Aldehydes, Ketones and Carboxylic Acids

Aldehydes and ketones both contain the carbonyl group, C=O. In an aldehyde the carbonyl carbon carries at least one hydrogen (–CHO, formyl group); in a ketone it is bonded to two carbon groups. Carboxylic acids contain the carboxyl group, –COOH, where the carbonyl carbon is also joined to –OH.

Naming aldehydes. The IUPAC name comes from the parent alkane with the ending –e replaced by –al: HCHO methanal, CH₃CHO ethanal, CH₃CH₂CHO propanal. The –CHO carbon is always C-1, so its locant is never written. When –CHO is attached to a ring the suffix becomes carbaldehyde (cyclohexanecarbaldehyde), and C₆H₅CHO keeps the name benzaldehyde (benzenecarbaldehyde). Common names use the trivial acid name: formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde; positions of substituents in common names are shown by Greek letters α, β, γ counting from the carbon next to –CHO.

Naming ketones. Replace –e by –one and give the carbonyl carbon the lowest locant: CH₃COCH₃ propanone (acetone), CH₃COCH₂CH₃ butanone, C₆H₅COCH₃ 1-phenylethanone (acetophenone), C₆H₅COC₆H₅ diphenylmethanone (benzophenone). Cyclic ketones are cycloalkanones with C=O at position 1, e.g. 2-methylcyclohexanone. Common names of ketones name the two alkyl groups followed by ketone (ethyl methyl ketone); a ketone with two identical groups is symmetrical, otherwise unsymmetrical.

When both –CHO and C=O are present, –CHO is the principal group (suffix –al) and the keto group becomes the prefix oxo: CH₃COCH₂CHO is 3-oxobutanal. Order of priority for suffixes that the board tests: –COOH > –CHO > C=O > –OH.

Structure of the carbonyl group. The carbonyl carbon is sp² hybridised, so the carbon and the three atoms attached to it lie in one plane with bond angles of about 120°. The C=O bond is a σ bond plus a π bond formed by sideways overlap of the unhybridised p orbitals of C and O. Oxygen is more electronegative, so the π electrons are pulled towards it: the bond is polarised, carbon carries a partial positive charge and oxygen a partial negative charge. This is why the carbonyl carbon is attacked by nucleophiles and the oxygen by electrophiles, and why these compounds have high dipole moments.

From alcohols. Primary alcohols give aldehydes and secondary alcohols give ketones on oxidation with PCC (pyridinium chlorochromate) or on dehydrogenation over heated copper at 573 K: CH₃CH₂OH → CH₃CHO (Cu, 573 K); (CH₃)₂CHOH → CH₃COCH₃. Tertiary alcohols dehydrate instead.

From hydrocarbons. Ozonolysis of an alkene (O₃ then Zn/H₂O) breaks C=C into two carbonyl fragments; zinc dust prevents the H₂O₂ formed from oxidising the aldehyde. Hydration of an alkyne with dilute H₂SO₄ and HgSO₄ gives ethanal from ethyne and ketones from all other alkynes.

Aldehydes from acyl chlorides: Rosenmund reduction. RCOCl + H₂ → RCHO + HCl over palladium deposited on barium sulphate (Pd/BaSO₄), usually with sulphur or quinoline as poison. The poisoned catalyst stops the reduction at the aldehyde stage instead of going on to the alcohol. Formaldehyde cannot be made this way because formyl chloride is unstable at room temperature.

Aldehydes from nitriles and esters. Stephen reaction: RCN + SnCl₂ + HCl → RCH=NH·HCl (imine hydrochloride), hydrolysed with water to RCHO. Alternatively DIBAL-H (diisobutylaluminium hydride) reduces a nitrile or an ester to an imine or hemiacetal that hydrolyses to the aldehyde; DIBAL-H at low temperature gives the aldehyde without touching a C=C elsewhere in the molecule.

Aromatic aldehydes from toluene. Etard reaction: toluene with chromyl chloride, CrO₂Cl₂, in CS₂ gives a brown chromium complex that is hydrolysed by water to benzaldehyde. With chromic oxide (CrO₃) in acetic anhydride, toluene gives benzylidene diacetate, C₆H₅CH(OCOCH₃)₂, which is hydrolysed by aqueous acid to benzaldehyde; the diacetate protects the aldehyde from further oxidation. Side-chain chlorination of toluene gives benzal chloride, C₆H₅CHCl₂, hydrolysed to benzaldehyde (a commercial method). Gatterman–Koch reaction: benzene with CO and HCl in the presence of anhydrous AlCl₃ or CuCl gives benzaldehyde directly.

