Reactions of Organic Acids and Bases

In short: Carboxylic acids (−COOH) are weak acids, and amines (−NH₂) are weak bases. A carboxylic acid reacts with an alcohol, with concentrated sulfuric acid as a catalyst, to make an ester and water, a reaction called esterification.

Carboxylic acids

Carboxylic acids have the carboxyl group, −COOH. They are weak acids:

$$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{CO}{\mathrm{O}}^{-}(\mathrm{aq})+{\mathrm{H}}_3{\mathrm{O}}^{+}(\mathrm{aq})$$

They react like other acids:

• with metals: $2C{\mathrm{H}}_3\mathrm{COOH}+\mathrm{Mg}\to (\mathrm{C}{\mathrm{H}}_3\mathrm{COO}{)}_2\mathrm{Mg}+{\mathrm{H}}_2$
• with carbonates (fizzing): $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+\mathrm{NaHC}{\mathrm{O}}_3\to \mathrm{C}{\mathrm{H}}_3\mathrm{COONa}+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$
• with bases: $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+\mathrm{NaOH}\to \mathrm{C}{\mathrm{H}}_3\mathrm{COONa}+{\mathrm{H}}_2\mathrm{O}$

Test: adding sodium carbonate or sodium hydrogen carbonate gives bubbles of $\mathrm{C}{\mathrm{O}}_2$. Alcohols, esters and aldehydes do not.

Amines and amides

Amines have an −NH₂ group (or N with carbon groups). Like ammonia, they are weak bases, because the lone pair on N accepts a proton:

$$\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_2(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3^{+}(\mathrm{aq})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{aq})$$

Amines react with acids to form salts: $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_2+\mathrm{HCl}\to \mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Cl}$.

Amides have the −CONH₂ group. They are made from a carboxylic acid and an amine (or ammonia). Amides are neutral: the C=O pulls the lone pair on N, so it cannot accept a proton.

Boiling points and solubility

For molecules of similar size:

All the small ones (up to about 4 carbons) dissolve in water because they can hydrogen bond with water. Esters can accept hydrogen bonds from water through their oxygens, so small esters are slightly soluble. As the carbon chain grows, solubility falls.

Esterification

An alcohol and a carboxylic acid react to form an ester and water. This is a condensation reaction (a small molecule, water, is lost).

$$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{COOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}$$

ethanoic acid + ethanol ⇌ ethyl ethanoate + water

• Catalyst: concentrated sulfuric acid. It also absorbs water, pushing the equilibrium right.
• The reaction is slow and reversible, so the yield is limited.
• To raise the yield: use an excess of the cheaper reactant, or remove water.

Refluxing

The reactants are heated under reflux: a condenser stands upright on the flask.

• Volatile reactants and products boil, condense and drip back, so nothing escapes. The mixture can be heated for a long time.
• Heat with a heating mantle or water bath, not a Bunsen flame: alcohols and esters are flammable.
• Add boiling chips for smooth boiling.
• Water flows in at the bottom of the condenser and out at the top.

Purifying the ester

1. Pour the cooled mixture into a separating funnel with water.
2. Add sodium carbonate solution to neutralise leftover acid ($\mathrm{C}{\mathrm{O}}_2$ bubbles; release the pressure often).
3. The ester is not very soluble, so it forms a separate layer (on top). Drain off the lower aqueous layer.
4. Dry the ester with anhydrous magnesium sulfate.
5. Distil, collecting the liquid at the ester's boiling point.

Hydrolysis of esters

Heating an ester with dilute acid or base breaks it back into an acid and an alcohol. With NaOH this is saponification, which is how soap is made from fats (esters of glycerol and long-chain fatty acids).

Soaps and detergents

A surfactant molecule has two ends:

• a long non-polar hydrocarbon tail that is hydrophobic (dissolves in grease),
• a charged head that is hydrophilic (dissolves in water).

Soap is the sodium salt of a long-chain fatty acid, such as sodium stearate, $\mathrm{C}{\mathrm{H}}_3(\mathrm{C}{\mathrm{H}}_2{)}_{16}\mathrm{CO}{\mathrm{O}}^{-}\mathrm{N}{\mathrm{a}}^{+}$.

How cleaning works:

1. The tails dissolve in a grease droplet; the heads stay in the water.
2. Many molecules surround the droplet, forming a micelle with negative heads on the outside.
3. The negative micelles repel each other, so the grease breaks into small droplets (an emulsion) that wash away.
4. Surfactants also lower the surface tension of water, so it wets surfaces better.

Soap in hard water: $\mathrm{C}{\mathrm{a}}^{2+}$ and $\mathrm{M}{\mathrm{g}}^{2+}$ ions form insoluble scum with soap. Detergents do not have this problem.

Quick summary

• Carboxylic acids (−COOH) are weak acids; they fizz with carbonates.
• Amines (−NH₂) are weak bases. Amides (−CONH₂) are neutral.
• Boiling points: ester < amine < alcohol < carboxylic acid < amide (similar size).
• Esterification: acid + alcohol ⇌ ester + water, conc. ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$, under reflux.
• Purify with sodium carbonate, a separating funnel and distillation.
• Surfactants have a hydrophobic tail and hydrophilic head and form micelles around grease.

Quiz