Question

The correct order of bond lengths for $\mathrm{C}-\mathrm{C}$ bonds in benzene, ethyne, ethene and ethane is

Answer 1

Bond length:

1. It is the distance between the centres of two covalently bound atoms.
2. Bond order is inversely proportional to bond length, as we all know. That is to say, the shorter the bond length, the higher the bond order.
3. The number of electrons participating in bonds between two atoms in a molecule is measured by bond order. Bond order is usually equal to the number of bonds between two atoms.
4. Here, ethane has a single bond, ethyne has a triple bond, and benzene has a double bond.
5. As ethyne has bond order $3$, it is the compound with the shortest bond length.
6. Because it possesses bond order $2$, benzene has a longer bond length than ethyne.
7. Ethane, which has bond order $1$, has the longest bond length.
8. In the case of benzene, however, the pi bonds are delocalized all over the ring, therefore the pi-electron density is scattered all over the ring hence alkenes have a shorter bond length than benzene.
9. Hence, the correct order is Ethyne$<$Ethene$<$Benzene$<$Ethane.

Therefore, the correct order of bond lengths for $\mathrm{C}-\mathrm{C}$ bonds in benzene, ethyne, ethene, and ethane are Ethyne$<$Ethene$<$Benzene$<$Ethane.