Minor students: the same paper (BCH2T03) is studied as Minor 3 in Sem IV by B.Sc students whose major is another subject (RTMNU syllabus 2025-26). The units are identical, so these notes are for you too.
Unit I: Orientation, Alkyl Halides and Aryl Halides
Syllabus topics (official RTMNU)
Orientation
Activating (–OH, –NH2) and deactivating (–Cl, –NO2, –COOH) substituents, their orientation and directive influence on further electrophilic substitution, o/p ratio.
Methods of formation and chemical reactions of alkyl benzene (Toluene) and biphenyl.
Alkyl halides : Nomenclature, classification, methods of formation, chemical reactions.
Mechanism of nucleophilic substitution reactions of alkyl halides SN1 and SN2 with energy profile diagrams.
Factors affecting SN1 and SN2 mechanisms.
Polyhalogen compounds : Chloroform and carbon tetrachloride – formation and chemical reactions.
Nuclear and side chain halogen derivatives of benzene: Halobenzene and benzyl halide preparation and reactions.
Orientation in electrophilic aromatic substitution
When benzene already carries one group (G), a second electrophile (E⁺) does not attack at random. The first group decides how fast the ring reacts (activating or deactivating) and where the new group goes (ortho/para or meta). This is called the directive influence.
Activating groups (ortho/para directing)
–OH, –NH₂, –OR, –NHCOCH₃, –CH₃: they push electrons into the ring.
+R (resonance) effect: the lone pair on O or N is delocalised into the ring. Resonance structures put a negative charge at the ortho and para carbons, so E⁺ attacks there.
The σ-complex (arenium ion) formed by o/p attack has an extra resonance form in which the positive charge sits on O or N (every atom has an octet), so it is more stable.
Example: phenol + Br₂ water → 2,4,6-tribromophenol (white ppt); aniline + Br₂ water → 2,4,6-tribromoaniline.
Deactivating groups
–NO₂, –COOH, –CHO, –CN, –SO₃H: withdraw electrons by –I and –R effects. Positive charge develops at ortho and para positions, so the meta position is the least deactivated. These are meta directors. Example: nitrobenzene + HNO₃/H₂SO₄ → m-dinitrobenzene.
Halogens (–Cl, –Br) are special: the strong –I effect deactivates the ring, but the lone pair (+R) still directs to ortho/para. Chlorobenzene nitrates slower than benzene but gives o- and p-chloronitrobenzene.
Group
Effect on rate
Directs to
–OH, –NH₂
Strongly activating
o/p
–CH₃
Weakly activating (+I, hyperconjugation)
o/p
–Cl, –Br
Weakly deactivating
o/p
–NO₂, –COOH
Strongly deactivating
m
Ortho/para ratio
Statistically there are two ortho positions and one para, so o:p should be 2:1.
In practice para is often major because of steric hindrance: a bulky group or bulky electrophile blocks the ortho position. Nitration of toluene gives about 58% ortho, 38% para; nitration of tert-butylbenzene gives mostly para.
Higher temperature and bulkier reagents favour para.
Reactions: electrophilic substitution occurs mainly at the para position (the other phenyl ring acts as an o/p-directing, activating group); e.g. nitration gives 4-nitrobiphenyl. Used as a heat-transfer fluid.
Classification: primary (1°, halogen on C attached to one carbon), secondary (2°), tertiary (3°).
Methods of formation
From alcohols: R–OH + HX (ZnCl₂ for HCl, Lucas reagent); R–OH + PCl₅ → RCl + POCl₃ + HCl; R–OH + SOCl₂ → RCl + SO₂ + HCl (best, by-products are gases).
From alkenes: CH₂=CH₂ + HBr → CH₃CH₂Br (Markovnikov addition).
Free-radical halogenation of alkanes; Finkelstein (RCl + NaI in acetone → RI) and Swarts (RBr + AgF → RF) reactions.
Chemical reactions
With aq. KOH → alcohol; with KCN → nitrile (R–CN); with AgCN → isocyanide; with NaOR → ether (Williamson); with NH₃ → amines.
With alc. KOH → alkene (dehydrohalogenation, Saytzeff product major).
With Na in dry ether → alkane (Wurtz); with Mg in dry ether → Grignard reagent RMgX.
Mechanism of SN1 and SN2
SN2 (substitution, nucleophilic, bimolecular)
Nucleophile attacks carbon from the side opposite to the leaving group (back-side attack).
A single transition state forms in which C is partly bonded to both Nu and X (five groups, trigonal bipyramidal).
X leaves; configuration is inverted (Walden inversion).
Rate = k[RX][Nu⁻] (second order). Energy profile: one hump (one transition state), no intermediate. Order of reactivity: CH₃X > 1° > 2° > 3°.
SN1 (unimolecular)
Slow step: C–X bond ionises to give a planar carbocation and X⁻.
Fast step: Nu⁻ attacks the carbocation from either face.
Result: racemisation (with some inversion) if the carbon was chiral.
Rate = k[RX] (first order). Energy profile: two humps with a carbocation in the valley between; the first hump is higher (rate-determining). Reactivity: 3° > 2° > 1° (carbocation stability).
