Question

The correct order of increasing $+I$ effect is :

• A. $-C{H}_3<-H<-D<-T$
• B. $-C{H}_3<-H<-T<-D$
• C. $-H<-C{H}_3<-T<-D$
• D. $-H<-D<-T<-C{H}_3$

Answer 1

The correct option is D $-H<-D<-T<-C{H}_3$

$-C{H}_3$ group has the highest tendency to donate electrons, so it has the highest $+I$ effect.
In case of isoptopes, heavier isotopes have more $+I$ effect.
Comparing $C-H$ and $C-D$ bonds:
A shorter bond i.e. $C-D$ bond has less volume to spread the electron density, resulting in a higher electron density throughout the bond Consequently, more is the electron density at carbon end of the bond and hence $-D$ has more $+I$ effect.

Similar arguments can be applied to tritium $\left(T\right)$ Since, $C-T$ bond is the shortest, it should be even more inductively electron donating towards carbon than deuterium.

So, the correct order is :
$-H<-D<-T<-C{H}_3$