Esters

Esters (R-CO-OR’) are the most common carboxylic acid derivatives in nature and industry. Triglycerides are triple esters of glycerol. Aspirin contains an ester. Biodiesel is a mix of fatty acid methyl esters. Every flavor and fragrance compound in fruit (banana, apple, pear) is an ester.

Structure

Esters have a carbonyl C=O and a C-OR single bond. The oxygen lone pair donates into the carbonyl via resonance, giving ester carbonyls less electrophilic character than ketones (and much less than acyl halides or anhydrides).

Formation

Four common routes:

1. Fischer esterification (Ch 8.8): carboxylic acid + alcohol + H⁺ cat. Reversible; drive forward with excess alcohol or water removal.
2. Acid chloride + alcohol + pyridine: fast and irreversible. Preferred for clean lab synthesis.
3. Anhydride + alcohol: acetic anhydride is a common acetylating agent for phenols and alcohols.
4. Carboxylate + alkyl halide (SN2): RCOO⁻ + R’X → RCOOR’ + X⁻. Works with primary halides in polar aprotic solvent. Slow with hindered substrates.

Hydrolysis

Acid-catalyzed hydrolysis: ester + water + H⁺ cat. → carboxylic acid + alcohol. This is the reverse of Fischer esterification. Reversible; pushed forward by excess water.

Base-promoted hydrolysis (saponification): ester + NaOH → sodium carboxylate + alcohol. Irreversible because the carboxylate is stable and cannot re-attack.

Saponification is the origin of the word “soap” (Latin “sapo”). Ancient Romans boiled animal fats with potash (potassium hydroxide from wood ashes) to make soap - a process that is still essentially the same today, though industrial scales have replaced the kettle.

Saponification Mechanism

1. OH⁻ attacks the ester carbonyl carbon.
2. Tetrahedral alkoxide intermediate forms.
3. Alkoxide (OR’⁻) leaves, reforming the carbonyl as a carboxylic acid.
4. Fast acid-base: the carboxylic acid is immediately deprotonated by hydroxide (or another OH⁻) to give the stable carboxylate anion.

Step 4 is irreversible because the carboxylate cannot re-attack the alcohol. This is why saponification is irreversible under basic conditions.

Transesterification

Swap one OR group for another:

R-CO-OR’ + R”-OH ⇌ R-CO-OR” + R’-OH (with acid or base catalyst)

Biodiesel production uses transesterification: triglycerides + methanol + base catalyst → fatty acid methyl esters (biodiesel) + glycerol. The glycerol is separated and used for other purposes.

Bioester Hydrolysis: Lipase

Pancreatic lipase and other esterases hydrolyze fats in the digestive tract. The enzyme active site uses a catalytic serine (nucleophile) and histidine/aspartate (general acid/base) to accelerate the hydrolysis billions of times.

Flavor and Fragrance Esters

Simple short-chain esters give fruit their characteristic smells: