21.3 Reactions of Carboxylic Acids

Carboxylic Acid Reactions

Carboxylic acids sit at a unique spot on the reactivity spectrum: they're more oxidized than aldehydes and ketones, but less reactive toward nucleophiles than acid chlorides or anhydrides. That combination means you need specific reagents and conditions to transform them. This section covers the major reactions you'll see: Fischer esterification, conversion to more reactive derivatives, and reduction to alcohols.

Fischer Esterification Reaction Mechanism

Fischer esterification is an acid-catalyzed condensation that combines a carboxylic acid and an alcohol to produce an ester and water. The acid catalyst (typically $H_2SO_4$) doesn't get consumed; it just activates the carbonyl.

Here's the mechanism step by step:

1. Protonation of the carbonyl oxygen by the acid catalyst. This makes the carbonyl carbon much more electrophilic.
2. Nucleophilic attack by the alcohol's oxygen on the activated carbonyl carbon, forming a tetrahedral intermediate.
3. Proton transfer from the incoming oxygen to one of the hydroxyl groups on the tetrahedral intermediate.
4. Loss of water (dehydration) collapses the tetrahedral intermediate and regenerates the carbonyl.
5. Deprotonation of the ester product regenerates the acid catalyst.

This reaction is reversible, so the equilibrium needs to be pushed toward products. Common strategies:

• Use a large excess of either the carboxylic acid or the alcohol (Le Chatelier's principle)
• Remove water as it forms, using azeotropic distillation with toluene or drying agents like molecular sieves
• Dean-Stark traps are another classic way to remove water continuously

Conversion of Carboxylic Acids to Other Derivatives

Carboxylic acids are relatively poor electrophiles because the $OH$ group is a bad leaving group. Converting them to more reactive derivatives (acid chlorides, anhydrides, amides) is a core strategy in synthesis.

To acid chlorides:

• Thionyl chloride ($SOCl_2$) is the most common reagent. The carboxylic acid attacks $SOCl_2$, forming a chlorosulfite intermediate. Chloride then acts as the nucleophile in an acyl substitution, displacing $SO_2$ and $HCl$ as gaseous byproducts. The fact that both byproducts leave as gases drives the reaction to completion.
• Oxalyl chloride ($(COCl)_2$) works similarly and is preferred when milder conditions are needed (often used with catalytic DMF).
• $PCl_3$ can also be used but is less common in practice.

To anhydrides:

• A dehydrating agent like DCC (dicyclohexylcarbodiimide) activates one carboxylic acid molecule, making its carbonyl susceptible to nucleophilic attack by a second carboxylate. The result is a symmetric anhydride plus dicyclohexylurea (DCU) as a byproduct.

To amides:

• Direct reaction of a carboxylic acid with an amine first forms a carboxylate salt (acid-base reaction), not an amide. Converting that salt to an amide requires high temperatures (above 200°C) to drive off water.
• The practical route is to first convert the acid to an acid chloride or anhydride, then treat with ammonia or a primary/secondary amine. The nucleophilic amine attacks the activated carbonyl, expelling chloride or carboxylate as the leaving group.

Reduction of Carboxylic Acids

Carboxylic acids are harder to reduce than aldehydes or ketones because the $OH$ group donates electron density into the carbonyl, making it less electrophilic. You need strong reducing agents.

1. Lithium aluminum hydride ($LiAlH_4$)

$LiAlH_4$ delivers hydride ($H^-$) to the carbonyl carbon. The mechanism proceeds through an aldehyde intermediate, but that aldehyde is immediately reduced again, so you can't stop at the aldehyde stage. The product after aqueous workup is a primary alcohol.

• $LiAlH_4$ is powerful but unselective. It also reduces esters, aldehydes, ketones, and amides. If your molecule has multiple reducible functional groups, $LiAlH_4$ will hit all of them.
• It reacts violently with water and must be used in anhydrous solvents like THF or diethyl ether.

2. Borane ($BH_3$)

$BH_3$ (often used as the THF complex, $BH_3 \cdot THF$) is the selective alternative. It coordinates to the carbonyl oxygen of the carboxylic acid, which facilitates hydride delivery. The product is again a primary alcohol.

• The key advantage: $BH_3$ reduces carboxylic acids faster than it reduces esters, amides, or even most ketones. This selectivity lets you reduce a $COOH$ group while leaving other carbonyl-containing functional groups intact.
• This selectivity is the opposite of $LiAlH_4$, which is why choosing the right reducing agent matters in multifunctional molecules.

Reactivity and Properties

The acidity of carboxylic acids (typical $pK_a$ around 4-5) comes from resonance stabilization of the carboxylate anion. When the proton leaves, the negative charge delocalizes equally over both oxygens, making the conjugate base unusually stable for an organic compound.

The carbonyl carbon is electrophilic because oxygen pulls electron density away from carbon through both induction and the $\pi$ bond. However, the lone pair on the $OH$ group partially donates back into the carbonyl through resonance, which is why carboxylic acids are less electrophilic than acid chlorides or aldehydes.

Carboxylic acids are already at a high oxidation state (the carbon bears bonds to two oxygens). Further oxidation is uncommon under normal conditions. Reduction, as covered above, moves the carbon to a lower oxidation state: carboxylic acid → aldehyde → primary alcohol. With $LiAlH_4$ or $BH_3$, you go all the way to the alcohol in one step.