Question

The correct sequence regarding the basic strength of the amines in aqueous solution is($R\text{is}C{H}_3$):

• A. $R_{3}N>R_{2}NH>RN{H}_2>N{H}_3$
• B. $R_{2}NH>RN{H}_2>R_{3}N>N{H}_3$
• C. $R_{2}NH>R_{3}N>RN{H}_2>N{H}_3$
• D. $RN{H}_2>R_{2}NH>R_{3}N>N{H}_3$

Answer 1

The correct option is B $R_{2}NH>RN{H}_2>R_{3}N>N{H}_3$

In aqueous phase, basic strength of amines depends on:​
1) Electron donating ​effect of alkyl group
Basicity of amines depend upon the number and type of alkyl groups attached to it. As the number of alkyl groups will increase, inductive effect will also increase and the electron density on N-atom will increase, therefore the basicity increases.

2) Steric factor​ (Crowding of alkyl group around N atom​)
Order of steric hindrance: tertiary > secondary > primary

3) Solvation effect
The process in which ions/molecules of solute get stabilized by the interaction with molecules of solvent is known as solvation effect. Stablization of ion is directly propotional to extent of solvation. Order of solvation​ of amines​​: primary > secondary > tertiary.
The basic stregth of amines in aqueous solution is the result of these factors mentioned above(solvation, elctron donation by
$+I$ and steric factors). Consedering these, the order for methylamines in aqueous solution is found as $R_{2}NH>RN{H}_2>R_{3}N>N{H}_3$