Ketones. From acyl chlorides with dialkylcadmium: 2RCOCl + R'₂Cd → 2RCOR' + CdCl₂ (the dialkylcadmium comes from a Grignard reagent and CdCl₂). From nitriles with a Grignard reagent: RCN + R'MgX → RC(=NMgX)R' → hydrolysis → RCOR'. Aromatic ketones by Friedel–Crafts acylation: benzene + CH₃COCl (anhydrous AlCl₃) → acetophenone + HCl.

Methanal is a gas at room temperature, ethanal is a volatile liquid, and the other aldehydes and ketones up to about eleven carbons are liquids; higher members are solids. The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of similar molar mass, because the polar C=O group gives dipole–dipole attractions between molecules. Their boiling points are lower than those of the corresponding alcohols and carboxylic acids, because aldehydes and ketones have no hydrogen bonded to oxygen and so cannot hydrogen bond with each other. The standard board ordering for compounds of similar mass is: alkane < ether < aldehyde or ketone < alcohol < carboxylic acid. For example, with molar mass 44 to 46: CH₃CH₂CH₃ (231 K) < CH₃OCH₃ (249 K) < CH₃CHO (293 K) < CH₃CH₂OH (351 K).

The lower members (methanal, ethanal, propanone) are miscible with water in all proportions because the carbonyl oxygen forms hydrogen bonds with water. Solubility falls rapidly as the hydrocarbon chain lengthens. All aldehydes and ketones dissolve in common organic solvents such as benzene, ether and chloroform. Lower aldehydes have sharp, pungent odours; as size increases the odour becomes fragrant, which is why many naturally occurring aldehydes and ketones are used in perfumery and flavouring.

The reasoning questions from this section are always the same three: why aldehydes and ketones boil above hydrocarbons (dipole–dipole forces), why they boil below alcohols and acids (no intermolecular hydrogen bonding), and why ethanal or propanone is soluble in water (hydrogen bonding with water).

Mechanism. A nucleophile attacks the electrophilic carbonyl carbon perpendicular to the plane of the sp² carbon. As the new bond forms, the hybridisation of carbon changes from sp² to sp³ and the π electrons move onto oxygen, giving a tetrahedral alkoxide intermediate. The alkoxide then picks up a proton (from the solvent or the acid present) to give the neutral addition product. The board expects the two steps drawn with curved arrows and the words planar, tetrahedral and alkoxide intermediate.

Reactivity. Aldehydes are more reactive than ketones for two reasons. Steric: a ketone has two bulky groups around the carbonyl carbon that hinder the approach of the nucleophile, while an aldehyde has one group and one small hydrogen. Electronic: two alkyl groups release electrons (+I effect) and reduce the partial positive charge on the carbonyl carbon more than one alkyl group does, so the ketone carbon is a weaker electrophile. Within aldehydes, reactivity falls as the alkyl group grows: HCHO > CH₃CHO > CH₃CH₂CHO. Aromatic carbonyls are less reactive than aliphatic ones because the ring delocalises electrons into the carbonyl carbon (resonance) and adds bulk. A typical order: ethanal > propanal > propanone > butanone > benzaldehyde > acetophenone.

Addition of HCN. HCN adds slowly on its own; a trace of base generates CN⁻, a much better nucleophile, which attacks the carbonyl carbon and gives a cyanohydrin, R₂C(OH)CN. Cyanohydrins are useful because the –CN group can be hydrolysed to –COOH or reduced to –CH₂NH₂.

Addition of sodium hydrogensulphite. NaHSO₃ adds to aldehydes and to methyl ketones to give crystalline bisulphite addition compounds. The equilibrium lies to the right for most aldehydes and to the left for most ketones. The adduct is hydrolysed back to the pure carbonyl compound by dilute acid or alkali, so the reaction is used to separate and purify aldehydes.

Addition of Grignard reagents gives alcohols after hydrolysis: methanal gives a primary alcohol, other aldehydes a secondary alcohol, ketones a tertiary alcohol.

