Question Video: Calculating the Number of 𝜎 and 𝜋 Bonds in 1,3-pentadiene Chemistry • Second Year of Secondary School

Video Transcript

How many 𝜎 bonds and 𝜋 bonds are in 1,3-pentadiene, H2C double bond CH single bond CH double bond CH single bond CH3? (A) 12 𝜎 bonds and two 𝜋 bonds, (B) 10 𝜎 bonds and two 𝜋 bonds, (C) two 𝜎 bonds and 12 𝜋 bonds, or (D) two 𝜎 bonds and 10 𝜋 bonds.

𝜎 bonds are covalent bonds that are formed through the head-on or end-to-end overlap of different types of hybridized or unhybridized bonding orbitals. 𝜋 bonds are a type of covalent bond that are formed through the sideways overlap of two adjacent p-subshell orbitals.

Let’s look at a molecule of ethene to see this more clearly. In a carbon–carbon double bond, the carbon atoms will be sp hybridized. Each carbon atom will have three sp orbitals. Two of the orbitals overlap to form a carbon–carbon 𝜎 bond. Four other 𝜎 bonds form between the carbon atoms and hydrogen atoms. A 𝜋 bond forms between the carbon 2p orbitals. It is now clear that a single bond is a 𝜎 bond and a double bond contains a 𝜎 and 𝜋 bond.

Let’s now redraw 1,3-pentadiene. To make it easier to determine the amount of 𝜎 bonds and 𝜋 bonds in 1,3-pentadiene, we can say that all the carbon–carbon single bonds and carbon–hydrogen bonds are 𝜎 bonds. The carbon–carbon double bonds are made up of one 𝜎 bond and one 𝜋 bond. It is clear that there are two 𝜋 bonds in total. Therefore, neither option (C) or (D) can be correct. If we count up the number of 𝜎 bonds, we find that there are 12.

Therefore, the number of 𝜎 and 𝜋 bonds in 1,3-pentadiene is (A), 12 𝜎 bonds and two 𝜋 bonds.