CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
Amides such as benzamide, C6H5CONH2, exhibit remarkable planarity around the amide C–N bond, and the lone pair on the amide nitrogen is far less basic than the lone pair on an amine nitrogen.
Consider only the CONH2 fragment (ignore the benzene ring) in answering the following questions.
(a) Draw the two most significant resonance contributors for the CONH2 fragment, clearly indicating all lone pairs and formal charges. Use curved arrows to connect the contributors.
(b) Justify why additional resonance contributors with a positive charge on oxygen make negligible contributions to the resonance hybrid.
(c) Using your resonance structures, explain why the C–N bond in an amide is shorter than a typical C–N single bond yet longer than a C=N double bond. Include a discussion of partial bond orders.
(d) Provide a detailed explanation, grounded in resonance, for why the basicity of the amide nitrogen is significantly lower than that of an amine nitrogen.