Question

Which of the following molecules has the shortest hydrogen-to-carbon bond?

• A. $C_2{H}_4$
• B. $C_2{H}_6$
• C. $C_2{H}_2$
• D. all are same

Answer 1

The correct option is C $C_2{H}_2$

Single bonds adjacent to double and triple bond are progressively shorter and stronger than normal 'single' bond. e.g, alkane. The carbon-carbon bond in ethane

(A) results from the overlap of two ${sp}^3$ orbital, In alkane. (B) However the carbon-carbon single bond is the result of overlap between ${sp}^2$ and an ${sp}^3$ orbital, while in Alkyne (C) the carbon-carbon single bond is the result of overlap between sp and a ${sp}^3$ orbital. These all are a single bond, but the bond in molecule Cis the shorter and stronger than the one in A.

An sp orbital is composed of one orbital and one p orbital thus it has 50% s charachter and 50% p character. ${sp}^2$ orbital in comparison have 33% s character and 67% p charachter., while ${sp}^3$ orbital have 25% s character and 75% p character. because of there spherical shape $2s$ orbitals are smaller, and hold electrons closer and tighter to the nucleus. compared to $2p$ orbitals. consequently bonds involving $sp$ - ${sp}^3$ overlap (as in alkyne C) are shorter stronger than bonds $sp$ - ${sp}^3$ involving overlap (as in alkene B). Bonds involving $sp$ - ${sp}^3$ (as in alkane A) are the longest and weakest of the group, because of the 75% 'p' character of hybrids.