What you'll learn

• How the acidity of carboxylic acids, phenols, water and alcohols compares.
• How carboxylic acids are made, reduced, decarboxylated and converted into derivatives.
• The key reagents, conditions and mechanisms for esters, acyl chlorides, amides, nitriles and hydroxynitriles.
• How to plan short synthetic routes and interpret practical observations safely.

The carboxyl group

The part $RCO-$ is called an acyl group. Many derivatives are made by replacing the $-OH$ of a carboxylic acid with another group, for example $-Cl$ in an acyl chloride or $-OR’$ in an ester.

Relative acidity

A Brønsted–Lowry acid is a proton donor. The easier it is for a compound to lose $H^{+}$, the stronger the acid.

The acidity order you need is:

carboxylic acids > phenols > water > alcohols

Typical $p{K}_a$ values support this: carboxylic acids are about 4–5, phenol is about 10, water is 15.7, and many alcohols are about 16–18. A lower $p{K}_a$ means a stronger acid.

Phenoxide ions are also resonance-stabilised, but less effectively. Alkoxide ions from alcohols have a localised negative charge on one oxygen, and alkyl groups tend to push electron density towards it, making them less stable.

Carboxylic acids show typical acid reactions:

$$\begin{array}{rl}RCOOH+NaOH& \to RCOONa+H_{2}O\\ 2RCOOH+{Na}_2{CO}_3& \to 2RCOONa+{CO}_2+H_{2}O\\ 2RCOOH+2Na& \to 2RCOONa+H_2\end{array}$$

Demonstrating the order

• Carbonates / hydrogencarbonates: carboxylic acids fizz, giving ${CO}_2$. Phenols, water and alcohols do not.
• Aqueous sodium hydroxide: carboxylic acids and phenols form salts; alcohols do not react appreciably.
• Sodium metal: carboxylic acids, water and alcohols can give hydrogen, so this is less selective.

Making carboxylic acids

Oxidation of alcohols and aldehydes

Primary alcohols are oxidised to aldehydes and then to carboxylic acids using acidified potassium dichromate(VI), $K_2{Cr}_2{O}_7/H^{+}$, under reflux. The colour changes from orange to green.

$$\begin{array}{rl}{RCH}_{2}OH+2[O]& \to RCOOH+H_{2}O\\ RCHO+[O]& \to RCOOH\end{array}$$

Use reflux when you want complete oxidation, because it allows heating without losing volatile reactants or products.

Oxidation of methyl side-chains on benzene rings

A methylbenzene side-chain can be oxidised to an aromatic carboxylic acid using hot alkaline potassium manganate(VII), then acidification.

$$C_6{H}_5{CH}_3+3[O]\to C_6{H}_{5}COOH+H_{2}O$$

So methylbenzene forms benzoic acid. The benzene ring is not oxidised under these conditions.

Reduction of carboxylic acids

Carboxylic acids are reduced to primary alcohols using lithium aluminium hydride, ${LiAlH}_4$, in dry ether, followed by acidified water.

$$RCOOH+4[H]\to {RCH}_{2}OH+H_{2}O$$

Decarboxylation

Decarboxylation means removal of carbon dioxide. Heating a sodium carboxylate with soda lime, a mixture of $NaOH$ and $CaO$, gives an alkane with one fewer carbon.

$$RCOONa+NaOH\to RH+{Na}_2{CO}_3$$

For example, sodium ethanoate forms methane.

Derivatives route map

A route map is useful because many questions ask you to choose reagents and conditions to move between families.

Esters

Carboxylic acid + alcohol ⇌ ester + water. The conditions are concentrated sulfuric acid catalyst and reflux.

$$RCOOH+R’OH\rightleftharpoons RCOOR’+H_{2}O$$

To draw the acid-catalysed esterification mechanism:

1. Protonate the carbonyl oxygen using $H^{+}$, making the carbonyl carbon more electron deficient.
2. Draw a curly arrow from the alcohol oxygen lone pair to the carbonyl carbon; move the $\pi$ electrons of $C=O$ onto oxygen.
3. Use proton transfers to turn an $-OH$ group into a good leaving group, $H_{2}O$.
4. Reform the carbonyl, eliminate water, then deprotonate to regenerate the acid catalyst.

