Aldehydes, Ketones and Carboxylic Acids: NEET notes

1. Names and the carbonyl group

NCERT §8.1

• The carbonyl group is >C=O. In an aldehyde its carbon is joined to one carbon and one hydrogen (methanal has two H); in a ketone it is joined to two carbons. Adding an –OH to the carbonyl carbon gives a carboxylic acid; –NH₂ gives an amide and a halogen gives an acyl halide. Esters and anhydrides are derivatives of the acids.
• Carbonyl compounds run through plant and animal life and give many natural smells: cinnamon owes its scent to cinnamaldehyde, vanilla beans to vanillin, and meadow sweet to salicylaldehyde. Acetone is made on a large scale as a solvent, and the family also goes into fabrics, plastics, perfumes, resins, paints and adhesives.
• Common names of aldehydes come from the common name of the matching acid, with the '-ic acid' ending changed to 'aldehyde' (formic acid → formaldehyde). Greek letters α, β, γ… mark substituent positions, α being the carbon next to –CHO.
• Common names of ketones name the two groups on the carbonyl (methyl n-propyl ketone); the simplest, dimethyl ketone, is acetone. Alkyl phenyl ketones take the acyl name plus 'phenone', as in acetophenone.
• IUPAC names change the final '-e' of the alkane to '-al' for aldehydes and '-one' for ketones. The aldehyde carbon is C-1; a ketone chain is numbered from the end nearer the C=O, and in a cyclic ketone the carbonyl carbon is C-1.
• When –CHO is attached to a ring the suffix 'carbaldehyde' follows the ring name, and the ring carbon bearing –CHO is C-1. The simplest aromatic aldehyde is benzenecarbaldehyde, also accepted by IUPAC as benzaldehyde.
• Name pairs (common = IUPAC): formaldehyde = methanal; acetaldehyde = ethanal; isobutyraldehyde = 2-methylpropanal; valeraldehyde = pentanal; acrolein = prop-2-enal; methyl n-propyl ketone = pentan-2-one; diisopropyl ketone = 2,4-dimethylpentan-3-one; mesityl oxide = 4-methylpent-3-en-2-one.
• The carbonyl carbon is sp² hybridised: three σ bonds lie in one plane about 120° apart, and the leftover p orbital overlaps a p orbital of oxygen to form the π bond, whose cloud lies above and below that plane. Oxygen also holds two lone pairs.
• Oxygen is more electronegative than carbon, so the C=O bond is polar: the carbon is an electrophilic (Lewis acid) centre and the oxygen a nucleophilic (Lewis base) centre. A dipolar resonance form, C⁺–O⁻, explains the large dipole moments; carbonyl compounds are more polar than ethers.

2. Preparing aldehydes and ketones

NCERT §8.2.1–8.2.2

• Oxidising alcohols: primary alcohols give aldehydes and secondary alcohols give ketones (Unit 7).
• Dehydrogenating alcohols: vapour of a volatile alcohol is passed over heated Ag or Cu. This industrial route turns 1° alcohols into aldehydes and 2° alcohols into ketones.
• Ozonolysis: an alkene treated with ozone and then zinc dust and water splits at the C=C to give aldehydes, ketones or both, depending on the groups around the double bond.
• Hydration of alkynes: water adds to ethyne in the presence of H₂SO₄ and HgSO₄ to give acetaldehyde. Every other alkyne gives a ketone this way.
• Rosenmund reduction: an acyl chloride is hydrogenated over palladium on barium sulphate, and the reaction stops at the aldehyde.
• Stephen reaction: a nitrile is reduced by stannous chloride and hydrochloric acid to an imine, and hydrolysing the imine gives the aldehyde.
• DIBAL-H, diisobutylaluminium hydride, reduces nitriles selectively to imines, which hydrolyse to aldehydes. Esters are also reduced to aldehydes by DIBAL-H.
• Worked reagents: hexan-1-ol → hexanal with PCC; cyclohexanol → cyclohexanone with anhydrous CrO₃; ethanenitrile → ethanal with DIBAL-H; allyl alcohol → propenal with PCC; but-2-ene → ethanal with O₃ then H₂O–Zn dust.

