Animated Solution for Chemistry - Organic Chemistry: The correct sequential addition of reagents in the preparation of 3-nitrobenzoic acid from benzene is

Imagine you are a molecular architect. Your task is to build 3-nitrobenzoic acid starting from a simple benzene ring. Look at the target: we have a nitro group ($-N{O}_2$) and a carboxylic acid group ($-COOH$), sitting meta to each other. How do we place them perfectly?

To design the synthesis, we must understand the directing nature of these groups. Both the nitro group and the carboxylic acid group are electron-withdrawing, meaning they are meta-directing. If we put the nitro group first, it will direct the next incoming group to the meta position.

Nitration of Benzene

So, let's start with nitration. We treat benzene with a mixture of concentrated nitric acid and sulfuric acid.

This generates the nitronium ion electrophile ($N{O}_2^{+}$), which attacks the ring to form nitrobenzene.

Bromination

Now, we need a carbon source at the meta position. But wait! We cannot perform a Friedel-Crafts reaction on nitrobenzene because the ring is highly deactivated. Instead, we perform halogenation.

Adding bromine with aluminum bromide places a bromine atom exactly at the meta position.

Grignard Formation

With the bromine in place, we have a perfect hook. We react this molecule with magnesium turnings in dry ether.

The magnesium inserts itself into the carbon-bromine bond, creating a highly reactive Grignard reagent.

Carboxylation

Finally, we introduce carbon dioxide. The nucleophilic carbon of the Grignard reagent attacks the electrophilic carbon of $C{O}_2$.

Following this with an acidic workup, the intermediate is protonated, yielding our target: 3-nitrobenzoic acid.

Looking at the complete sequence, we successfully navigated the directing effects and reactivity constraints. The correct order of reagents is nitration, followed by bromination, Grignard formation, and finally carboxylation. This matches option (d) perfectly.