Carboxylic acids (18)

Carboxylic acids are weak acids, meaning they only partially dissociate (ionise) when dissolved in water. An equilibrium is established where most of the acid molecules remain undissociated. For ethanoic acid, the equilibrium is: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq). The position of this equilibrium lies far to the left. They are, however, stronger acids than alcohols. This increased acidity is due to two main factors. Firstly, the electron-withdrawing carbonyl group (C=O) pulls electron density away from the O-H bond, weakening it and making the proton (H⁺) easier to release. Secondly, the resulting carboxylate anion (e.g., CH₃COO⁻) is stabilised by resonance. The negative charge is delocalised (spread out) over the two oxygen atoms, which makes the ion more stable and less likely to re-form the acid. The acidity can be further increased by attaching electron-withdrawing groups (like chlorine) to the carbon atom next to the -COOH group. These groups pull electron density away from the carboxylate ion, spreading the negative charge even more and increasing its stability. The more electron-withdrawing groups present, the stronger the acid.

There are two principal laboratory methods for preparing carboxylic acids. The first is the oxidation of primary alcohols or aldehydes. This is achieved by heating the primary alcohol or aldehyde with an excess of a strong oxidising agent, typically acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄), under reflux. Refluxing ensures that any volatile intermediates (like the aldehyde) do not escape and are fully oxidised to the carboxylic acid. The colour change from orange (Cr₂O₇²⁻) to green (Cr³⁺) indicates the reaction has occurred. The second method is the hydrolysis of nitriles (R-C≡N). This involves refluxing the nitrile with a dilute acid (e.g., HCl(aq)) or alkali (e.g., NaOH(aq)). Acid hydrolysis produces the carboxylic acid and an ammonium salt (e.g., NH₄Cl), while alkaline hydrolysis produces the sodium salt of the carboxylic acid, which must then be acidified to form the carboxylic acid itself. This nitrile route is useful as it increases the length of the carbon chain by one carbon atom.

Acyl chlorides (RCOCl) are highly reactive derivatives of carboxylic acids, making them useful intermediates in synthesis. They are formed by replacing the hydroxyl (-OH) group of a carboxylic acid with a chlorine atom. This is a substitution reaction. There are three common chlorinating agents used in the lab:

1. Thionyl chloride (SOCl₂): This is often the best choice. The reaction produces the acyl chloride, sulfur dioxide gas, and hydrogen chloride gas. Since the by-products are gaseous, they escape from the reaction mixture, leaving a relatively pure acyl chloride, which simplifies purification.
2. Phosphorus(V) chloride (PCl₅): This reacts vigorously at room temperature to produce the acyl chloride, phosphoryl chloride (POCl₃), and hydrogen chloride gas. The by-products can be difficult to separate from the desired product.
3. Phosphorus(III) chloride (PCl₃): This reaction requires heating. It produces the acyl chloride and phosphorous acid (H₃PO₃). The equation requires balancing with a 3:1 ratio of carboxylic acid to PCl₃.

Acyl chlorides are powerful acylating agents, meaning they are excellent at transferring their acyl group (R-C=O) to other molecules. Their high reactivity stems from the highly electron-deficient (δ+) carbonyl carbon, which is attacked by nucleophiles. These reactions are vigorous and proceed via a nucleophilic addition-elimination mechanism. Key reactions include:

1. Hydrolysis (with water): A violent reaction occurs with cold water, producing the corresponding carboxylic acid and steamy/misty fumes of hydrogen chloride gas. Example: CH₃COCl + H₂O → CH₃COOH + HCl.
2. Reaction with Alcohols: A vigorous reaction produces an ester and HCl. This is a much faster and more efficient way to make esters than traditional acid-catalysed esterification, and the reaction goes to completion. Example: CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl.
3. Reaction with Phenols: Similar to alcohols, acyl chlorides react with phenols to form phenolic esters. This reaction is important because phenols do not readily form esters with carboxylic acids directly. Example: C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ + HCl.
4. Reaction with Ammonia and Amines: A violent reaction occurs with ammonia to form a primary amide. With a primary amine, an N-substituted amide is formed. In both cases, two moles of ammonia/amine are required per mole of acyl chloride: one acts as the nucleophile, and the second acts as a base to mop up the HCl produced, forming an ammonium salt. Example: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl.

The ease with which a chlorine-containing compound is hydrolysed by water depends greatly on its structure. The order of reactivity is: Acyl Chloride > Halogenoalkane > Halogenoarene.

• Acyl Chlorides (e.g., CH₃COCl): Hydrolyse extremely rapidly, even with cold water. The carbonyl carbon is very electron-deficient (highly δ+) because it is bonded to two highly electronegative atoms (O and Cl). This makes it very susceptible to attack by a nucleophile like water.
• Halogenoalkanes (e.g., CH₃CH₂Cl): Hydrolyse much more slowly, typically requiring heating with aqueous alkali (a stronger nucleophile than water). The carbon bonded to the chlorine is δ+, but less so than in an acyl chloride as it's only attached to one electronegative atom.
• Halogenoarenes (e.g., Chlorobenzene, C₆H₅Cl): Are extremely resistant to hydrolysis and require very harsh conditions (e.g., high temperature and pressure with NaOH). This is because a lone pair of electrons from the chlorine atom is delocalised into the benzene ring's π-system. This gives the C-Cl bond partial double bond character, making it much stronger and harder to break.

The mechanism for acyl chloride hydrolysis is nucleophilic addition-elimination. In step 1 (addition), a water molecule attacks the δ+ carbonyl carbon. In step 2 (elimination), the C=O bond reforms, and the chloride ion is expelled as a good leaving group.