Carboxylic acids (33)

Carboxylic acids contain the carboxyl functional group, -COOH. They are classified as weak acids because they only partially dissociate (ionise) when dissolved in water. An equilibrium is established where most molecules remain undissociated. For example, with ethanoic acid: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq). The acidity arises from two main factors: 1) The highly electronegative oxygen atom of the carbonyl group (C=O) withdraws electron density from the O-H bond, weakening it and making the proton (H⁺) easier to release. 2) The resulting carboxylate anion (R-COO⁻) is stabilised by resonance. The negative charge is delocalised (spread out) over the two oxygen atoms, which makes the anion more stable and less likely to re-join with a proton.

The strength of a carboxylic acid is significantly influenced by other atoms or groups attached to the carbon chain, particularly on the carbon atom adjacent to the -COOH group (the alpha-carbon). Electron-withdrawing groups, such as chlorine atoms, pull electron density away from the carboxylate group through the carbon chain (a negative inductive effect). This further spreads out the negative charge on the carboxylate ion, increasing its stability. A more stable anion means the acid is more likely to dissociate, so the acid becomes stronger. The more electron-withdrawing groups present, or the more electronegative they are, the stronger the acid. For example, trichloroethanoic acid (CCl₃COOH) is a much stronger acid than ethanoic acid (CH₃COOH). Conversely, electron-donating groups, like alkyl groups, push electron density towards the carboxylate group, destabilising the anion and making the acid weaker.

Because they release H⁺ ions in solution, carboxylic acids undergo the typical reactions of any acid, although the reactions may be slower than with strong acids like HCl. They react with reactive metals, bases (alkalis), and carbonates to form a carboxylate salt. The name of the salt is derived from the acid, for example, ethanoic acid forms ethanoate salts.

1. With reactive metals (e.g., Mg, Zn, Na): Produces a salt and hydrogen gas.
2. With bases/alkalis (e.g., NaOH, CaO): A neutralisation reaction occurs, producing a salt and water.
3. With carbonates (e.g., Na₂CO₃, K₂CO₃): Produces a salt, water, and carbon dioxide gas. The production of CO₂ (effervescence) is a useful test for the carboxylic acid group, as alcohols do not react this way.

There are two principal methods for preparing carboxylic acids in the laboratory.

1. Oxidation of Primary Alcohols and Aldehydes: Primary alcohols can be oxidised to aldehydes, which can then be further oxidised to carboxylic acids. To ensure the final product is the carboxylic acid, a strong oxidising agent (like acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄) is used, and the mixture is heated under reflux. Refluxing prevents any volatile aldehyde from escaping, ensuring it is fully oxidised. The colour change observed is from orange (Cr₂O₇²⁻) to green (Cr³⁺).
2. Hydrolysis of Nitriles: Nitriles (compounds containing the -C≡N group) can be converted to carboxylic acids by heating them under reflux with either dilute acid (e.g., HCl) or dilute alkali (e.g., NaOH). This reaction is called hydrolysis. Acid hydrolysis produces the carboxylic acid directly, along with an ammonium salt. Alkaline hydrolysis produces a carboxylate salt, which must then be acidified in a separate step to form the free carboxylic acid.

Carboxylic acids can undergo two other important transformations: reduction and esterification.

1. Reduction: Carboxylic acids are resistant to reduction by common reducing agents. They can only be reduced to primary alcohols using a very powerful reducing agent, lithium tetrahydridoaluminate (LiAlH₄). The reaction is typically carried out in a dry ether solvent, as LiAlH₄ reacts violently with water. The overall reaction can be represented as adding four hydrogen atoms. Note that sodium borohydride (NaBH₄) is not strong enough to reduce carboxylic acids.
2. Esterification: This is a condensation reaction between a carboxylic acid and an alcohol to form an ester and water. The reaction is reversible and has a slow rate at room temperature. To improve the yield and rate, the reaction is heated under reflux with a strong acid catalyst, typically concentrated sulfuric acid. The catalyst both speeds up the reaction and absorbs the water produced, helping to shift the equilibrium to the right.

Acyl chlorides (or acid chlorides) are highly reactive derivatives of carboxylic acids, with the general formula R-COCl. They are formed by replacing the -OH group of the carboxylic acid with a -Cl atom. This is a key step in synthesis as acyl chlorides are much more reactive than the parent carboxylic acids. There are three common reagents used for this conversion:

1. Thionyl chloride (SOCl₂): This is often the preferred method as the other products, sulfur dioxide (SO₂) and hydrogen chloride (HCl), are gases and can be easily removed from the reaction mixture, leaving a purer product.
2. Phosphorus(V) chloride (PCl₅): This solid reagent reacts vigorously at room temperature, producing the acyl chloride, phosphorus oxychloride (POCl₃) and hydrogen chloride gas.
3. Phosphorus(III) chloride (PCl₃): This liquid reagent also works, but requires gentle heating. The stoichiometry is different, with 3 moles of carboxylic acid reacting per mole of PCl₃.

Acyl chlorides are among the most reactive organic functional groups. Their high reactivity is due to the large partial positive charge (δ+) on the carbonyl carbon, which is made highly electron-deficient by both the electronegative oxygen and chlorine atoms. This makes it an excellent target for nucleophiles. The reactions are typically nucleophilic addition-elimination reactions and are much faster and more vigorous than with carboxylic acids. Key reactions include:

1. Hydrolysis: A violent reaction with cold water to form the parent carboxylic acid and misty fumes of hydrogen chloride gas. RCOCl + H₂O → RCOOH + HCl.
2. Reaction with Alcohols: A vigorous, irreversible reaction to form an ester and HCl. This is a more efficient way to make esters than direct esterification, giving a higher yield. RCOCl + R'OH → RCOOR' + HCl.
3. Reaction with Phenols: Similar to alcohols, phenols react with acyl chlorides to form phenyl esters and HCl.
4. Reaction with Amines and Ammonia: A violent reaction with ammonia or primary amines to form amides and HCl. The HCl produced will then react with any excess amine to form a salt. RCOCl + 2NH₂R' → RCONHR' + R'NH₃⁺Cl⁻.

The rate at which different types of organic chlorides hydrolyse (react with water) varies dramatically. The order of reactivity is: Acyl chloride >> Alkyl chloride > Aryl chloride.

• Acyl Chlorides (e.g., ethanoyl chloride, CH₃COCl): These hydrolyse extremely rapidly, even in cold water. The carbonyl carbon is strongly δ+ because it is bonded to two highly electronegative atoms (O and Cl). This makes it very attractive to the lone pair on the oxygen of a water molecule (a nucleophile). The reaction proceeds via a fast nucleophilic addition-elimination mechanism.
• Alkyl Chlorides (e.g., chloroethane, CH₃CH₂Cl): These hydrolyse much more slowly, usually requiring heat and often a catalyst (like NaOH(aq) in nucleophilic substitution). The carbon bonded to chlorine is δ+, but less so than in an acyl chloride as it's only attached to one electronegative atom.
• Aryl Chlorides (e.g., chlorobenzene, C₆H₅Cl): These are extremely resistant to hydrolysis and do not react with water under normal conditions. This is because the lone pair of electrons on the chlorine atom is delocalised into the pi-system of the benzene ring. This gives the C-Cl bond partial double bond character, making it much stronger and harder to break.