Addition of alcohols. An aldehyde with one equivalent of a monohydric alcohol under dry HCl gas gives a hemiacetal, and with a second equivalent an acetal, RCH(OR')₂. Ketones do not react with monohydric alcohols easily but react with ethylene glycol under dry HCl to give cyclic products called ethylene glycol ketals. Dry HCl protonates the carbonyl oxygen and makes the carbon more electrophilic. Acetals and ketals are stable to base but are hydrolysed back by aqueous acid, so they are used to protect a carbonyl group during other reactions.

Addition of ammonia and its derivatives (H₂N–Z). The nucleophile adds to C=O and the adduct loses water to give a compound with C=N–Z. Z = H gives an imine; Z = R gives a substituted imine (Schiff's base); Z = OH (hydroxylamine) gives an oxime; Z = NH₂ (hydrazine) gives a hydrazone; Z = NHC₆H₅ (phenylhydrazine) gives a phenylhydrazone; Z = NHC₆H₃(NO₂)₂ gives a 2,4-dinitrophenylhydrazone (yellow, orange or red precipitate, the 2,4-DNP test); Z = NHCONH₂ (semicarbazide) gives a semicarbazone. These reactions work best at a weakly acidic pH of 3.5: at lower pH the amine is protonated and loses its nucleophilicity, at higher pH the carbonyl is not sufficiently protonated and activated.

Reduction to alcohols. Aldehydes are reduced to primary alcohols and ketones to secondary alcohols by sodium borohydride (NaBH₄), lithium aluminium hydride (LiAlH₄) or catalytic hydrogenation (H₂/Ni, Pt or Pd). NaBH₄ is the milder reagent and is selective for C=O; it does not reduce esters, acids or C=C. LiAlH₄ reduces almost every carbonyl group and needs dry ether because it reacts violently with water.

Reduction to hydrocarbons. Two named reactions convert C=O into CH₂, and both are asked every year. Clemmensen reduction: the carbonyl compound is heated with zinc amalgam and concentrated hydrochloric acid, R₂C=O + 4[H] → R₂CH₂ + H₂O (Zn–Hg/conc. HCl). It is suited to compounds that are stable in strong acid. Wolff–Kishner reduction: the carbonyl compound is first converted to its hydrazone with hydrazine, R₂C=O + H₂NNH₂ → R₂C=NNH₂ + H₂O, and the hydrazone is heated with KOH in a high-boiling solvent such as ethylene glycol, giving R₂CH₂ + N₂. It is suited to compounds that are stable in base. So acetophenone gives ethylbenzene by either method, and cyclohexanone gives cyclohexane.

Choose the method by what else is in the molecule: an acid-sensitive group (an acetal, a tertiary alcohol that would dehydrate) calls for Wolff–Kishner, a base-sensitive group (an ester that would hydrolyse) calls for Clemmensen. A common 2-mark question asks for the reagent to convert benzaldehyde or acetophenone to the hydrocarbon, and an equal number ask for the product of NaBH₄ on a given ketone. Write the hydrazone as an intermediate when you give the Wolff–Kishner equation; the marking scheme rewards the two-step form.

Aldehydes are oxidised to carboxylic acids very easily, by strong oxidants (HNO₃, KMnO₄, K₂Cr₂O₇) and even by mild ones like Tollens' reagent and Fehling's solution, because the C–H bond on the carbonyl carbon is broken without touching a C–C bond. Ketones resist oxidation; only under vigorous conditions (strong oxidant, heat) do they break a C–C bond to give a mixture of smaller carboxylic acids. The C–C bond on either side of C=O can break, so butanone gives ethanoic acid (from both fragments) together with some methanoic and propanoic acid; the major cleavage keeps the C=O with the smaller alkyl group.

Tollens' test. Tollens' reagent is ammoniacal silver nitrate, [Ag(NH₃)₂]⁺. Warming with an aldehyde deposits a silver mirror on the tube: RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 2H₂O + 4NH₃. Both aliphatic and aromatic aldehydes respond; ketones do not.

Fehling's test. Fehling A is aqueous CuSO₄, Fehling B is alkaline sodium potassium tartrate (Rochelle salt); the mixture contains a deep blue Cu(II)–tartrate complex. Heating with an aliphatic aldehyde gives a reddish-brown precipitate of Cu₂O: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O. Aromatic aldehydes do not respond to Fehling's test, which is how benzaldehyde is distinguished from ethanal or propanal.

