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Major Reactions of Carboxylic Acids

Carboxylic acids undergo reactions such as esterification, reduction, alpha-halogenation through the HVZ reaction, and decarboxylation under suitable conditions.

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Student-friendly explanation

Carboxylic acid reactions are best understood by recognising which bond or position is changing. In esterification, the -OH of the acid is involved in forming an ester with an alcohol under acid catalysis. Strong reducing agents such as LiAlH4 convert carboxylic acids to primary alcohols. The HVZ reaction halogenates the alpha carbon of carboxylic acids that have alpha hydrogen. Decarboxylation removes carbon dioxide from carboxylate salts under suitable heating conditions.

How to write this in exams

  1. 1

    Start with the exact idea

    Carboxylic acids undergo reactions such as esterification, reduction, alpha-halogenation through the HVZ reaction, and decarboxylation under suitable conditions.

  2. 2

    Then show how to use it

    Locate the -COOH group, identify the requested product class, check alpha hydrogen if HVZ is involved, select the reagent, then verify carbon count and functional group change.

  3. 3

    Add one concrete example

    Ethanoic acid reacts with ethanol in the presence of concentrated acid to form ethyl ethanoate. Ethanoic acid on reduction with LiAlH4 gives ethanol.

  4. 4

    Avoid this incomplete answer

    Writing aldehyde as the final product of LiAlH4 reduction of a carboxylic acid; the usual product is a primary alcohol.

Definition

Carboxylic acids undergo reactions such as esterification, reduction, alpha-halogenation through the HVZ reaction, and decarboxylation under suitable conditions.

Example

Ethanoic acid reacts with ethanol in the presence of concentrated acid to form ethyl ethanoate. Ethanoic acid on reduction with LiAlH4 gives ethanol.

Rule to remember

Esterification: RCOOH + R'OH in acid catalysis → RCOOR' + H2O. Reduction: RCOOH + LiAlH4 → RCH2OH. HVZ: RCH2COOH + X2/red P → RCHXCOOH. Decarboxylation: carboxylate salt under suitable conditions → hydrocarbon with one less carbon.

Sequence to remember

Esterification: RCOOH + R'OH in acid catalysis>
RCOOR' + H2O. Reduction: RCOOH + LiAlH4>
RCH2OH. HVZ: RCH2COOH + X2/red P>
RCHXCOOH. Decarboxylation: carboxylate salt under suitable conditions>
hydrocarbon with one less carbon. A reaction summary map is useful to connect -COOH with ester, alcohol, alpha-halo acid, and decarboxylation product. Locate the -COOH group, identify the requested product class, check alpha hydrogen if HVZ is involved, select the reagent, then verify carbon count and functional group change

Memory hook

Carboxylic acid reactions answer four questions: ester, alcohol, alpha-halogen, or CO2 loss.

Examples and method

Worked example

Convert propanoic acid to 2-bromopropanoic acid. Propanoic acid has alpha hydrogen, so treat it with bromine in the presence of red phosphorus under HVZ conditions to replace an alpha hydrogen by bromine.

Method to apply

Locate the -COOH group, identify the requested product class, check alpha hydrogen if HVZ is involved, select the reagent, then verify carbon count and functional group change.

Diagram support

A reaction summary map is useful to connect -COOH with ester, alcohol, alpha-halo acid, and decarboxylation product.

How CBSE asks it

Asked as conversions, reagent identification, named reaction, product prediction, and reason questions about alpha hydrogen or reduction products.

Avoid common mistakes

Common confusion

Students often apply HVZ reaction to acids without alpha hydrogen, where the required alpha-halogenation cannot occur.

Common wrong answer

Writing aldehyde as the final product of LiAlH4 reduction of a carboxylic acid; the usual product is a primary alcohol.

Exam tip

Before writing a carboxylic acid reaction, check whether the question asks for ester formation, reduction to alcohol, alpha substitution, or loss of CO2.

Quick check

What product is formed when CH3COOH is reduced with LiAlH4?

Ethanol is formed because LiAlH4 reduces the -COOH group to -CH2OH.

Answer writing and exam use

1-mark answer

Carboxylic acids undergo reactions such as esterification, reduction, alpha-halogenation through the HVZ reaction, and decarboxylation under suitable conditions.

2-mark answer

Carboxylic acids undergo reactions such as esterification, reduction, alpha-halogenation through the HVZ reaction, and decarboxylation under suitable conditions. Esterification: RCOOH + R'OH in acid catalysis → RCOOR' + H2O. Reduction: RCOOH + LiAlH4 → RCH2OH. HVZ: RCH2COOH + X2/red P → RCHXCOOH. Decarboxylation: carboxylate salt under suitable conditions → hydrocarbon with one less carbon. Ethanoic acid reacts with ethanol in the presence of concentrated acid to form ethyl ethanoate. Ethanoic acid on reduction with LiAlH4 gives ethanol.

3-mark answer

Carboxylic acid reactions are best understood by recognising which bond or position is changing. In esterification, the -OH of the acid is involved in forming an ester with an alcohol under acid catalysis. Strong reducing agents such as LiAlH4 convert carboxylic acids to primary alcohols. The HVZ reaction halogenates the alpha carbon of carboxylic acids that have alpha hydrogen. Decarboxylation removes carbon dioxide from carboxylate salts under suitable heating conditions. Esterification: RCOOH + R'OH in acid catalysis → RCOOR' + H2O. Reduction: RCOOH + LiAlH4 → RCH2OH. HVZ: RCH2COOH + X2/red P → RCHXCOOH. Decarboxylation: carboxylate salt under suitable conditions → hydrocarbon with one less carbon. Convert propanoic acid to 2-bromopropanoic acid. Propanoic acid has alpha hydrogen, so treat it with bromine in the presence of red phosphorus under HVZ conditions to replace an alpha hydrogen by bromine. Asked as conversions, reagent identification, named reaction, product prediction, and reason questions about alpha hydrogen or reduction products. Writing aldehyde as the final product of LiAlH4 reduction of a carboxylic acid; the usual product is a primary alcohol.
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