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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?

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?

Grade:12

5 Answers

SAGAR SINGH - IIT DELHI
878 Points
13 years ago

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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Sagar Singh

B.Tech, IIT Delhi

Deblina Roy
14 Points
5 years ago
Due to greater number of Hydrogen bonding with water molecule secondary and primary amines are stable in aqueous medium...but there is more +I effecting alkyl groups present in  secondary amine compared to primary amine... Therefore the basicity order of amines in aqueous medium is secondary amine> Primary amine>tertiary amine> NH3
 
Deblina Roy
14 Points
5 years ago
Due to greater number of Hydrogen bonding with water molecule secondary and primary amines are stable in aqueous medium.But there is more +I effecting alkyl groups present in secondary amine compared to primary amine.Therefore the basicity  order of amines in aqueous medium is secondary amine>Primary amines>tertiary amines>NH3...       By ; Deblina Roy                                                              class 12 studying student          
Mokshagna
31 Points
5 years ago
As also the due to the hydrophobic nature of alkyl group present in the aliphatic amines and the strict hindrance present in tertiary these both combined favour for 2°amines therefore the order in aqueous solution is 2°>1°>3°>nh3 for ch3 and for c2h5 the order is 2°>3°>1°>nh3 and the expected order would be same in the gaseous or non gaseous solution 
Ajeet Tiwari
askIITians Faculty 86 Points
3 years ago
Hello A Shreekhare,
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
Thanks

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