Discussion
Amongst the following, the compound that will show IR absorption band at 1780 cm-1 is :
- A.
- B.
- C.
- D.
Answer: D
The compound absorbing at 1780 cm-1 is (D), cyclobutanone.
Ring strain raises the C=O stretching frequency of cyclic ketones, because squeezing the ring angle gives the C-C bonds more p character and the C=O bond more s character, stiffening it. Cyclohexanone absorbs near 1715 cm-1, cyclopentanone (A) near 1745 cm-1 and cyclobutanone near 1780 cm-1.
Amides absorb lower than ketones because nitrogen's lone pair gives the C=O partial single-bond character. The six-membered lactam (B) absorbs near 1660 cm-1, and even the strained four-membered beta-lactam (C) comes near 1745 cm-1, so of the four only cyclobutanone reaches 1780 cm-1.
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