Question

The correct order of $+I$ strength for the following groups
$-C{H}_3,-C{H}_{2}R,-C{D}_3$ is :
(Here $R$ is any alkyl group ).

• A. $-C{H}_{2}R>-C{H}_3>-C{D}_3$
• B. $-C{H}_3>-C{D}_3>-C{H}_{2}R$
• C. $-C{D}_3>-C{H}_{2}R>-C{H}_3$
• D. $-C{H}_{2}R>-C{D}_3>C{H}_3$

Answer 1

The correct option is D $-C{H}_{2}R>-C{D}_3>C{H}_3$

Since alkyl group is an electron donating group :
More the number of alkyl groups attached more is the $+I$ strength.
So, $-C{H}_{2}R$ has the highest $+I$ strength.
In case of isotopes, heavier isotopes show more $+I$ effect.
Also, bond length of $C-D$ is less than $C-H$ bond length.
So electron density becomes higher due to shorter bond length.
$\therefore +I$ effect is more for $-C{D}_3$
The correct order of decreasing $+I$ strength is :
$-C{H}_{2}R>-C{D}_3>C{H}_3$