Question

Which of the following is the correct order of bond length?

• A. \( \text{C–H}<\text{C}\equiv\text{N}<\text{C}=\text{O}<\text{C–O} \)
• B. \( \text{C}\equiv\text{N}<\text{C–H}<\text{C–O}<\text{C}=\text{O} \)
• C. \( \text{C–H}<\text{C}\equiv\text{N}<\text{C–O}<\text{C}=\text{O} \)
• D. \( \text{C–O}<\text{C}\equiv\text{N}<\text{C}=\text{O}<\text{C–H} \)

Hint:

For bond length comparison, first check \textbf{bond order}, then \textbf{atomic size}. Small atoms like hydrogen can form extremely short bonds even if the bond is single.

Answer 1

The Correct Option is A

Concept: Bond length is defined as the average distance between the nuclei of two bonded atoms. It mainly depends on:
Bond order: As bond order increases, bond length decreases.
Atomic size: Smaller atoms form shorter bonds.
Type of bond: Triple bonds are shorter than double bonds, which are shorter than single bonds. Thus, the general trend is: \[ \text{Triple bond}<\text{Double bond}<\text{Single bond} \]
Step 1: Analyze each bond individually.
\(C–H\): This is a single bond, but hydrogen is the smallest atom. Due to its very small atomic radius, the C–H bond has a very short bond length (shorter than many multiple bonds involving larger atoms).
\(C\equivN\): This is a triple bond between carbon and nitrogen. Triple bonds have the highest bond order (3), resulting in very strong attraction between nuclei and hence a short bond length.
\(C=O\): This is a double bond. Its bond order is 2, so its bond length is longer than a triple bond but shorter than a single bond.
\(C–O\): This is a single bond with bond order 1. Single bonds have the least overlap between orbitals, making them the longest among the given bonds.
Step 2: Compare all bonds based on bond order and atomic size.
Shortest bond: \(\text{C–H}\) (small hydrogen atom)
Next: \(\text{C}\equiv\text{N}\) (triple bond)
Next: \(\text{C}=\text{O}\) (double bond)
Longest: \(\text{C–O}\) (single bond)
Step 3: Arrange in increasing order of bond length: \[ \text{C–H}<\text{C}\equiv\text{N}<\text{C}=\text{O}<\text{C–O} \] This order exactly matches Option (A).