Ans.1.
(a) CHROMOPHORE: The term chromophore was previously used to denote a functional group of some other structural feature of which gives a color to compound. For example- Nitro group is a chromophore because its presence in a compound gives yellow color to the compound. But these days the term chromophore is used in a much broader sense which may be defined as “any group which exhibit absorption of electromagnetic radiation in a visible or ultra-visible region “It may or may not impart any color to the compound. Some of the important chromophores are: ethylene, acetylene, carbonyls, acids, esters and nitrile groups etc.
A carbonyl group is an important chromophore, although the absorption of light by an isolated group does not give rise to any colour in the ultra-violet spectroscopy.
Types of chromophores: Two types of chromophores are known.
1. Chromophores in which the groups have π electrons undergo π-π* transitions. For examples:-ethylenes, acetylenes etc.
2. Chromophores having both π- electrons and n (non-bonding) electrons undergo two types of transitions. i.e., π-π* and n-π*, for examples: - carbonyls, nitriles, azo compounds and nitro compounds etc.
Identification of chromophores:
There is no set rule for the identification of a chromophore. The change in position as well as the intensity of the absorption depends upon a large number of factors. Following points may be useful.
1. Spectrum having a band near 300 mµ may possess two or three conjugated units.
2. Absorption bands near 270-350 mµ with very low intensity ɛmax 10-100 are because of n-π* transitions of carbonyl group.
3. Simple conjugated chromophores likes dienesor,α β –unsaturated ketones have εmax values, i.e., from 10,000 to 20,000.
4. The absorption with ɛmax value between1, 000-10,000 reveals the presence of an aromatic system. If aromatic nucleus is substituted with groups which can extends the chromophore, the absorption take place at still higher value of extinction coefficients.
AUXOCHROMES: It is a group which itself does not act as a chromophore but when attached to a chromophore, it shifts the adsorption towards longer wavelength along with an increase in the intensity of absorption. Some commonly known auxochromic groups are: -OH, -NH2, -OR, -NHR, and –NR2. For example: When the auxochrome –NH2 group is attached to benzene ring. Its absorption change from λ max 225 (ɛmax 203) to λmax 280 (εmax1430)
All auxochromes have one or more non-bonding pairs of electrons. If an auxochromes is attached to a chromophore, it helps is extending the conjugation by sharing of non-bonding pair of electrons as shown below.
CH2 = CH – NR2 ------------- > CH2-CH-NH2
The extended conjugation has been responsible for bathochromic effect of auxochromes.
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