Question

If $R=C_2{H}_5,$ the correct order of basic character of ethyl substituted amines in aqueous medium is:

• A. $2^o>3^o>1^o>N{H}_3$
• B. $2^o>1^o>3^o>N{H}_3$
• C. $3^o>2^o>1^o>N{H}_3$
• D. $N{H}_3>1^o>2^o>3^o$

Answer 1

The correct option is A $2^o>3^o>1^o>N{H}_3$

In aqueous phase, the basic strength of the amines is based on three factor.
(i) Electron donating effect of alkyl group:
Here more the number of +I group, more will be the basic strength of amine and this is because +I group will donate the electron density. More the electron density on nitrogen will make them ease to donate the electron. Hence they will be more basic.
Order of basic strength here will be:
$3^{o}amine>2^{o}amine>1^{o}amine>N{H}_3$
(ii) Steric factor :
More the number of bulky groups, more they will hinder the donation of lone pair of electron and lesser tha basic strength.
Order of steric hindrance,
$3^{o}amine>2^{o}amine>1^{o}amine>N{H}_3$
(iii) Solvation effect:
Here the amines are solvated by the solvent$\left(H_{2}O\right)$ molecule. Solvation effect is more in primary amine because they don't have steric groups to hinder and it has more $N-H$ bonds so the solvation is high in primary amine and less in tertiary amine and solvation make the conjugate acid more stable, hence making it more basic.
Order of solvation effect
$1^{o}amine>2^{o}amine>3^{o}amine$
Overall in aqueous phase we have many factor to determine the basic strength of amine. Combining all the three effect, the basic strength of amine follows the order
For $C_2{H}_5$ substituent,
$(C_2{H}_5{)}_{2}NH>(C_2{H}_5{)}_{3}N>C_2{H}_{5}N{H}_2>N{H}_3$