Question

Which of the following molecule has longest $C=C$ bond length?

• A. $C{H}_2=C=C{H}_2$
• B. $C{H}_3-CH=C{H}_2$
• C. $CH-\underset{C{H}_3}{\underset{|}{\stackrel{C{H}_3}{\stackrel{|}{C}}}-}CH=C{H}_2$
• D. $C{H}_3-\underset{C{H}_3}{\underset{|}{C}=}C{H}_2$

Answer 1

The correct option is C $CH-\underset{C{H}_3}{\underset{|}{\stackrel{C{H}_3}{\stackrel{|}{C}}}-}CH=C{H}_2$

Larger the positive inductive effect of the neighbouring atoms, higher will be the electron density around the $C=C$ bond and hence higher the repulsion between the atoms sharing the double bond.

This will give rise to a longer $C=C$ bond.

Among the given alternatives, option $C$ will have the longest $C=C$ bond length due to the larger positive inductive effect offered by the three neighbouring methyl groups.

Hence, the correct option is C.