Why in ethene angle between carbon hydrogen bonds is slightly less than angle between carbon hydrogen and carbon carbon Bond?
Why in ethene angle between carbon hydrogen bonds is slightly less than angle between carbon hydrogen and carbon carbon Bond?
Ethene
Hybridisation = Sp2
Shape is planar.
The H-C-H bond angle in ethene is 117° and the H-C-C angle is 121.5°
There are two reasons that combine to explain this angular deformation in ethene.
● First, from these bond angles we can determine that the C-H sigma bonds are (Sp2.2) hybridized and the C-C sigma bond is (Sp1.7) hybridized.
From this hybridization we see that the C-C sigma bond has higher s-character content (1 part s to 1.7 parts p - 37% s) than the C-H bonds (1 part s to 2.2 parts p - 31% s). Since there is more s character in the C-C bond,
it is lower in energy and the carbon sigma electrons will tend to flow towards this lower energy C-C bond. Consequently, the C-C sigma bond will contain more electron density than the C-H bonds. Therefore, the electron repulsion between the C-C sigma bond and C-H sigma bonds will be greater than the electron repulsion between the two C-H bonds. Hence the H-C-C bond angle will open up slightly from the sp2 ideal of 120° and the H-C-H angle will close down slightly in order to minimize the bond-bond electrostatic repulsions.
● Second, steric factors (which are also really just another way of describing electron-electron repulsion) may also come into play. To whatever extent the cis H-C-C-H hydrogen-hydrogen repulsion is more destabilizing than the geminal H-C-H hydrogen-hydrogen repulsion, it will also serve to increase the C-C-H bond angle and shrink the H-C-H bond angle.
Ethene
Hybridisation = Sp2
Shape is planar.
The H-C-H bond angle in ethene is 117° and the H-C-C angle is 121.5°
There are two reasons that combine to explain this angular deformation in ethene.
● First, from these bond angles we can determine that the C-H sigma bonds are (Sp2.2) hybridized and the C-C sigma bond is (Sp1.7) hybridized.
From this hybridization we see that the C-C sigma bond has higher s-character content (1 part s to 1.7 parts p - 37% s) than the C-H bonds (1 part s to 2.2 parts p - 31% s). Since there is more s character in the C-C bond,
it is lower in energy and the carbon sigma electrons will tend to flow towards this lower energy C-C bond. Consequently, the C-C sigma bond will contain more electron density than the C-H bonds. Therefore, the electron repulsion between the C-C sigma bond and C-H sigma bonds will be greater than the electron repulsion between the two C-H bonds. Hence the H-C-C bond angle will open up slightly from the sp2 ideal of 120° and the H-C-H angle will close down slightly in order to minimize the bond-bond electrostatic repulsions.
● Second, steric factors (which are also really just another way of describing electron-electron repulsion) may also come into play. To whatever extent the cis H-C-C-H hydrogen-hydrogen repulsion is more destabilizing than the geminal H-C-H hydrogen-hydrogen repulsion, it will also serve to increase the C-C-H bond angle and shrink the H-C-H bond angle.
