 (5). The measured values of V, and of the intensity relative to phenol (A) for the phenols studied here have therefore been compared with the values calculated from the Haininett equation to show up any departures from the predicted values.…”

Arrhenius Parameters for Reaction of tert-Butylperoxy Radicals with some Hindered Phenols and Aromatic Amines

“…The 2,6-di-ieri-butyl-4-substituted phenols, a-naphthol, and the aromatic amines were either commercial products 01. samples which had been prepared previously in our laboratory (6,7). They were purified before use by recrystallization, sublimation or zone refining.…”

Rotational Isomerism in O‐Methylphenol: Matrix‐Isolation Infrared Study and MNDO Calculations

“…The results of the present matrrxisolation spectroscopic studies and MNDO calculations on the rotational isomerism in o-methyphenoI can be summarized as follows. (1) The resolution of a weak band from the main hydroxyl band in the stretching and torsional regions clearly indicates the existence of: two stable isomers. (2) The MNDO calculations predict that only two rotational 'isomers can exist with the trans' (stag) conformation being more stable than the cis (stag) conformation by 3.32Kd/mole.…”