Ch_322b_20.05

Ch_322b_20.05 - Inductive Effect The adjacent polar...

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Amines and Amides The decreased base strength of amides is explained by both resonance and inductive influences, as was discussed with aryl amines. Amides are much less basic than amines even though they appear to be structurally similar. R-NH 2 : amine RC-NH 2 amide O = pK a (conjugate acid) ~9 (aliphatic) pK b ~5 (aliphatic) ~0 ~14
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Resonance Stabilization of Amides Stabilization by ! -electron delocalization is important in amides, but relatively unimportant in their conjugate acids, as illustrated in the resonance structures below. RC-NH 2 : :O: = RC-NH 2 :O: + - RC=NH 2 :O: - + large contribution from resonance stabilization conjugate acid RC-NH 3 :O: + RC-NH 3 :O: + - + little resonance stabilization small contribution Therefore, resonance lowers the energy of the free base state relative to the conjugate acid state leading to a smaller K b in comparison with aliphatic amines.
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Unformatted text preview: Inductive Effect The adjacent polar carbonyl group exerts a strong electron-withdrawing effect that stabilizes the nonbonding electrons on the nitrogen. RC-NH 2 :O: !" ! + Quiz 20.05 While amines are considered organic bases, amides are neutral compounds as illustrated below. RNH 2 + H 2 O RNH 3 + HO-+ when R is alkyl K b 10-5 ~ ~ RCNH 2 + H 2 O RCNH 3 + HO-+ K b 10-14 ~ ~ O = O Which statement below is correct? (A) Amides are weaker bases than amines because of resonance stabilization of the starting state. (B) Amides are weaker bases than amines because of the inductive effect (electron-withdrawing influence) of the carbonyl function that stabilizes the starting state. (C) Amides are weaker bases than amines because of both (A) and (B)....
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_20.05 - Inductive Effect The adjacent polar...

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