In a C≡C triple bond, how many sigma and pi bonds are present?

• A. 1 sigma, 2 pi
• B. 2 sigma, 2 pi
• C. 1 sigma, 1 pi
• D. 0 sigma, 3 pi

Answer: (A)

In a C≡C triple bond, there is one sigma bond and two pi bonds. This arises because each carbon uses sp hybridization to form the sigma bond along the line between the two nuclei. The remaining unhybridized p orbitals on each carbon (two perpendicular p orbitals) overlap sideways to form two pi bonds. So the triple bond consists of a single sigma bond plus two pi bonds, giving a bond order of 3. The sigma bond provides the main head-on connection, while the two pi bonds add additional bonding interaction but are weaker and require the p orbitals to remain properly aligned, which also restricts rotation about the bond.