Question

(ii)
In a UV spectrum phenol show absorption band at 210, 270 nm in aqueous solution
but in NaOH solution show at 235 and 287 nm.
​

Answer 1

Answer: This may help you

Explanation:

Anisoles, compared with phenols, possess similar spectral properties, except that properties

determined mainly by the hydrogen atom of the hydroxy group are modified. 111 this way,

comparison of the ultraviolet absorption spectra of phenols and anisoles permits, for example,

the study of the spectral effects of hydrogen bonding and the study of steric interactions

involving the hydroxy and methoxy groups.

INTRODUCTION

In previous parts of this series (1) it mas found that ultraviolet B-band spectra can

often be interpreted by assuiniilg that the B-bands of ultraviolet spectra are determined

predominantly by nlesomeric and steric interactions. Phenolic spectra, which were

examined in the previous paper (la), were found to be of interest since they provide good

examples of secondary interactions such as intramolecular hydrogen bonding. The

phenolic B-band is also of interest, since it is believed to be determined by so-called

locally excited states; that is, in the absence of appreciable direct mesomeric interaction

between the hydroxy group and the benzene ring, the benzene ring is assunled to deter-

mine the observed absorption band with the hydroxy group exerting only a secondary

influence. The anisole spectra provide an evident extension of this investigation, and we

have therefore deterinined, or redetermined, a number of the relevant anisole spectra.

THE SPECTRUM OF AXISOLE

Table I lists the inain absorptioil inaxinla of anisole in the region 210-330 mp.

Anisole in cyclohexane absorbs maxiinally at 220 inp, that is, at longer wavelength

than the corresponding absorption of pheilol at 210 inp (la). We deduce from this that

the methoxy group does not merely exert a "secondary" interaction on the benzene

ring, as has been postulated for the spectrum of phenol, since such a secondary inter-

action would cause a wavelength displaceinent of only ca. 7 mp relative to benzene (see

ref. la and section on metasubstituted anisoles). This in turn suggests that some addi-

tional mesomeric interaction occurs between the inethoxy group and the benzene ring

or, in other words, that the transitions leading to the B-band in anisole involve a s~llaller

proportion of locally excited states compared with the transitions leading to the B-band

in phenol.

The important question arises why locally excited states should preferelltially con-

tribute to this absorption in phenol. One possible explanation is that in phenol the

0-H bond is aligned approximately parallel to the plane of the benzeile ring (Ia), whereas

in anisole, partly for steric reasons, theO-CH3 bond is directed out of the plane of the

benzene ring as in Ib. To satisfy this explanation, it must further be assumed that this

direction of the 0-CH3 bond as in Ib favors the overlap of the unshared electro~zs of

the oxygen atom with the a-electrons of the benzene ring, and that in this way additional

resonance interaction is possible. This explanation is consistent with the observation

that the spectrum of pheliol in ether or ethanol solution, which presuinably involves the

attachment of a solvent molecule to the phenolic hydrogen atom and hence causes an increase in the effective size of the hydroxy group, shows maximal absorption at or near

218 mp (la), na~nely nluch like that of anisole in nlost solvents (see Table I). This there-

fore may imply that under these conditions structures such as Ia, partly for steric reasons,

tend to change into structures of type Ib which facilitate orbital overlap and which tend

to give rise to a rnaxinlal absorption near 218 rnp."