The Visible Spectrum and Color

An auxochrome is a substituent that, when attached to a chromophore, alters both the Amax aiid the intensity of the absorption, usually increasing both. For example, OH and NH2 groups are auxochromes. The lone-pair electrons on oxygen and nitrogen in the compounds shown here are available to interact with the tt electron cloud of the benzene ring such an interaction increases Amax- 

Removing a proton from phenol increases the Amax because the phenolate ion has an additional lone pair. Protonating aniline decreases the Amax because the lone pair is no longer available to interact with the tt cloud of the benzene ring. Because the anilininm ion does not have an auxochrome, its Amax is similar to that of benzene. 

The relationship between the wavelengths of the light that a substance absorbs and the substance s observed color is shown in Table 14.7. Notice that two absorption bands 

Chlorophyll a and b are highly conjugated compounds that absorb visible light, causing green light to be reflected from the surface tissues of plants. 

When margarine was first produced, it was colored with butter yellow to make it look more like butter. (White margarine would not be very appetizing.) This dye was abandoned after it was found to be carcinogenic. j8-Carotene (page 635) is now used to color margarine. Methyl orange is a commonly used acid-base indicator (see Problem 71). 

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