3. Aromatic aldehydes and ketones

NCERT §8.2.2–8.2.3

• Strong oxidising agents take toluene all the way to benzoic acid. To stop at benzaldehyde the methyl group is first turned into an intermediate that resists further oxidation.
• Etard reaction: chromyl chloride, CrO₂Cl₂, converts the methyl group of toluene into a chromium complex, and hydrolysis of the complex gives benzaldehyde.
• With chromic oxide, CrO₃, in acetic anhydride, toluene gives benzylidene diacetate, which aqueous acid hydrolyses to benzaldehyde.
• Side-chain chlorination of toluene gives benzal chloride, C₆H₅CHCl₂, and hydrolysing it gives benzaldehyde. This is a commercial method for benzaldehyde.
• Gatterman-Koch reaction: benzene or a derivative treated with carbon monoxide and hydrogen chloride, with anhydrous aluminium chloride or cuprous chloride, gives benzaldehyde or a substituted benzaldehyde.
• Ketones from acyl chlorides: a Grignard reagent and cadmium chloride give a dialkylcadmium, R₂Cd, which turns an acyl chloride into a ketone.
• Ketones from nitriles: a nitrile treated with a Grignard reagent and then hydrolysed gives a ketone.
• Friedel-Crafts acylation: benzene or a substituted benzene with an acid chloride and anhydrous AlCl₃ gives the aromatic ketone, for example acetophenone from benzene and ethanoyl chloride.

4. Physical properties of aldehydes and ketones

NCERT §8.3

• Methanal is a gas at room temperature and ethanal a volatile liquid. The other aldehydes and ketones are liquids or solids.
• Dipole–dipole attraction between C=O groups gives weak association, so aldehydes and ketones boil above hydrocarbons and ethers of similar mass. With no O–H they cannot hydrogen-bond to each other, so they boil below alcohols of similar mass.
• Boiling points for masses 58–60: n-butane 273 K (58), methoxyethane 281 K (60), propanal 322 K (58), acetone 329 K (58), propan-1-ol 370 K (60).
• Worked order for masses 72–74, lowest first: n-butane < ethoxyethane < butanal < butan-1-ol. Butanal is more polar than the ether; only the alcohol is hydrogen-bonded.
• Methanal, ethanal and propanone mix with water in all proportions because their oxygen accepts hydrogen bonds from water. Solubility falls quickly as the alkyl chain grows; all of them dissolve well in benzene, ether, methanol and chloroform.
• The lower aldehydes smell sharp and pungent. Larger molecules smell less pungent and more fragrant, and many natural aldehydes and ketones are blended into perfumes and flavours.

5. Nucleophilic addition

NCERT §8.4

• Unlike alkenes, which add electrophiles, aldehydes and ketones add nucleophiles across C=O.
• Mechanism: the nucleophile attacks the carbonyl carbon from a direction roughly perpendicular to the plane of its sp² orbitals. The carbon becomes sp³, giving a tetrahedral alkoxide intermediate, which takes a proton from the medium. Net result: Nu⁻ and H⁺ add across C=O.
• Aldehydes are generally more reactive than ketones. Sterically, a ketone's two larger groups crowd the approach to the carbon; electronically, two alkyl groups lower the carbon's positive character more than one does.
• Benzaldehyde is less reactive than propanal: resonance with the ring reduces the polarity of its C=O, so its carbonyl carbon is less electrophilic.
• Worked order of reactivity, lowest first: butanone < propanone < propanal < ethanal; and acetophenone < p-tolualdehyde, benzaldehyde < p-nitrobenzaldehyde.
• HCN adds to give cyanohydrins. Pure HCN reacts very slowly, so a base is added to generate CN⁻, a stronger nucleophile. Cyanohydrins are useful intermediates in synthesis.
• Sodium hydrogensulphite (NaHSO₃) adds to give water-soluble addition products. For most aldehydes the equilibrium lies to the right, for most ketones to the left (steric reasons). Dilute mineral acid or alkali regenerates the carbonyl compound, so the reaction is used to separate and purify aldehydes.
• With dry HCl, an aldehyde adds one molecule of a monohydric alcohol to give a hemiacetal (an alkoxyalcohol) and a second to give an acetal, a gem-dialkoxy compound. Ketones with ethylene glycol give cyclic ethylene glycol ketals. Dry HCl protonates the carbonyl oxygen and makes the carbon more electrophilic.
• Acetals and ketals are hydrolysed by aqueous mineral acid back to the aldehyde or ketone. Grignard reagents also add to C=O (Unit 7).

