Question

Please provide a detailed explaination of how and why stretching frequencies of carbonyl groups are affected by ring size and conjugation...

Answer 1

Solution :-

C=O bond is the strong bond and more polar bond therefore it absorbs at about the 1715 cm-1

But when the electron withdrawing groups are present near to the carbonyl then its frequency can increase to the about 1750 cm-1 because the C=O bond becomes more stronger due to the electron withdrawing effect of the other group or atom.

When the conjugation is present then pi electrons can resonate therefore it decreases the bond strength of the C=O bond which causes to decrease in the carbonyl frequency the conjugated C=O shows absorption peak at 1700 cm-1

When the ring size changes then it have different strain in the rings therefore the carbonyl bond frequency is affected.