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order of basicity in aqueous medium in methylamines is secondary>primary>tertiary.But in the case of ethylamines it is secondary>tertiary>primary.why?explain?

6 years ago


Answers : (1)


Dear student,

When we compare the basicity of the aliphatic amines, we would expect the basicity of tertiary amines to be the greatest, followed by secondary amine and then primary amine.

But this is not so. The order of basicity is:

NH3< primary amine ~ tertiary amine < secondary amine

This “unexpected” order of basicity can be understood from the below stated reasons:

a)    Steric hindrance:

The size of an alkyl group is more than that of a hydrogen atom. So, an alkyl group would hinder the attack of a hydrogen atom, thus decreasing the basicity of the molecule. So, the more the number of alkyl groups attached, lesser will be its basicity.

So, the decreasing order of basicity will be:

Primary amine > secondary amine > tertiary amine

b)    Solvation of ions:

When amines are dissolved in water, they form protonated amines. Also, the number of possibilities of hydrogen bonding also increases. More the number of hydrogen bonding, more is the hydration that is released in the process of the formation of hydrogen bonds.

The more the hydration energy of the molecule, more is the stability of the amine. Experiments have indicated, in terms of hydration energy, that the tertiary amines are the least stable followed by the secondary amines followed by the primary amines.

This can be explained on the basis of the following reactions:

In primary,

RNH2 +H2O → RNH3 + + OH-

In secondary,
R2NH +H2O  → R2NH2+ + OH-
In tertiary,
R3N + H2O  → R2NH+ + OH-

So, the number of hydrogen bonds possible when primary amines are dissolved in water is the greatest, implying that they are most stable species of amine, the least being the tertiary amines.

The combined effect of the pushing effect of the alkyl group (+I effect), steric hindrance and the salvation of amines causes the basicity order to be: (basicity of tertiary is almost the same as that of primary).

NH3< primary amine ~ tertiary amine < secondary amine


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B.Tech, IIT Delhi

5 years ago

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