6. Ammonia derivatives and reduction

NCERT §8.4

• Ammonia and its derivatives, H₂N–Z, add to C=O in a reversible, acid-catalysed reaction. The intermediate loses water quickly to form >C=N–Z, which pulls the equilibrium towards product.
• Table 8.2 (Z → reagent → product): –H, ammonia → imine; –R, amine → substituted imine (Schiff's base); –OH, hydroxylamine → oxime; –NH₂, hydrazine → hydrazone; –NHC₆H₅, phenylhydrazine → phenylhydrazone; –NHCONH₂, semicarbazide → semicarbazone; 2,4-dinitrophenylhydrazine → 2,4-dinitrophenylhydrazone.
• With 2,4-DNP an aldehyde or ketone gives a coloured solid (yellow, orange or red), which chemists use to identify it.
• Semicarbazide has two –NH₂ groups, but only the one joined to the other nitrogen forms semicarbazones; the –NH₂ next to C=O has its lone pair shared with the carbonyl by resonance.
• NaBH₄, LiAlH₄ or catalytic hydrogenation reduce aldehydes to primary alcohols and ketones to secondary alcohols (Unit 7).
• Clemmensen reduction: zinc amalgam and concentrated hydrochloric acid turn C=O into CH₂.
• Wolff-Kishner reduction: the hydrazone formed with hydrazine is heated with sodium or potassium hydroxide in a high-boiling solvent such as ethylene glycol, and C=O again becomes CH₂.

7. Oxidation and the tests

NCERT §8.4

• Aldehydes are oxidised easily to carboxylic acids by nitric acid, potassium permanganate, potassium dichromate and similar reagents, and even by mild oxidising agents such as Tollens' and Fehling's reagents.
• Ketones are oxidised only under vigorous conditions (strong oxidants, high temperature). C–C bonds break, giving a mixture of carboxylic acids with fewer carbons than the ketone.
• Tollens' test: an aldehyde warmed with freshly prepared ammoniacal silver nitrate gives a bright silver mirror. The aldehyde becomes the carboxylate anion; the medium is alkaline. RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 2H₂O + 4NH₃.
• Fehling's test: Fehling solution A (aqueous copper sulphate) and solution B (alkaline sodium potassium tartrate, Rochelle salt) are mixed in equal amounts. Heating with an aldehyde gives a reddish-brown precipitate of Cu₂O. Aromatic aldehydes do not give this test.
• Haloform reaction: aldehydes and ketones with at least one CH₃ on the carbonyl carbon (methyl ketones) are oxidised by sodium hypohalite to the sodium salt of an acid with one carbon fewer, and the CH₃ becomes haloform. A C=C in the molecule is not attacked.
• The iodoform test (sodium hypoiodite, yellow CHI₃) detects a CH₃CO group, or a CH₃CH(OH) group that is oxidised to CH₃CO under the test conditions.
• Worked example: C₈H₈O gives an orange-red 2,4-DNP precipitate and a positive iodoform test, reduces neither Tollens' nor Fehling's reagent, does not decolourise bromine water or Baeyer's reagent, and on drastic oxidation gives C₇H₆O₂. It is acetophenone; the acid is benzoic acid.

8. α-Hydrogen reactions and uses

NCERT §8.4–8.5

• Hydrogens on the carbon next to C=O (α-hydrogens) are acidic: the carbonyl group withdraws electrons strongly, and the conjugate base is resonance-stabilised.
• Aldol reaction: an aldehyde or ketone with at least one α-hydrogen, with dilute alkali as catalyst, gives a β-hydroxy aldehyde (aldol) or β-hydroxy ketone (ketol). Ethanal gives 3-hydroxybutanal.
• The aldol or ketol loses water easily to give an α,β-unsaturated carbonyl compound, the aldol condensation product: 3-hydroxybutanal → but-2-enal. Ketones are included under the same name.
• Cross aldol condensation uses two different aldehydes and/or ketones. If both have α-hydrogens, four products form: ethanal with propanal gives but-2-enal, 2-methylpent-2-enal, pent-2-enal and 2-methylbut-2-enal.
• Cannizzaro reaction: an aldehyde with no α-hydrogen, heated with concentrated alkali, disproportionates. One molecule is reduced to the alcohol and another oxidised to the carboxylic acid salt: 2HCHO + conc. KOH → CH₃OH + HCOOK.
• In aromatic aldehydes and ketones the carbonyl group deactivates the ring and directs electrophiles to the meta position.
• Formalin, a 40% solution of formaldehyde, preserves biological specimens; formaldehyde also makes bakelite (a phenol-formaldehyde resin) and urea-formaldehyde glues.
• Acetaldehyde is the raw material for drugs, polymers, vinyl acetate, ethyl acetate and, mainly, acetic acid. Perfume and dye makers use benzaldehyde; acetone and ethyl methyl ketone are industrial solvents; camphor, acetophenone, vanillin and butyraldehyde are prized for their smell and taste.