Haloform (iodoform) reaction. Compounds containing the CH₃CO– group (ethanal, all methyl ketones) or the CH₃CH(OH)– group that is oxidised to it (ethanol, propan-2-ol) react with sodium hypohalite (NaOX, made from X₂ and NaOH) to give a haloform and the sodium salt of the acid with one carbon fewer: CH₃COR + 3NaOI → CHI₃ + RCOONa + 2NaOH. With iodine the yellow precipitate of iodoform is the test. Sodium hypohalite is itself the oxidant: the methyl group leaves as the haloform and the rest of the molecule becomes the carboxylate with one carbon fewer. The iodoform test separates ethanal from propanal, propanone from pentan-3-one, and acetophenone from benzaldehyde.

The α-hydrogens of an aldehyde or ketone are acidic (pK_a about 20) because the electron-withdrawing carbonyl group pulls electron density from the α C–H bond, and because the carbanion (enolate) formed on losing the proton is stabilised by resonance: the negative charge is shared between the α-carbon and the carbonyl oxygen.

Aldol reaction. Aldehydes and ketones that have at least one α-hydrogen, when treated with dilute alkali, give β-hydroxy aldehydes (aldols) or β-hydroxy ketones (ketols). Two molecules of ethanal with dilute NaOH give 3-hydroxybutanal, CH₃CH(OH)CH₂CHO; on heating this loses water to give but-2-enal (crotonaldehyde), CH₃CH=CHCHO, the aldol condensation product. Propanone gives 4-hydroxy-4-methylpentan-2-one (diacetone alcohol), which dehydrates to 4-methylpent-3-en-2-one (mesityl oxide). The three steps the board wants: (1) OH⁻ removes an α-hydrogen to give the enolate; (2) the enolate attacks the carbonyl carbon of a second molecule to give an alkoxide; (3) the alkoxide takes a proton from water to give the aldol; then heat dehydrates it to the α,β-unsaturated carbonyl compound.

Cross aldol condensation. When two different carbonyl compounds, both with α-hydrogens, are mixed, four products result (two self-condensation and two cross products). Ethanal plus propanal, for example, gives but-2-enal, 2-methylpent-2-enal, pent-2-enal and 2-methylbut-2-enal. Ketones can also be used as one component: benzaldehyde (no α-H) plus acetophenone gives a single product, benzalacetophenone (C₆H₅CH=CHCOC₆H₅), because only one partner can form an enolate.

Cannizzaro reaction. Aldehydes that have no α-hydrogen (methanal, benzaldehyde, 2,2-dimethylpropanal) undergo self oxidation and reduction (disproportionation) with concentrated alkali: one molecule is reduced to the alcohol and another oxidised to the salt of the acid. 2HCHO + conc. NaOH → CH₃OH + HCOONa; 2C₆H₅CHO + conc. KOH → C₆H₅CH₂OH + C₆H₅COOK. Formaldehyde undergoes Cannizzaro but not aldol; ethanal undergoes aldol but not Cannizzaro. Between two different aldehydes without α-H (a crossed Cannizzaro), formaldehyde is always the one oxidised because it is the stronger reducing agent.

Electrophilic substitution in aromatic aldehydes and ketones. The –CHO and –COR groups are deactivating and meta-directing, because the carbonyl withdraws electrons from the ring by resonance and the ortho and para positions are left electron-poor. So benzaldehyde with conc. HNO₃/H₂SO₄ gives 3-nitrobenzaldehyde, and acetophenone nitrates to 3-nitroacetophenone. This matters in conversions: to make m-nitrobenzoic acid from benzene you must acylate or oxidise to the carbonyl first and nitrate later; to make p-nitrobenzoic acid you nitrate toluene first (–CH₃ is ortho/para directing) and then oxidise the methyl group.

Uses. Formaldehyde as 40% aqueous solution (formalin) preserves biological specimens and is used to make Bakelite (with phenol), urea–formaldehyde glues and other polymers. Acetaldehyde is used to make acetic acid, ethyl acetate, vinyl acetate, polymers and drugs. Benzaldehyde is used in perfumery and in dye intermediates. Acetone and ethyl methyl ketone are common industrial solvents. Butyraldehyde, vanillin, acetophenone, camphor and other aldehydes and ketones are used in blending perfumes and flavouring agents. A single 1-mark question on formalin or on the solvent use of acetone is the most this part ever fetches; spend your time on the reactions instead.

