Question

(a) explain the basic character of amines in terms of their K_b and pK_b values.

(b) Explain the basic character of amines in gaseous phase.

(c) explain the basic character of amines in aqueous solution.
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Answer 1

Answer for (a) part

The basic character of amines can be understood in terms of their $\large\rm { K_{b}}$ and $\large\rm { pK_{b}}$ values as illustrated below:

$\small\sf { R - NH_{2} + H_{2} O ⇌ R - NH_{3} + OH}$

$\large\sf { K = \frac{ [R- NH_{3}][OH^{-}]}{[R-NH_{2}][H_{2} O]}}$

$\large\sf { K[H_{2} = \frac{ \bigg [ R - NH_{3} \bigg ] \bigg [ OH \bigg ]}{ [ R - NH_{2}]}}$

$\large\sf{ K_{b} = \frac{ \bigg [ R - NH_{3} \bigg ] \bigg [ OH \bigg ]}{ [ R - NH_{2}]}}$

$\large\sf { pK_{b} = -log \ K_{b}}$

Larger is the value of K_b or smaller is the value of pK_b, the stronger is the basic character of amine. Aliphatic amines are stronger bases than ammonia due of +I effect of alkyl group while aromatic amines are weaker bases than ammonia due to electron withdrawing nature of aryl group. here, it should be noted that besides inductive effect, there are other factors also like solvation effect, stearic hindrance etc. which affect the basic strength of amines.

answer for (b) part

The basis character of an amine depends upon the ease with which it forms a cation by accepting a proton from an acid. the more stable is the cation, the more is the basic character.

$\bigg\lgroup\sf$ refer to Attachment {i} $\large\rm \bigg\rgroup$

here the substituted ammonium ion gets stabilised due to dispersal of +ve charge by the +I effect of alkyl group. Hence RNH₂ is stronger base than NH₃. Further, the basic character of aliphatic amines should increase with increase in the number of alkyl groups, but it is so only in gaseous state.

Answer for (c) part

in aqueous solution, the substituted ammonium ion gets stabilised not only by +I effect but also by solvation with water molecules. the greater is the size of the ion, lesser will be the solvation and less is the stability of the ion and the order stability of ions is as follows:

$\bigg\lgroup\sf$ refer to Attachment {ii} $\large\rm \bigg\rgroup$

Greater is the stability of the substituted cation, stronger will be the base. thus the order of basicity of aliphatic amines should be 1°>2°>3°. but steric hinderance due to alkyl groups also ay a significant role to H-bonding. Thus it is combined effect of inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of aliphatic amines in aqueous solution. the order found is as follows:

(CH₃)₂ NH > CH₃NH₂ > (CH₃)₃N > NH₃

(C₂H₅)₂ NH > (C₂H₅)₃ N > C₂H₅NH₂ > NH₃

Answer 2

Answer:

Greater is the stability of the substituted cation, stronger will be the base. thus the order of basicity of aliphatic amines should be 1°>2°>3°. but steric hinderance due to alkyl groups also ay a significant role to H-bonding. Thus it is combined effect of inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of aliphatic amines in aqueous solution. the order found is as follows: