intro to amines and base strength

what is an amine?

compounds based on ammonia where H atoms have been replaced by alkyl or aryl groups

what is an aliphatic amine?

amines with alkyl substituents

what is an aromatic amine?

amines with aryl substituents 

describe the features of an quaternary ammonium salt:

• not an amine!

• +vely charged N atom bonded to 4 alkyl groups 

• has a counter ion (to balance charge)

• formed when lone pair on 3^o amine bonds with a 4th alkyl group

how do we name amines?

• usually use “amine” as a suffix (e.g. propylamine)

• but sometimes use “amino” as a prefix e.g. 1-aminopropane

what is a base?

proton acceptor

are amines strong or weak bases? why?

• weak bases - lone pair on N atom can accept a H⁺

• ability of ammonia/amine to act as a base depends on how well the N’s lone pair can accept a proton

• the higher the e^- density on the N, the more easily it will accept a proton

describe and explain the pattern for base strength in amines/ammonia:

• aromatic amine < NH₃ < 1^o < 2^o < 3^o

• the more alkyl groups, the more available the lp on the N atom is

• so there is a greater +ve inductive effect

why is the base strength of an aromatic amine low?

N lone pair partially delocalised into the benzene ring, decreasing base strength