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Grade: 12
Is the order of the basicity of methyl-substituted amines and ethyl-substituted amines change the 'same' in an 'AQUEOUS' and 'NON-AQUEOUS' medium?
7 months ago

Answers : (2)

24735 Points

The ammonium cation (protonated form) of aliphatic amine is solvation-stabilised by H2O molecules via Hydrogen-bonding in aqueous solution.

Ammonium cation of primary amine (RNH3+) is maximum solvated of all types (primary, secondary & tertiary) due to greater number of hydrogens directly attached to nitrogen and hence their hydrogen bonding with oxygen of water(solvent) molecules.

In total, three effects are most useful to decide basic character of simple aliphatic amines in aqueous solution. (a) Inductive effect – which favors tertiary amine to be more basic due to greater + I effect of three alkyl groups directly attached to nitrogen (b) Solvation effect- which favor primary amine to be more basic due to greater solvation of its cation. (c) Steric factor – which causes tertiary amine to be less basic due to steric hindrance to approach of proton towards nitrogen.

As a combined result of above three factors, in aq solution, the order of basic character (pKb values in brackets) in isomeric aliphatic amines are different in different cases-

  1. (CH3)2NH [3.27] > CH3NH2 [3.38] > (CH3)3N [4.22] > NH3 [4.75]
  2. (C2H5)2NH [3.0] > (C2H5)3N [3.25] > C2H5NH2 [3.29] > NH3 [4.75]
  3. Me2CH-NH2 > NH3 > (Me2CH)2NH > (Me2CH)3N
  4. NH3 > Me3C-NH2 > (Me3C)2NH > (Me3C)3N
7 months ago
Vikas TU
11678 Points
Dear student
Please find the solution to the asked query:
the compounds , NH3, CH3NH2, (CH3)2NH , (CH3)3N
when considered on the basis of inductive effect and steric hindrance can be arranged in the increasing order as follows:
Ammonia shows no steric hindrance and no inductive effect, so:
the increasing order is :
(CH3)2NH > CH3NH2> (CH3)3N> NH3
In (CH3)2NH two alkyl groups increase the e- density. This is decreased in (CH3)3NH. However  due to more steric hindrance in (CH3)3N the order of basicity gets reversed , thus (CH3)2NH >  (CH3)3N
Hope this information will clear your doubts regarding the topic. If you have any other doubts please ask here on the forum and our experts will try to solve them as soon as possible.
7 months ago
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