Animated Solution for Chemistry - Organic Chemistry: For the reaction given below. The compound which is not formed as a product in the reaction is a

The problem presents us with an interesting organic transformation. At first glance, the reactant might seem complex, but breaking it down into its functional groups reveals the elegant chemistry at play.

Let's look closely at the reactant provided in the question. We have a benzene ring substituted with two functional groups: an aldehyde group ($-CHO$) at the top and a hydroxymethyl group ($-C{H}_{2}OH$) at the bottom.

The most critical observation here is the nature of the carbon atom adjacent to the aldehyde group. Because the aldehyde is directly attached to the benzene ring, the adjacent carbon (the $\alpha$-carbon) is part of the aromatic system and already has four bonds. Therefore, there are no $\alpha$-hydrogens present.

Cannizzaro Reaction

When an aldehyde lacking $\alpha$-hydrogens is treated with a concentrated base like sodium hydroxide ($NaOH$) and heated, it cannot undergo an Aldol condensation. Instead, it undergoes a classic disproportionation reaction known as the Cannizzaro reaction.

In a disproportionation reaction, two molecules of the same reactant participate simultaneously. One molecule is oxidized, while the other is reduced.

Let's apply this to our specific reactant. Two molecules of $p$-(hydroxymethyl)benzaldehyde will react.

First, let's consider the reduction half of the reaction. The aldehyde group of one molecule gains hydrogen atoms and is reduced to a primary alcohol group ($-C{H}_{2}OH$). This transforms the molecule into 1,4-benzenedimethanol. Notice that this product contains two alcohol groups, making it a diol.

Next, let's look at the oxidation half. The aldehyde group of the second molecule loses a hydrogen and gains an oxygen, initially forming a carboxylate salt. The second step of the reaction involves acidification with $H_3{O}^{+}$, which protonates the salt to form a carboxylic acid group ($-COOH$). This yields 4-(hydroxymethyl)benzoic acid. This product is a monocarboxylic acid and notably contains both an acid and an alcohol functional group.

Now that we have identified our two products, let's evaluate the given options to see which compound is not formed.

- Option (a): A compound with both alcohol and acid functional groups. This perfectly describes our oxidized product, 4-(hydroxymethyl)benzoic acid. - Option (b): A monocarboxylic acid. Again, this describes our oxidized product. - Option (d): A diol. This describes our reduced product, 1,4-benzenedimethanol.

This leaves us with Option (c): a dicarboxylic acid. Neither of our products contains two carboxylic acid groups. The original hydroxymethyl group ($-C{H}_{2}OH$) remains unaffected by the Cannizzaro conditions. Therefore, a dicarboxylic acid is not formed in this reaction.