9. Carboxylic acids: names and structure

NCERT §8.6

• A carboxylic acid carries the carboxyl group, –COOH: a carbonyl joined to a hydroxyl, which gives the name. Acids are aliphatic (RCOOH) or aromatic (ArCOOH).
• Higher aliphatic acids with C₁₂–C₁₈ chains, the fatty acids, occur in natural fats as glycerol esters. Acids are the starting point for anhydrides, esters, acid chlorides and amides.
• Many acids were isolated early and keep common names ending in '-ic acid' taken from their source: formic acid from red ants (Latin formica), acetic acid from vinegar (acetum), butyric acid from rancid butter (butyrum).
• IUPAC names change the '-e' of the alkane to '-oic acid', with the carboxyl carbon as C-1. For two or more –COOH groups the chain without them is named and 'dicarboxylic acid', 'tricarboxylic acid' is added, with numbers for the positions.
• Name pairs: methanoic (formic), ethanoic (acetic), propanoic (propionic), butanoic (butyric) and 2-methylpropanoic (isobutyric) acids; ethanedioic (oxalic), propanedioic (malonic), butanedioic (succinic), pentanedioic (glutaric) and hexanedioic (adipic) acids; benzenecarboxylic acid (benzoic acid), 2-phenylethanoic (phenylacetic) acid and benzene-1,2-dicarboxylic (phthalic) acid.
• The carboxyl carbon is planar, its three bonds roughly 120° apart. The –OH oxygen shares a lone pair with C=O by resonance, so this carbon is a weaker electrophile than the carbon of an aldehyde or ketone.

10. Preparing carboxylic acids

NCERT §8.7

• Primary alcohols are oxidised to carboxylic acids by KMnO₄ in neutral, acidic or alkaline solution, or by K₂Cr₂O₇ or CrO₃ in acid (Jones reagent). Aldehydes give acids even with mild oxidants.
• Alkylbenzenes are oxidised vigorously by chromic acid or by acidic or alkaline KMnO₄. The whole side chain, however long, ends up as –COOH; primary and secondary alkyl groups react, but a tertiary group is untouched. Suitably substituted alkenes also give acids.
• Nitriles are hydrolysed with H⁺ or OH⁻ first to amides and then to acids; milder conditions stop the reaction at the amide.
• Grignard reagents react with carbon dioxide (dry ice) to give carboxylate salts, and mineral acid then releases the acid.
• Because nitriles and Grignard reagents are made from alkyl halides, these two routes turn an alkyl halide into an acid with one carbon more (ascending the series).
• Acyl chlorides hydrolyse with water, and faster with aqueous base, which gives carboxylate ions that acid converts to the acid. Anhydrides hydrolyse with water.
• Esters give the acid directly by acidic hydrolysis; basic hydrolysis gives the carboxylate, which is then acidified.
• Worked: 4-methylacetophenone → benzene-1,4-dicarboxylic acid and ethylbenzene → benzoic acid by KMnO₄ side-chain oxidation; cyclohexene → hexane-1,6-dioic acid by KMnO₄/H⁺; bromobenzene → benzoic acid via Mg (Grignard), CO₂, then H₃O⁺.

