Amines

Amines

Question

Give reasons:
(CH3)2NH is more basic than (CH3)3N in an aqueous solution.

Answer

The order of basic strength of aliphatic amines in the aqueous phase is based on the following factors:

Steric factor: Alkyl group is larger than hydrogen atom that causes steric hindrance to attack of acid. As the number of alkyl group increases from primary to tertiary. The steric hindrance also increases in the same direction.

Solvation of ions: when amines are dissolved in water, they form hydrogen bonds with a water molecule and release hydration energy; in the process converting themselves to the protonated amines to get stabilized. Greater the extent of hydrogen bonding, greater is the hydration energy released and more is the stability of the protonated amine; thus greater is the tendency of the amine to form cation leading to the greater basic strength of the amine. So, the actual order of basic strength in the aqueous phase is found to be:

Secondary > Tertiary > Primary

Therefore, secondary amines are the strongest base in aqueous solution.

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