Esters are hydrolysed in two main ways:

• Acid hydrolysis: dilute acid, water, reflux; reversible, giving carboxylic acid and alcohol.
• Alkaline hydrolysis: aqueous sodium hydroxide, reflux; gives carboxylate salt and alcohol. Acidification afterwards gives the carboxylic acid.

Acyl chlorides and amides

Carboxylic acids are converted to acyl chlorides using sulfur dichloride oxide, ${SOCl}_2$.

$$RCOOH+{SOCl}_2\to RCOCl+{SO}_2+HCl$$

Acyl chlorides react rapidly because the carbonyl carbon is very electron deficient and chloride is a good leaving group.

For hydrolysis:

$$RCOCl+H_{2}O\to RCOOH+HCl$$

For amide formation:

$$RCOCl+2{NH}_3\to {RCONH}_2+{NH}_{4}Cl$$

The curly-arrow mechanism is nucleophilic addition–elimination:

1. A nucleophile, such as water or ammonia, attacks the carbonyl carbon; the $C=O$ $\pi$ electrons move onto oxygen.
2. The oxygen lone pair reforms the carbonyl; the $C-Cl$ bond breaks to release ${Cl}^{-}$.
3. A proton is removed. With ammonia, a second ammonia molecule accepts the proton to form ${NH}_4^{+}$.

Amides can also be made by reacting a carboxylic acid with ammonia to form an ammonium carboxylate salt, then heating to dehydrate it.

$$RCOOH+{NH}_3\to {RCOO}^{-}{NH}_4^{+}$$ $${RCOO}^{-}{NH}_4^{+}\to {RCONH}_2+H_{2}O$$

Amides can be dehydrated to nitriles using phosphorus(V) oxide, $P_2{O}_5$, with heat.

$${RCONH}_2\to RCN+H_{2}O$$

Nitriles and hydroxynitriles

From halogenoalkanes

Halogenoalkanes form nitriles by nucleophilic substitution with ethanolic potassium cyanide, $KCN$, under reflux.

$$R-X+{CN}^{-}\to R-CN+X^{-}$$

For the curly-arrow mechanism, draw the arrow from the carbon lone pair of ${CN}^{-}$ to the carbon bonded to the halogen, and another arrow from the $C-X$ bond to the halogen. This is an $S_{N}2$ reaction for primary halogenoalkanes.

From aldehydes and ketones

Aldehydes and ketones react with hydrogen cyanide, usually with $KCN$ as a catalyst, to form hydroxynitriles.

To draw the mechanism:

1. ${CN}^{-}$ attacks the carbonyl carbon; the $C=O$ $\pi$ electrons move onto oxygen.
2. The alkoxide ion formed is protonated by $HCN$.
3. ${CN}^{-}$ is regenerated, so only a small amount is needed as catalyst.

Hydrolysis and reduction

Nitriles and amides are hydrolysed under acidic or alkaline reflux.

$$\begin{array}{rl}RCN+2H_{2}O+H^{+}& \to RCOOH+{NH}_4^{+}\\ RCN+H_{2}O+{OH}^{-}& \to {RCOO}^{-}+{NH}_3\\ {RCONH}_2+H_{2}O+H^{+}& \to RCOOH+{NH}_4^{+}\\ {RCONH}_2+{OH}^{-}& \to {RCOO}^{-}+{NH}_3\end{array}$$

Nitriles are reduced to primary amines using ${LiAlH}_4$ in dry ether, followed by acidified water.

$$RCN+4[H]\to {RCH}_2{NH}_2$$

Practical data: yields and evidence

In organic preparations, you often reflux to complete a reaction, then purify by separation, drying and distillation or recrystallisation. IR spectroscopy can support identification: carboxylic acids show a broad O–H absorption around 2500–3300 cm⁻¹ and a strong C=O absorption near 1700 cm⁻¹; nitriles show a sharp C≡N absorption around 2220–2260 cm⁻¹.