Also from the chemical reactions list: aldehydes and ketones react with alcohols to form acetals and ketals (covered above), and with Grignard reagents to form alcohols. The complete set of nucleophilic additions, reductions, oxidations and α-hydrogen reactions is what the 5-mark question draws on, so make one page with every reagent and every product for ethanal, propanone and benzaldehyde, and revise from that page.

Carboxylic acids contain the –COOH group. Aliphatic acids are named by replacing the –e of the alkane by –oic acid, with the –COOH carbon as C-1: HCOOH methanoic acid (formic), CH₃COOH ethanoic acid (acetic), CH₃CH₂COOH propanoic acid (propionic), CH₃(CH₂)₂COOH butanoic acid (butyric), CH₃(CH₂)₃COOH pentanoic acid (valeric), CH₃(CH₂)₁₄COOH hexadecanoic acid (palmitic). Dicarboxylic acids take the suffix –dioic acid and the parent chain includes both carboxyl carbons: HOOC–COOH ethanedioic (oxalic), HOOCCH₂COOH propanedioic (malonic), HOOC(CH₂)₂COOH butanedioic (succinic), HOOC(CH₂)₄COOH hexanedioic (adipic), C₆H₄(COOH)₂ (1,2) benzene-1,2-dicarboxylic (phthalic). When –COOH is on a ring the suffix is carboxylic acid: cyclohexanecarboxylic acid; C₆H₅COOH is benzoic acid (benzenecarboxylic acid).

Common names come from the natural source (formic from ants, acetic from vinegar, butyric from rancid butter) and label substituent positions with α, β, γ from the carbon next to –COOH, so CH₃CH(OH)COOH is α-hydroxypropionic acid (lactic acid) and 2-hydroxypropanoic acid in IUPAC.

Structure. The carboxyl carbon is sp² hybridised and the C=O, C–OH and R groups lie in a plane with angles of about 120°. The group is best described by three resonance structures: the neutral form, a form with C=O⁺–H (positive on the hydroxyl oxygen) and a form with C⁺–O⁻. The lone pair on the –OH oxygen is delocalised into the carbonyl, so the carbonyl carbon is less electrophilic than in an aldehyde or ketone. That is why carboxylic acids do not give the typical nucleophilic addition reactions of carbonyls (no reaction with HCN, NaHSO₃, 2,4-DNP, no Tollens' test) even though they contain C=O. The same delocalisation, and more strongly the resonance of the carboxylate ion, explains the acidity discussed in section 8.9.

From primary alcohols and aldehydes. Oxidation with acidified or alkaline KMnO₄, K₂Cr₂O₇/H₂SO₄, or CrO₃ converts RCH₂OH and RCHO to RCOOH. Mild oxidants such as Tollens' reagent oxidise aldehydes but not alcohols.

From alkylbenzenes. Vigorous oxidation with alkaline KMnO₄ followed by acidification, or with acidic K₂Cr₂O₇, converts the whole side chain to –COOH whatever its length, provided the benzylic carbon has at least one hydrogen. Toluene and ethylbenzene both give benzoic acid; p-xylene gives terephthalic acid. Tertiary alkylbenzenes (tert-butylbenzene) are not oxidised because there is no benzylic hydrogen.

From nitriles and amides. Nitriles are hydrolysed by dilute acid or alkali first to amides and then to acids: RCN + H₂O → (H⁺) RCONH₂ → (H⁺, H₂O) RCOOH + NH₄⁺. Under mild conditions the hydrolysis stops at the amide. This is the route that adds a carbon: R–X → (KCN) R–CN → (H₃O⁺) R–COOH, for example bromomethane to ethanoic acid.

From Grignard reagents. The Grignard reagent adds to solid carbon dioxide (dry ice) to give the magnesium salt of the acid, which is acidified: RMgX + CO₂ → RCOOMgX → (H₃O⁺) RCOOH + Mg(OH)X. Like the nitrile route this gives an acid with one more carbon than the halide. Bromobenzene → phenylmagnesium bromide → benzoic acid is the standard conversion.

From acyl halides and anhydrides. Acid chlorides are hydrolysed by water to acids, or by aqueous base to the carboxylate which is then acidified. Anhydrides on hydrolysis give two molecules of the acid.

From esters. Acidic hydrolysis gives the acid and the alcohol directly. Basic hydrolysis (saponification) gives the carboxylate salt and the alcohol; the salt is acidified to release the acid.

For the exam, know which routes lengthen the chain (nitrile and Grignard) and which keep it (oxidation and hydrolysis); most conversion questions turn on that.

