What is the bond length of C = C in benzene?
(A) 139 pm
(B) 133 pm
(C) 154 pm
(D) 111 pm
The correct answer and explanation is:
The correct answer is (B) 133 pm.
Benzene (C₆H₆) is a well-known aromatic compound that consists of six carbon atoms arranged in a hexagonal ring with alternating single and double bonds. However, these bonds are not fixed as single or double bonds. Due to the phenomenon of resonance, the electrons in the π bonds are delocalized across the entire ring, meaning the actual bond between each pair of carbon atoms is a hybrid of single and double bonds. This resonance effect leads to bond lengths that are intermediate between a typical single bond and a typical double bond.
In a regular single bond (such as C–C in ethane), the bond length is typically about 154 pm. For a typical double bond (like C=C in ethene), the bond length is shorter, around 134 pm. However, due to resonance in benzene, the bond length of each C–C bond is approximately 133 pm. This value is closer to the bond length of a double bond but not exactly the same, reflecting the shared electron density between the C–C bonds in the benzene ring.
The bond length of 133 pm is indicative of the fact that all six C–C bonds in benzene are equivalent, which is a direct result of the resonance structure that distributes the electron density uniformly across the ring. This gives benzene its unique stability and characteristic properties, such as its resistance to reactions that would normally break a double bond.
Thus, the bond length in benzene represents a compromise between the lengths of single and double C–C bonds due to the resonance effect.