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I learnt that basic character is directly proportional to the positive inductive effect and inversely proportional to steric hindrance

Based on the above mentioned properties how is $\ce{(CH3)2NH}$ is more basic than $\ce{CH3NH2}$ while $\ce{(CH3)3N}$ is less basic than the secondary amine $\ce{(CH3)2NH}$?

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closed as off-topic by jerepierre, Asker123, user15489, Freddy, Jun-Goo Kwak Apr 26 '15 at 4:22

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    $\begingroup$ Welcome to Chemistry SE! This looks like a homework question, please show your work into this, so that we are not doing the homework for you. :) $\endgroup$ – Asker123 Apr 25 '15 at 21:22
  • $\begingroup$ Actually to convey it to you..this is a MCQ question that i came across in a competitive exam :) the answer as i have mentioned above to my knowledge is (CH3)2NH i just don't understand as to why it has to be the secondary amine that is more basic than the tertiary amine or primary amine :) $\endgroup$ – user33031 Apr 25 '15 at 21:24
  • $\begingroup$ @Asker123 I believe he did show sufficient work into it. He explained what he knows and if he didn't know anything about the solvation effect there is not much more he can say. I feel like this should be re-opened as it is a very valid question and one I have been wondering about myself $\endgroup$ – sab hoque Apr 29 at 4:44
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what we expect here would be the tertiary amine to have the greatest basicity and the least would be the primary amine.

But through experiments it has proven that secondary amines to have the greatest basicity. the order of basicity is as follows.

NH3 < primary amine < secondary amine

The basicity of the tertiary amine is approx. equal to that of primary amine.

The reasons behind this is:

  1. Steric hindrance

  2. Solvation of ions

3 + inductive effect

When the alkyl group attached increases the basicity reduces. It is because the size of the alkyl group is so large that it prevents an attack from Hydrogen ion. So according to this theory decreasing order would be

Primary amine > Secondary amine > Tertiary amine

But there's another thing you've got to consider. That is the solvation of ions. Amines dissolve in water to form protonated amines thus increasing the number of possibilities of hydrogen bonding. More the H bondings, greater the dissolution in water. Higher the H bonding, greater is their stability and greater is their basicity

Experiments have indicated that primary amines are the ones who have more H bonds and tertiary amines are the least.

So by combining all those reasons scientists have shown that basicity of amines vary as follows.

NH3 < primary amine ~ tertiary amine < secondary amine

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