Question

The C-H bond distance is longest in:

• A. ${\mathrm{C}}_2{\mathrm{H}}_2$
• B. ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{Br}}_2$
• C. ${\mathrm{C}}_2{\mathrm{H}}_4$
• D. ${\mathrm{C}}_2{\mathrm{H}}_6$

Answer 1

The correct option is D

${\mathrm{C}}_2{\mathrm{H}}_6$

Explanation for correct answer:-

Option (D) ${\mathrm{C}}_2{\mathrm{H}}_6$:

• Bond length is defined as the distance between two atoms that are participating in a bond.
• We can also understand the bond length with the help of the s character.
• Higher the s-character lesser is the bond length.
• This is because higher s character implies that the electrons that are bonded pairs are closely held together, thus making the bond length small as well as strong.
• Hybridisation of ${\mathrm{C}}_2{\mathrm{H}}_6$is ${\mathrm{sp}}^3$
• The s character in ${\mathrm{C}}_2{\mathrm{H}}_6$(Ethane) is: $\frac{1}{4}\times 100=25\%$ which is the least amongst the other given compounds.
• Therefore the $\mathrm{C}-\mathrm{H}$ bond length of ${\mathrm{C}}_2{\mathrm{H}}_6$is the longest.
• Hence it is the correct option.

Explanation for incorrect answer:-

Option (A) ${\mathrm{C}}_2{\mathrm{H}}_2$

• Hybridisation of ${\mathrm{C}}_2{\mathrm{H}}_2$is $\mathrm{sp}$
• The s character in ${\mathrm{C}}_2{\mathrm{H}}_2$(Ethyne) is: $\frac{1}{2}\times 100=50\%$ which is the highest amongst the other given compounds.
• Therefore the $\mathrm{C}-\mathrm{H}$ bond length of ${\mathrm{C}}_2{\mathrm{H}}_6$is the least.
• Hence it is the incorrect option.

Option (B) ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{Br}}_2$

• Hybridisation of ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{Br}}_2$is ${\mathrm{sp}}^2$
• The s character in ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{Br}}_2$(Dibromoethane) is: $\frac{1}{3}\times 100=33.3\%$ which is not the lowest.
• Therefore the $\mathrm{C}-\mathrm{H}$ bond length of ${\mathrm{C}}_2{\mathrm{H}}_6$is not the longest.
• Hence it is the incorrect option.

Option (C) ${\mathrm{C}}_2{\mathrm{H}}_4$

• Hybridisation of ${\mathrm{C}}_2{\mathrm{H}}_4$is ${\mathrm{sp}}^2$
• The s character in ${\mathrm{C}}_2{\mathrm{H}}_4$(Ethene) is: $\frac{1}{3}\times 100=33.3\%$ which is not the lowest.
• Therefore the $\mathrm{C}-\mathrm{H}$ bond length of ${\mathrm{C}}_2{\mathrm{H}}_4$is the least.
• Hence it is the incorrect option.

Therefore, The $\mathrm{C}-\mathrm{H}$ bond distance is the longest in option (D) ${\mathrm{C}}_2{\mathrm{H}}_6$.