Question

The number of sigma and pi-bonds in 1-butene 3-yne are:
(A) 5 sigma and 5 pi
(B) 7 sigma and 3 pi
(C) 8 sigma and 2 pi
(D) 6 sigma and 4 pi

Answer 1

Hint: Pi Bonds can form in double and triple bonds but do not form in single bonds in most cases. Pi bonds are generally weaker than sigma bonds because they are formed of lateral overlapping, however, sigma bonds are formed by axial overlapping.

Complete step by step answer:
The structure of 1-butene 3-yne is as follows:

Sigma bonds ($\sigma $bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Single bonds are formed by sigma bonds. Axial overlapping of s-s orbitals, and s-p and p-p orbitals results in formation of sigma bonds.
Pi bonds ($\pi $ bonds) are covalent chemical bonds where two lobes of an orbital on one atom overlap two lobes of an orbital on another atom and this overlap occurs laterally.
The compound 1-butene 3-yne has 4 C-H single bonds, that is 4 sigma bonds, and 3 C-C sigma bonds, therefore, there are a total of 7 sigma bonds.
The compound has one double bond and one triple bond. Each double bond has one sigma and one pi bond , and each triple bond has one sigma and two pi bonds, therefore, the compound has 3 pi bonds. So, the correct answer is “Option B”.

Note: Each of these atomic orbitals in $\pi $ bonds has zero electron density at a shared nodal plane, passing through the two bonded nuclei. The same plane is also a nodal plane for the molecular orbital of the pi bond.