11. Physical properties and acidity

NCERT §8.8–8.9.1

• Aliphatic acids with up to nine carbons are colourless liquids with unpleasant smells; the higher acids are wax-like, nearly odourless solids because they hardly evaporate.
• Acids boil higher than aldehydes, ketones and even alcohols of similar mass because their molecules are held together more extensively by hydrogen bonds. These bonds survive even in the vapour, where most acids, like those in aprotic solvents, exist as dimers.
• Acids with up to four carbons mix with water through hydrogen bonding; solubility falls as the chain grows, and higher acids are practically insoluble. Benzoic acid is nearly insoluble in cold water. All dissolve in benzene, ether, alcohol and chloroform.
• Acids give hydrogen with electropositive metals and salts with alkalis. Unlike phenols they also react with carbonates and hydrogencarbonates to release CO₂, the test for a carboxyl group.
• In water an acid gives a resonance-stabilised carboxylate ion and H₃O⁺. Strength is quoted as pKa = −log Ka; a smaller pKa means a stronger acid. pKa < 1 is strong, 1–5 moderately strong, 5–15 weak and above 15 extremely weak.
• pKa values: hydrochloric acid −7.0; trifluoroacetic acid (the strongest carboxylic acid) 0.23; benzoic acid 4.19; acetic acid 4.76; phenol 10; ethanol about 16.
• Acids are stronger than phenols: the carboxylate ion has two equivalent resonance forms with the charge on the two electronegative oxygens, while phenoxide's forms are unequal and put charge on less electronegative ring carbons.
• Electron-withdrawing groups stabilise the carboxylate and raise acidity; electron-donating groups lower it. Group effect, increasing: Ph < I < Br < Cl < F < CN < NO₂ < CF₃. Strongest first: CF₃COOH > CCl₃COOH > CHCl₂COOH > NO₂CH₂COOH > NC–CH₂COOH > FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > ClCH₂CH₂COOH > C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH.
• A phenyl or vinyl group attached directly to –COOH raises acidity, because the sp² carbon it bonds to is more electronegative. On the ring: 4-nitrobenzoic acid pKa 3.41 < benzoic acid 4.19 < 4-methoxybenzoic acid 4.46.

12. Reactions and uses of carboxylic acids

NCERT §8.9.2–8.10

• Heating with mineral acids such as H₂SO₄, or with P₂O₅, gives the anhydride.
• Esterification: an acid and an alcohol or phenol give an ester with concentrated H₂SO₄ or HCl gas as catalyst. The mechanism is nucleophilic acyl substitution: protonated C=O is attacked by the alcohol, a proton shift turns –OH into –OH₂⁺, water leaves, and the protonated ester loses H⁺. Removing water or ester as it forms drives the reversible reaction.
• PCl₅, PCl₃ or SOCl₂ replace –OH by Cl to give the acyl chloride. Thionyl chloride is preferred because its by-products, SO₂ and HCl, are gases that escape, leaving the product easy to purify.
• Ammonia gives the ammonium salt, which on strong heating loses water to give the amide.
• LiAlH₄, or better diborane, reduces acids to primary alcohols. Diborane leaves ester, nitro and halo groups largely alone; NaBH₄ does not reduce –COOH.
• Decarboxylation: the sodium salt heated with sodalime (NaOH and CaO in the ratio 3 : 1) loses CO₂ to give a hydrocarbon. Kolbe electrolysis of an aqueous alkali-metal salt gives a hydrocarbon with twice the carbons of the acid's alkyl group.
• Hell-Volhard-Zelinsky reaction: an acid with an α-hydrogen, treated with chlorine or bromine and a little red phosphorus, gives the α-halocarboxylic acid.
• In aromatic acids –COOH deactivates the ring and directs to meta. They do not undergo Friedel-Crafts reactions: the ring is deactivated and the AlCl₃ catalyst binds to the carboxyl group.
• Uses: methanoic acid in rubber, textile, dyeing, leather and electroplating; ethanoic acid as a solvent and as vinegar; hexanedioic acid for nylon-6,6; benzoate esters in perfumes; sodium benzoate as a food preservative; higher fatty acids for soaps and detergents.

