Discussion
The correct order for stretching frequency of carbonyl group of the following molecules is :
I. CH3-CO-O-CH=CH2
II. CH3-CO-CH2CH3
III. CH2=CH-CO-OCH3
Answer: D
The order is I > III > II: vinyl acetate > methyl acrylate > butanone.
A ketone such as butanone (II) absorbs near 1715 cm-1. In an ester the alkoxy oxygen withdraws electrons inductively, which strengthens the C=O bond, so saturated esters absorb near 1740 cm-1.
Vinyl acetate (I) is an enol ester: the oxygen's lone pair is shared with the C=C bond, so it donates less to the carbonyl, and the C=O frequency rises to about 1770 cm-1. Methyl acrylate (III) is an alpha,beta-unsaturated ester: conjugation of C=C with C=O lowers its frequency to about 1725 cm-1, still above the ketone. So I > III > II, and the orders placing the ketone first or the acrylate above the enol ester are wrong.
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