Show the products of the reactions of these carboxylic acids with PBr₃/Br₂ before and after hydrolysis.
(a) pentanoic acid
(b) phenylacetic acid
(c) succinic acid
(d) oxalic acid

Verified step by step guidance

1. Step 1: Understand the reaction mechanism. The reaction involves the conversion of carboxylic acids into α-bromo acids using PBr3 and Br2, followed by hydrolysis. PBr3 activates the carboxylic acid, forming an acyl bromide intermediate, which reacts with Br2 to introduce a bromine atom at the α-carbon.
2. Step 2: Analyze the structure of each carboxylic acid. For (a) pentanoic acid, the α-carbon is adjacent to the carboxylic group. For (b) phenylacetic acid, the α-carbon is next to the benzyl group. For (c) succinic acid, there are two α-carbons due to the dicarboxylic nature. For (d) oxalic acid, there are no α-hydrogens, so bromination does not occur.
3. Step 3: Predict the intermediate products after the reaction with PBr3/Br2. For (a), the α-carbon of pentanoic acid will be brominated, forming α-bromopentanoic acid. For (b), phenylacetic acid will form α-bromophenylacetic acid. For (c), succinic acid will form α,α'-dibromosuccinic acid. For (d), oxalic acid will remain unchanged as it lacks α-hydrogens.
4. Step 4: Consider the hydrolysis step. Hydrolysis converts the acyl bromide intermediate back into the carboxylic acid, retaining the bromine substitution at the α-carbon. For (a), (b), and (c), the brominated carboxylic acids remain intact. For (d), no change occurs.
5. Step 5: Summarize the products. (a) α-bromopentanoic acid, (b) α-bromophenylacetic acid, (c) α,α'-dibromosuccinic acid, and (d) oxalic acid (unchanged).

Key Concepts

Carboxylic Acid Reactivity

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH). They are reactive due to the polar nature of the carbonyl and hydroxyl groups, allowing them to undergo various reactions, including halogenation and esterification. Understanding their reactivity is crucial for predicting the products formed when they react with reagents like PBr3 and Br2.

PBr3 and Halogenation

Phosphorus tribromide (PBr3) is a reagent commonly used to convert alcohols and carboxylic acids into their corresponding bromides. In the presence of Br2, PBr3 facilitates the halogenation of the carboxylic acid, leading to the formation of acyl bromides. This step is essential for understanding the initial products before hydrolysis occurs.

Hydrolysis of Acyl Bromides

Hydrolysis is the reaction of a compound with water, leading to the breakdown of that compound. In the context of acyl bromides formed from carboxylic acids, hydrolysis results in the regeneration of the carboxylic acid and the release of bromide ions. This step is critical for determining the final products of the reaction sequence involving PBr3 and Br2.