Must-know facts

1. Carbonyl carbon: sp², trigonal planar, ~120°; C=O polar (C electrophilic, O nucleophilic).
2. Aldehyde from nitrile: Stephen (SnCl₂ + HCl, then hydrolysis) or DIBAL-H; DIBAL-H also turns esters into aldehydes.
3. Rosenmund: RCOCl + H₂ over Pd–BaSO₄ → RCHO.
4. Benzaldehyde from toluene: Etard (CrO₂Cl₂), CrO₃ in acetic anhydride, or side-chain chlorination then hydrolysis; from benzene: Gatterman-Koch (CO + HCl, anhydrous AlCl₃ or CuCl).
5. Ketones: dialkylcadmium + acyl chloride; nitrile + Grignard then hydrolysis; Friedel-Crafts acylation.
6. b.p. (K) at mass 58–60: n-butane 273 < methoxyethane 281 < propanal 322 < acetone 329 < propan-1-ol 370.
7. Nucleophilic addition: attack perpendicular to the plane, sp² → sp³ tetrahedral alkoxide, then H⁺. Aldehydes > ketones (steric + electronic).
8. HCN addition needs base (CN⁻); NaHSO₃ adducts purify aldehydes; dry HCl + alcohol → hemiacetal → acetal; ketone + ethylene glycol → ketal.
9. H₂N–Z products: oxime (NH₂OH), hydrazone (N₂H₄), phenylhydrazone, semicarbazone, 2,4-DNP derivative (yellow/orange/red), Schiff's base (RNH₂).
10. C=O → CH₂: Clemmensen (Zn–Hg, conc. HCl) or Wolff-Kishner (NH₂NH₂, then KOH/NaOH in ethylene glycol, heat).
11. Tollens' (silver mirror) and Fehling's (red-brown Cu₂O) are given by aldehydes, not ketones; aromatic aldehydes fail Fehling's.
12. Iodoform (yellow CHI₃) = CH₃CO– or CH₃CH(OH)–; haloform gives an acid salt with one C fewer.
13. Aldol: needs α-H, dilute alkali → β-hydroxy carbonyl → α,β-unsaturated on losing water. Cannizzaro: no α-H, concentrated alkali, disproportionation.
14. Formalin = 40% formaldehyde.
15. Acids from Grignard + CO₂ and from nitrile hydrolysis add one carbon to the alkyl halide.
16. Side-chain oxidation turns any 1° or 2° alkyl group on a ring into –COOH; a tertiary group is not oxidised.
17. Carboxylic acids boil above alcohols of similar mass; dimers in vapour; up to 4 C miscible with water.
18. pKa: HCl −7.0; CF₃COOH 0.23; 4-nitrobenzoic 3.41; benzoic 4.19; 4-methoxybenzoic 4.46; acetic 4.76; phenol 10; ethanol ~16.
19. Acids release CO₂ from NaHCO₃; phenols do not.
20. SOCl₂ is preferred for RCOCl because SO₂ and HCl escape as gases.
21. LiAlH₄ or B₂H₆ reduce –COOH to –CH₂OH; NaBH₄ does not.
22. Sodalime = NaOH : CaO = 3 : 1; decarboxylation loses one carbon. HVZ: X₂ + red P → α-halo acid.
23. –CHO, –COR and –COOH on a ring are deactivating and meta-directing; benzoic acid gives no Friedel-Crafts reaction.

Common traps

Formulas

Key terms

Carbonyl group

>C=O, a carbon double-bonded to oxygen.

Cyanohydrin

The product of HCN adding to C=O: a carbon carrying both –OH and –CN.

Hemiacetal

An alkoxyalcohol from an aldehyde and one molecule of alcohol.

Acetal / ketal

A gem-dialkoxy compound from an aldehyde (or ketone) and two alcohol groups.

Schiff's base

A substituted imine, >C=N–R, from a carbonyl compound and a primary amine.

2,4-DNP derivative

Coloured solid formed with 2,4-dinitrophenylhydrazine; identifies aldehydes and ketones.

α-Hydrogen

A hydrogen on the carbon next to the carbonyl group; acidic.

Aldol

A β-hydroxy aldehyde (or ketol, β-hydroxy ketone) from the aldol reaction.

Disproportionation

One species both oxidised and reduced, as in the Cannizzaro reaction.

Fatty acids

Aliphatic carboxylic acids with C₁₂–C₁₈ chains, found as glycerol esters in fats.

Jones reagent

CrO₃ in aqueous acid, an oxidant for alcohols.

Sodalime

NaOH and CaO in the ratio 3 : 1, used for decarboxylation.

Hell-Volhard-Zelinsky reaction

α-Halogenation of a carboxylic acid with Cl₂ or Br₂ and red phosphorus.