Aldehydes Ketones And Carboxylic Acids

The given conversion involves the bromination of propanoic acid $CH_3CH_2COOH$ to 2-bromopropanoic acid $CH_3CHBrCOOH$ . The bromine atom is introduced at the $%5Calpha$ -carbon (the carbon adjacent to the carboxyl group). This type of $%5Calpha$ -halogenation of carboxylic acids is known as the Hell-Volhard-Zelinsky (HVZ) reaction.
The Hell-Volhard-Zelinsky reaction proceeds in two steps:
Step 1: The carboxylic acid reacts with bromine ( $Br_2$ ) in the presence of red phosphorus (P red.). The red phosphorus reacts with bromine to form phosphorus tribromide ( $PBr_3$ ), which then converts the carboxylic acid to an acyl bromide. The acyl bromide undergoes tautomerization to form an enol, which is then brominated at the $%5Calpha$ -position.
Step 2: The $%5Calpha$ -bromoacyl bromide is hydrolyzed with water ( $H_2O$ ) to yield the $%5Calpha$ -bromocarboxylic acid.
The overall reaction sequence is:
$ $CH_3CH_2COOH%20%5Cxrightarrow%7BBr_2%2FP%7D%20%5C%2C%20red.CH_3CHBrCOBr%20%5Cxrightarrow%7BH_2O%7D%20CH_3CHBrCOOH$ $%3Cbr%3ETherefore%2C%20the%20reagent%20'X'%20required%20for%20this%20conversion%20is%20(i)%20Br$ _2 $%2FP%20red.%2C%20followed%20by%20(ii)%20H$ _2 $O.%20%3Cbr%3EAnalyzing%20the%20other%20options%3A%3Cbr%3EOption%20(2)%20Br$ _2 $%2FCCl$ _4 $is%20used%20for%20the%20bromination%20of%20alkenes%20or%20alkanes%20(radical%20substitution)%2C%20not%20for$ \alpha $-halogenation%20of%20carboxylic%20acids.%3Cbr%3EOption%20(3)%20Br$ _2 $%2F%20OH$ ^- $is%20used%20for%20the%20haloform%20reaction%20with%20methyl%20ketones%20or%20secondary%20alcohols%20that%20can%20be%20oxidized%20to%20methyl%20ketones.%3Cbr%3EOption%20(4)%20PBr$ _3 $is%20used%20to%20convert%20carboxylic%20acids%20to%20acyl%20bromides%20or%20alcohols%20to%20alkyl%20bromides%2C%20but%20it%20does%20not%20directly%20introduce%20a%20bromine%20at%20the$ \alpha $-position%20of%20a%20carboxylic%20acid.%20%3Cbr%3EThus%2C%20the%20correct%20reagent%20'X'%20is$ \boxed{(i) Br_2/P \, red., (ii) H_2O} $.

The conversion of a carboxylic acid to an acyl chloride involves replacing the -OH group with a -Cl group. Common reagents for this transformation include thionyl chloride (SOCl $_2$ ), phosphorus pentachloride (PCl $_5$ ), and phosphorus trichloride (PCl $_3$ ). Among these, thionyl chloride is often preferred due to its relatively clean reaction and gaseous byproducts (SO $_2$ and HCl), which are easily removed.
The reaction with thionyl chloride proceeds as:
$ $CH_3CH_2COOH%20%2B%20SOCl_2%20%5Crightarrow%20CH_3CH_2COCl%20%2B%20SO_2%20%5Cuparrow%20%2B%20HCl%20%5Cuparrow$ $%3Cbr%3EThe%20other%20options%20are%20not%20suitable%20for%20this%20direct%20conversion%3A%3Cbr%3EHCl%20can%20react%20with%20carboxylic%20acids%20to%20form%20acyl%20chlorides%20under%20specific%20conditions%2C%20but%20it%20is%20not%20the%20preferred%20method.%3Cbr%3EHOCl%20is%20hypochlorous%20acid%2C%20an%20oxidizing%20agent%20and%20a%20source%20of%20chlorine%2C%20but%20not%20used%20for%20acyl%20chloride%20formation%20from%20carboxylic%20acids.%3Cbr%3ENaOCl%20is%20sodium%20hypochlorite%2C%20also%20an%20oxidizing%20agent%20and%20not%20typically%20used%20for%20this%20conversion.%3Cbr%3EThus%2C%20the%20preferred%20reagent%20is$ \boxed{SOCl_2} $.

High-Yield Trend

Chapter Questions
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Aldehydes Ketones And Carboxylic Acids

High-Yield Trend

Chapter Questions 2 MCQs

Official Solution

Official Solution

Chapter Questions
2 MCQs