Question

Explain the bond formation in ethylene and acetylene.

Answer 1

Bonding in Ethylene, C₂H₄

1. Bonding in ethylene can he explained by hybridisation concept.

2. The valency of carbon is 4. The electronic configuration of carbon is 1s² 2s² 2p_x¹ 2p_y¹ 2p_z⁰ . One electron from 2s orbital is promoted to 2p . orbital in the excited state to satisfy the valency of carbon.

3. In ethylene both the carbon atoms undergo sp² hybridisation involving 2s, 2p_x and sp_y orbitals resulting in 3 equivalent sp² hybridised orbitals lying in the XY plane at an angle of 120º to each other. The unhybridised 2p_z orbital lies perpendicular to the xy plane.

4. One of the sp² hybndised orbitals of each carbon atoms lying along the X – axis linearly overlaps with each other resulting in the formation of C – C sigma bond. The other two sp² hybridised orbitals of both carbon atom linearly overlap with the four is orbitals of four hydrogen atoms leading to the formation of two C – H sigma bonds on each carbon atom.

5. The unhybridised 2p_z orbital of both carbon atoms can overlap only sideways as they are not in the molecular axis. This lateral overlap results in the formation of a pi bond between the two carbon atoms.

Bonding in acetylene (C₂H₂):

1. The electronic configuration of valence shell of carbon atom in the ground state is [He] 2s² 2p_x¹ 2p_z⁰ . One electron from 2s orbital is promoted to 2p_z orbital in the excited state to satisfy the valency of carbon.

2. In acetylene molecule, both the carbon atoms are in sp hybridised state. The 2s and 2p_x orbitals resulting in two equivalent sp hybridised orbitals are formed lying in a straight line along the X – axis. The unhybridised 2p_y , and 2p_z orbitais lie perpendicular to the X-axis.

3. One of the two sp hybridised orbitals of each carbon atom linearly overlaps with each other resulting in the formation of a C – C sigma bond. The other sp hybridised orbital of both carbon atoms linearly overlap with the two is orbitals of two hydrogen atoms leading to the formation of one C – H sigma bond on each carbon atom.

4. The unhybridised 2p_y and 2p_z orbitals of each carbon atom overlap sideways. This lateral overlap results in the formation of two pi bonds. (p_y – p_y ) and (p_z – p_z ) between the two carbon atoms.