Vanillin compound obtained from vanilla beans, has total sum of oxygen atoms and π electrons is __________.

Solution

Chemistry ma’am · 3 steps

Draw vanillin first, then count its three oxygens and every electron sitting in a double bond.

1. Structure. Vanillin is 4-hydroxy-3-methoxybenzaldehyde, C₈H₈O₃. A benzene ring carries a -CHO group, an -OCH₃ group next to it, and an -OH group para to the -CHO.

   Part of vanillin	Oxygen atoms	π electrons
   Benzene ring (3 C=C)	0	6
   Aldehyde -CHO (one C=O)	1	2
   Methoxy -OCH₃	1	0
   Hydroxy -OH	1	0

2. Count each. There are 3 oxygen atoms. There are four π bonds in all, three in the ring and one in C=O, so 4 × 2 = 8 π electrons. Lone pairs on oxygen are not π-bond electrons, so they are not counted.
3. So. The sum is 3 + 8 = 11.

The common slip: 9. Counting only the six ring π electrons and forgetting the aldehyde C=O gives 3 + 6 = 9, not 11.

The idea to keep. Each double bond has exactly one π bond, and each π bond holds 2 electrons. Count every C=C and C=O, ring or not, and leave the lone pairs out.

Answer from the NTA final answer key. Solution by PrepWiser.