19.6 Preparation of Carboxylic Acids

This section outlines the primary methods for synthesizing carboxylic acids from various organic starting materials.

1. By the Hydrolysis of Nitriles

Organic compounds containing a cyanide group ($-C\equiv N$) are known as nitriles or cyanides. Carboxylic acids can be prepared by hydrolyzing an alkyl nitrile, which is typically done by boiling it with a dilute mineral acid (e.g., $HCl$, $H_{2}S{O}_4$) or an alkali (e.g., $NaOH$).

Reaction Pathway: The process starts from an alkyl halide, which is converted to a nitrile, and then hydrolyzed to the carboxylic acid.

$R-X\stackrel{KCN}{\to }R-C\equiv N\stackrel{H_3{O}^{+}\text{ or }O{H}^{-},\Delta }{\to }R-COOH$

Acidic Hydrolysis:

$R-C\equiv N+2H_{2}O+H^{+}\stackrel{\Delta }{\to }R-COOH+N{H}_4^{+}$

Basic Hydrolysis:

$R-C\equiv N+O{H}^{-}+H_{2}O\stackrel{\Delta }{\to }R-CO{O}^{-}+N{H}_3$

The resulting carboxylate salt is then acidified to yield the carboxylic acid:

$R-CO{O}^{-}+H_3{O}^{+}\to R-COOH+H_{2}O$

Key Feature: This method is useful for increasing the length of a carbon chain, as the final carboxylic acid contains one more carbon atom than the original alkyl halide.

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2. By the Oxidation of Primary Alcohols

Primary alcohols can be oxidized to carboxylic acids using strong oxidizing agents. The reaction proceeds through an intermediate aldehyde, which is further oxidized.

Oxidizing Agents: Common agents include potassium permanganate ($KMn{O}_4$) or potassium dichromate ($K_{2}C{r}_2{O}_7$) in an acidic medium (e.g., dilute $H_{2}S{O}_4$).

General Reaction:

$R-C{H}_{2}OH\stackrel{[O]}{\to }[R-CHO]\stackrel{[O]}{\to }R-COOH$

Where [O] represents the oxidizing agent.

Example: Oxidation of ethanol to acetic acid.

$C{H}_{3}C{H}_{2}OH\stackrel{K_{2}C{r}_2{O}_7/H^{+}}{\to }C{H}_{3}COOH$

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3. By the Hydrolysis of Esters

Esters undergo hydrolysis to produce a carboxylic acid and an alcohol. This reaction can be catalyzed by either an acid or a base.

Acidic Hydrolysis: An ester is refluxed with a dilute mineral acid like $HCl$ or $H_{2}S{O}_4$. This is a reversible reaction.

$R-COO{R}^{\prime }+H_{2}O\rightleftharpoons R-COOH+R^{\prime }OH$

Basic Hydrolysis (Saponification): An ester is heated with a dilute alkali, such as $NaOH$. This process is called saponification because it is used to make soap (alkali salts of fatty acids).

1. The ester is hydrolyzed to form a carboxylate salt.

$R-COO{R}^{\prime }+NaOH\to R-COONa+R^{\prime }OH$

2. The salt is then acidified to produce the final carboxylic acid.

$R-COONa+HCl\to R-COOH+NaCl$

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4. From Grignard Reagents (Carbonation)

Carboxylic acids can be prepared by reacting Grignard reagents with dry ice (solid $C{O}_2$) followed by acid hydrolysis.

$R-MgX+O=C=O\to R-COOMgX\stackrel{H_3{O}^{+}}{\to }R-COOH+Mg(OH)X$

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5. Oxidative Cleavage of Alkenes

Alkenes react with hot, concentrated alkaline $KMn{O}_4$ to undergo cleavage at the double bond, forming carboxylic acids (if the double bond carbon is attached to a hydrogen).

$R-CH=CH-R^{\prime }\stackrel{KMn{O}_4/O{H}^{-},\Delta }{\to }R-COOH+R^{\prime }-COOH$

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Concept Assessment Exercise 19.1

Q: How can you convert $C{H}_{3}Cl$ into $C{H}_{3}COOH$?

A: This conversion involves increasing the carbon chain by one atom (from chloromethane to acetic acid). The nitrile hydrolysis method is suitable for this.

1. Step 1: Formation of the Nitrile

   React chloromethane ($C{H}_{3}Cl$) with potassium cyanide ($KCN$) in an alcoholic solution. This is a nucleophilic substitution reaction where the chloride is replaced by a cyanide group.

   $C{H}_{3}Cl+KCN\stackrel{\text{Alcohol}}{\to }C{H}_{3}CN+KCl$

   The product is methyl cyanide, also known as ethanenitrile or acetonitrile.

2. Step 2: Hydrolysis of the Nitrile

   Hydrolyze the methyl cyanide ($C{H}_{3}CN$) by boiling it with a dilute mineral acid (e.g., $H_3{O}^{+}$).

   $C{H}_{3}CN+2H_{2}O\stackrel{H^{+},\Delta }{\to }C{H}_{3}COOH+N{H}_3$

The overall two-step conversion is an effective way to synthesize a carboxylic acid from an alkyl halide with an additional carbon atom.