C-x bond in haloalkenes is shorter than C-X bond in haloalkanes. Why?
The C-X bond in haloalkenes is shorter than haloalkanes.
This can be understood in two ways.
1. In case of haloalkane, the carbon attached to halogen atom is sp3 hybridised where as in case of haloalkenes, the carbon attached to halogen atom is sp2 hydridised. In sp2 hybridisation s character is 33 % whereas in case of sp3 hybridisation, s character is 25 %.
We know higher is the s character higher will be the electronegativity, therefore the carbon having sp2 hybridisation is more electronegative and hence the bond pair of electron would be more closer to carbon, which results in the shortening of the C-Cl bond.
2. In case of haloalkene, the lone pair of halogen atom participate in resonance with the conjugated double bond. It leads to development of a partial double bond character in the C-X bond, thereby decreasing the C-X bond length in haloalkene.
This can be understood in two ways.
1. In case of haloalkane, the carbon attached to halogen atom is sp3 hybridised where as in case of haloalkenes, the carbon attached to halogen atom is sp2 hydridised. In sp2 hybridisation s character is 33 % whereas in case of sp3 hybridisation, s character is 25 %.
We know higher is the s character higher will be the electronegativity, therefore the carbon having sp2 hybridisation is more electronegative and hence the bond pair of electron would be more closer to carbon, which results in the shortening of the C-Cl bond.
2. In case of haloalkene, the lone pair of halogen atom participate in resonance with the conjugated double bond. It leads to development of a partial double bond character in the C-X bond, thereby decreasing the C-X bond length in haloalkene.