Aliphatic carboxylic acids up to nine carbons are colourless liquids with unpleasant odours; higher acids are waxy solids that are almost odourless because of low volatility. Their boiling points are higher than those of aldehydes, ketones and even alcohols of comparable molar mass. The reason is extensive intermolecular hydrogen bonding: in the vapour phase and in aprotic solvents carboxylic acids exist as cyclic dimers in which two O–H···O hydrogen bonds hold two molecules in an eight-membered ring. Acetic acid, for example, has a vapour-density that corresponds to the dimer (CH₃COOH)₂. Alcohols form hydrogen bonds too, but only one per molecule on average and less strongly, because the O–H of an acid is more polarised.

The simple aliphatic acids up to four carbons are miscible with water because they hydrogen bond with water; solubility falls with increasing chain length as the hydrophobic hydrocarbon part grows. Benzoic acid, the simplest aromatic acid, is nearly insoluble in cold water but dissolves in hot water. All carboxylic acids are soluble in less polar organic solvents such as benzene, ether, alcohol and chloroform.

Expect the ordering question in this form: arrange CH₃COOH, CH₃CH₂OH, CH₃CHO and CH₃OCH₃ by boiling point and give the reason. The answer is CH₃COOH > CH₃CH₂OH > CH₃CHO > CH₃OCH₃, with dimeric hydrogen bonding, single hydrogen bonding, dipole–dipole and weak dipole–dipole interactions respectively.

Carboxylic acids react with active metals to give hydrogen and with alkalis to give salts. They also liberate CO₂ from weaker acids' salts: RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂. Phenols do not do this, so the brisk effervescence with sodium hydrogencarbonate is the standard test that distinguishes a carboxylic acid (benzoic acid) from a phenol.

Why acids are acidic. RCOOH ⇌ RCOO⁻ + H⁺, with K_a = [RCOO⁻][H⁺]/[RCOOH] and pK_a = −log K_a. Smaller pK_a means the stronger acid. Carboxylic acids have pK_a around 4–5, weaker than mineral acids but far stronger than alcohols (pK_a about 16–18) and phenols (pK_a about 10). The reason is the carboxylate ion, RCOO⁻, in which the negative charge is delocalised equally over two electronegative oxygen atoms; the two resonance structures are equivalent. In phenoxide the charge is delocalised over the ring carbons, which are less electronegative, and the contributing structures are not equivalent, so phenoxide is less stabilised. Alkoxide has no resonance at all.

Effect of substituents. Anything that stabilises the carboxylate ion increases acidity. Electron-withdrawing groups (–NO₂, –CN, –X, –C₆H₅) disperse the negative charge by their −I effect and raise K_a; electron-releasing alkyl groups (+I) intensify the charge and lower it. So HCOOH > CH₃COOH > CH₃CH₂COOH, and for halogens the effect increases with electronegativity, number and closeness to –COOH: FCH₂COOH > ClCH₂COOH > BrCH₂COOH; Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH; 2-chlorobutanoic > 3-chlorobutanoic > 4-chlorobutanoic > butanoic acid.

In aromatic acids, electron-withdrawing groups on the ring (–NO₂) increase acidity and electron-donating ones (–OCH₃, –CH₃) decrease it, so 4-nitrobenzoic acid > benzoic acid > 4-methoxybenzoic acid. Benzoic acid is stronger than acetic acid because the sp² phenyl carbon is electron-withdrawing relative to a methyl group.

Anhydride formation. Two molecules of an acid lose water on heating with a dehydrating agent such as P₂O₅, or one molecule of acid reacts with an acid chloride: 2CH₃COOH → (P₂O₅, Δ) (CH₃CO)₂O + H₂O.

Esterification. An acid heated with an alcohol or a phenol in the presence of a mineral acid (conc. H₂SO₄ or dry HCl gas) gives the ester: RCOOH + R'OH ⇌ RCOOR' + H₂O. The reaction is reversible; the ester is distilled off or the water removed to drive it. The mechanism is asked as a 3-mark question and has five steps: (1) protonation of the carbonyl oxygen by H⁺, making the carbonyl carbon a better electrophile; (2) nucleophilic attack by the alcohol oxygen on that carbon to give a tetrahedral intermediate; (3) proton transfer from the –OR'H⁺ oxygen to one of the –OH groups; (4) loss of a water molecule to regenerate the C=O with a proton on it; (5) loss of H⁺ to give the ester. The oxygen of the alcohol ends up in the ester (shown by ¹⁸O labelling); the water comes from the acid's –OH.

