is a primary amine or quartanery amine more acidic?

[kfcman289]

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Is a primary or quartanary amine more acidic? I know that the N has a positive charge in a quartanary, but it doesn't really have any hydrogens to give?

 

[kfcman289]

bump

 

[Czarcasm]

It depends on what type of amine you're referring to. If it's entirely alkylated, the only Hydrogen available for deprotonation is one of the 4 methyl groups, which is barely acidic at all. If it's a hydrogen directly bound to the nitrogen, it's considerably more acidic. Also keep in mind that alkyl groups are electron donating, which means, the central atom has less 'desire' for the lone pair when a proton dissociates. So this would mean that a ammonium salt NH4Cl is more acidic than a primary amine: NH3(CH3)+, a secondary amine: NH2(CH3)2+, a tertiary amine: NH(CH3)3+, and a quaternary amine: N(CH3)4+. If you recall, NH4+ groups of aminoacids have a pKa of around 9-11, which makes it even more acidic than water and alcohols.

 

[DrknoSDN]

Tetramethylammonium has a pKa of 42 according to (link). About the only thing that it could lose a hydrogen to is a methyl cation.
Without implicating strange R groups, a tertiary amine is the weakest acid of the 4 types of amines because like Czarcasm said, it would need to give up a hydrogen from one of the methyl R groups.

Normally questions compare primary secondary and tertiary because they have the same charge on nitrogen and follow the trend mentioned about electron donating alkyl groups.

 

[kfcman289]

Thanks for your replies. My question comes from an Exam Krackers question:

Which amine is the least basic:
a) primary
b)secondary
C) tertiary
d) quartanary

the answer was d. can anyone explain why?

 

[DrknoSDN]

Least basic is easier to answer here because a quarternary amine has almost no ability to accept a proton.
Answers A-C have a lone pair that can accept a hydrogen atom while D does not.