Question

Bond length of ethane (I), ethene (II), acetylene (III) and benzene (IV) follows the order 

(a) I > II > III > IV 

(b) I > II > IV > III 

(c) I > IV > II > III 

(d) III > IV> II > I

Answer 1

(c) I > IV > II > III 

The trend for bond order is opposite to bond length.

Let's find the trend for bond length first.

Consider the two points:

Bond length of a single bond (alkane) is more than that of a double bond (alkene).

Alkene bonds feature one sigma and one pi bond. Benzene features one sigma and one pi bond as well. However, the pi bonds are delocalized all around the ring in the case of benzene, so the pi electron density is distributed around the ring. Thus, the C-C in Alkenes are shorter because the pi electrons are not distributed over several atoms.

So the increasing order of bond length is,

Ethane < Benzene < Ethene < Acetylene

And finally, the bond length in increasing order would be,

acetylene(120pm) < Ethene(133pm) < Benzene(139pm) < Ethane(154pm)