Which of the following statements is incorrect ?

Options

A. The `C-H` bond length in both ethene and ethane is identical.
B. The `C-H` bond in ethane is longer than in ethene.
C. The `C-H` bond in ethene is shorter than in ethane.
D. Both (2) and (3)

Answer

C

Explanation

The `C-H` bond in ethane is shorter and stronger than in ethane because it is formed by the overlap of `sp^(3)` hybrid orbital `(HO)` of `C`, instead of `sp^(3)HO` as in ethane. Compared with an `sp^(3)HO`, and `sp^(2)HO` has less `p` character and more `s` character. A `p` orbital extends some distance form the nucleus while an `s` orbital lies close about the nucleus. Thus, as the `s` character of a hybrid orbital increases, the effective size of the hybrid orbital decreases resulting in corresponding decrease in the length of the bond to a given second atom. Thus, an `sp^(2)-s-C-H` bond is shorter than an `sp^(3)-sC-H` bond. The `C-H` bond length is `1.09Å` in ethene compared with `1.112 Å` in ethane.
Simililarly, the carbon-carbon bond distance in propylene `(1.501Å)` is shorter in comparison with the carbon-carbon bond distance in ethane `(1.534Å)` beacuse an `sp^(2)-sp^(3)` bond has more `s` character than an `sp^(3)-sp^(3)` bond,
Since shorter bond are stronger bonds, the `C-H` bond dissociation energy in ethylene `(108 Kcal)` is larger than energy in propylene `(92Kcal)`, and the `C-C` bond dissociation enrgy in propylene `(92 Kcal)` is greater than that in ethane `(88 Kcal)`