Question

Propose a synthesis for 3,5 -dibromo-2-hydroxybenzoic acid (3,5-dibromosalicylic acid) from phenol.

Short answer

Question: Propose a synthesis for 3,5-dibromo-2-hydroxybenzoic acid (3,5-dibromosalicylic acid) starting from phenol. Answer: The synthesis of 3,5-dibromo-2-hydroxybenzoic acid can be achieved through the following steps: 1. Bromination of phenol with Br2 in acetic acid to form 2,4,6-tribromophenol. 2. Hydroxylation of 2,4,6-tribromophenol with an aqueous solution of NaOH to form 3,5-dibromo-2-hydroxyphenol. 3. Oxidation of 3,5-dibromo-2-hydroxyphenol using KMnO4 in an aqueous alkaline solution to form 3,5-dibromo-2-hydroxybenzoic acid (3,5-dibromosalicylic acid).

Step-by-step solution

Bromination of Phenol

In the first step, we will introduce the bromine atoms at positions 3 and 5 of the phenol ring. To achieve this, we will brominate phenol by treating it with bromine (Br2) in the presence of acetic acid (CH3COOH) as a solvent. The bromination of phenol proceeds through electrophilic aromatic substitution, giving the 2,4,6-tribromophenol as the major product. The reaction proceeds as follows: Phenol + Br2 (in AcOH) → 2,4,6-Tribromophenol

Hydroxylation of 2,4,6-Tribromophenol

Now, we need to remove one bromine atom from the position 2 of the 2,4,6-tribromophenol to restore the hydroxyl group. This can be done by treating the 2,4,6-tribromophenol with an aqueous solution of sodium hydroxide (NaOH) followed by hydrolysis with water. This results in the formation of 3,5-dibromo-2-hydroxyphenol (3,5-dibromophenol). The reaction proceeds as follows: 2,4,6-Tribromophenol + NaOH (aq) → 3,5-Dibromo-2-hydroxyphenol + H2O

Oxidation of 3,5-Dibromo-2-hydroxyphenol to form 3,5-Dibromo-2-hydroxybenzoic acid

Finally, we need to convert the hydroxyl group at position 2 into a carboxylic acid group. This can be done by oxidizing the 3,5-dibromo-2-hydroxyphenol using an oxidizing agent, such as potassium permanganate (KMnO4) in an aqueous alkaline solution. The oxidation of the phenol to a carboxylic acid is an important transformation that will give us the desired product, 3,5-dibromo-2-hydroxybenzoic acid (3,5-dibromosalicylic acid). The reaction proceeds as follows: 3,5-Dibromo-2-hydroxyphenol + KMnO4 (in aq. alkaline solution) → 3,5-Dibromo-2-hydroxybenzoic acid In conclusion, the synthesis of 3,5-dibromo-2-hydroxybenzoic acid (3,5-dibromosalicylic acid) can be achieved through a sequential bromination, hydroxylation, and oxidation of phenol, with proper selection of reagents for each step.