Acyl chlorides. The hydroxyl is replaced by chlorine with PCl₅, PCl₃ or thionyl chloride: RCOOH + PCl₅ → RCOCl + POCl₃ + HCl; 3RCOOH + PCl₃ → 3RCOCl + H₃PO₃; RCOOH + SOCl₂ → RCOCl + SO₂ + HCl. Thionyl chloride is preferred because both by-products are gases and the acid chloride is left pure.

Amides. The acid reacts with ammonia to give the ammonium salt, which loses water on heating to give the amide: CH₃COOH + NH₃ → CH₃COONH₄ → (Δ, −H₂O) CH₃CONH₂. Benzoic acid similarly gives benzamide, and phthalic acid gives phthalimide through the diammonium salt.

Reduction. Carboxylic acids are reduced to primary alcohols by LiAlH₄ in dry ether followed by hydrolysis, or by diborane, B₂H₆. Diborane is selective: it reduces –COOH but does not reduce ester, nitro or halogen groups present in the molecule. Sodium borohydride does not reduce carboxylic acids at all. Catalytic hydrogenation does not reduce the –COOH group under ordinary conditions. Example: CH₃COOH → (i) LiAlH₄/ether, (ii) H₃O⁺ → CH₃CH₂OH.

Decarboxylation. Carboxylic acids lose carbon dioxide when their sodium salts are heated with soda lime (NaOH + CaO in the ratio 3:1), giving the hydrocarbon with one carbon fewer: RCOONa + NaOH → (CaO, Δ) R–H + Na₂CO₃. Sodium acetate gives methane; sodium benzoate gives benzene. This is the way to shorten a chain by one carbon in conversion questions (propanoic acid to ethane, for example).

Kolbe electrolysis. Electrolysis of a concentrated aqueous solution of the sodium or potassium salt of a carboxylic acid gives an alkane at the anode with twice the number of alkyl carbons: 2RCOO⁻ → R–R + 2CO₂ + 2e⁻ at the anode, with H₂ and NaOH formed at the cathode. Sodium acetate gives ethane, sodium propanoate gives butane. Mechanistically the carboxylate loses an electron to give an acyloxy radical, which loses CO₂ to give an alkyl radical, and two alkyl radicals couple. (Kolbe electrolysis is listed under decarboxylation in NCERT; it is a low-frequency 1- or 2-mark item.)

Hell–Volhard–Zelinsky (HVZ) reaction. Carboxylic acids that have α-hydrogens are halogenated at the α-carbon by chlorine or bromine in the presence of a small amount of red phosphorus, giving α-halo acids: CH₃COOH → (Cl₂/red P; then H₂O) ClCH₂COOH, and with excess Cl₂ further to Cl₂CHCOOH and Cl₃CCOOH. Red phosphorus forms PX₃ which converts the acid to the acyl halide; the acyl halide enolises and is halogenated at the α-position. The α-halo acid is the entry to α-hydroxy acids (with aqueous alkali) and α-amino acids (with ammonia), which appear in conversions such as acetic acid to glycine.

Ring substitution in aromatic acids. The –COOH group is deactivating and meta-directing because it withdraws electrons from the ring by resonance and induction. Benzoic acid nitrates with conc. HNO₃/H₂SO₄ to 3-nitrobenzoic acid and brominates with Br₂/FeBr₃ to 3-bromobenzoic acid. Carboxylic acids do not undergo Friedel–Crafts alkylation or acylation because the deactivated ring will not react and because the –COOH group forms a complex with the AlCl₃ catalyst (the acid's –OH is a Lewis base and ties up the Lewis acid). This is a standard 2-mark reason.

Uses. Methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Ethanoic acid is used as a solvent and as vinegar in food. Hexanedioic acid is used for nylon-6,6. Esters of benzoic acid are used in perfumery; sodium benzoate is a food preservative. Higher fatty acids are used for soaps and detergents.

Put HVZ next to the aldol and Cannizzaro reactions on your name-reaction page; together with Rosenmund, Etard, Stephen, Gatterman–Koch, Clemmensen and Wolff–Kishner these are the reactions the 3-mark 'write the following name reactions' question is drawn from, and the examiner expects the reagent, the condition and a balanced example for each.