Reactions of CHCl₃: oxidises in air and light to poisonous phosgene: 2CHCl₃ + O₂ → 2COCl₂ + 2HCl (stored in dark bottles with 1% ethanol). Carbylamine test: CHCl₃ + RNH₂ + 3KOH → RNC (foul smell) + 3KCl + 3H₂O. With HNO₃ → chloropicrin CCl₃NO₂. With Ag powder → acetylene.
CCl₄: from CS₂ + 3Cl₂ —(AlCl₃)→ CCl₄ + S₂Cl₂, or by exhaustive chlorination of methane. Reactions: hydrolysis with steam at high temperature gives phosgene; with HF/SbCl₅ gives freons (CCl₂F₂). Used as solvent, earlier as fire extinguisher (pyrene).
Aryl halides are unreactive to nucleophiles: C–Cl has partial double-bond character (resonance), carbon is sp², and the phenyl cation is unstable. Dow's process needs NaOH at 623 K and 300 atm to give phenol. –NO₂ groups at o/p make substitution easy.
Electrophilic substitution of chlorobenzene goes o/p (nitration gives mostly p-chloronitrobenzene).
Benzyl halides are very reactive in SN1 and SN2 because the benzyl carbocation is resonance-stabilised. C₆H₅CH₂Cl + aq. NaOH → benzyl alcohol; + KCN → benzyl cyanide; oxidation with Cu(NO₃)₂ gives benzaldehyde.
Remember: Ring halogen = FeCl₃, dark, cold (electrophilic). Side-chain halogen = light/heat, no catalyst (free radical).
Likely exam questions
Long answer
Explain the mechanism of SN1 and SN2 reactions of alkyl halides with energy profile diagrams. Discuss the factors affecting them.
What are activating and deactivating groups? Explain the directive influence of –OH and –NO₂ groups in electrophilic substitution with resonance structures.
Short answer
Why are aryl halides less reactive than alkyl halides towards nucleophilic substitution?
How is chloroform prepared? Why is it stored in dark coloured bottles?
Classification and nomenclature, Monohydric alcohols : Methods of formation by reduction of aldehydes, ketones using H2/Ni, hydrolysis of alkyl halides, addition of Grignard reagent to aldehydes and ketones.
Reactions of alcohol.
Dihydric alcohols : Methods of formation, chemical reactions of vicinal glycols, oxidative cleavage (Pb(OAc)4 and HIO4) and Pinacol – pinacolone rearrangement.
Trihydric alcohols : Methods of formation, chemical reactions of glycerol.
(B) Phenols
Structure and bonding, Preparation of phenols from cumene, chlorobenzene (Dows and Raschig process) and diazonium salts.
Physical properties and acidic character, Resonance stabilization of phenoxide ion, Reactions of phenols, Electrophilic aromatic substitution, acetylation and carboxylation, Claisen rearrangement, Gatterman synthesis reaction Mechanism of (i) Fries Rearrangement, (ii) Reimer-Tiemann reaction.
Alcohols: classification and nomenclature
Alcohols contain the –OH group attached to an sp³ carbon. They are classified by the number of –OH groups and by the type of carbon carrying –OH.
With HX / PCl₅ / SOCl₂: gives alkyl halides. Lucas test (conc. HCl + ZnCl₂): 3° turbidity at once, 2° in about 5 min, 1° no turbidity at room temperature.
Dehydration: CH₃CH₂OH —(conc. H₂SO₄, 443 K)→ CH₂=CH₂ + H₂O; at 413 K excess alcohol gives diethyl ether.
Oxidation (K₂Cr₂O₇/H⁺): 1° → aldehyde → acid; 2° → ketone; 3° resist oxidation. Catalytic dehydrogenation over Cu at 573 K gives the same carbonyl products; 3° alcohol gives alkene.
Dihydric alcohols (vicinal glycols)
Formation: from ethene with cold dilute alkaline KMnO₄ (Baeyer's reagent): CH₂=CH₂ + H₂O + [O] → HOCH₂CH₂OH. Also by hydrolysis of ethylene oxide (H₃O⁺), or of 1,2-dibromoethane with aqueous Na₂CO₃.
Reactions: with Na gives monosodium and disodium glycolate; with HCl gives ethylene chlorohydrin; on heating with ZnCl₂ gives acetaldehyde; with HNO₃ oxidation gives glycolic acid and finally oxalic acid.
Oxidative cleavage: a C–C bond carrying two –OH groups is broken.
HOCH₂–CH₂OH + HIO₄ → 2HCHO + HIO₃ + H₂O
Lead tetraacetate does the same: HOCH₂CH₂OH + Pb(OAc)₄ → 2HCHO + Pb(OAc)₂ + 2CH₃COOH. Used to find the position of –OH groups in sugars.
Pinacol–pinacolone rearrangement
Pinacol (2,3-dimethylbutane-2,3-diol) on warming with acid gives pinacolone (3,3-dimethylbutan-2-one).
One –OH is protonated by H⁺ and leaves as water, giving a 3° carbocation.
A methyl group shifts (1,2-shift) with its electron pair to the positive carbon.
The new carbocation is stabilised by the lone pair of the remaining oxygen (oxocarbenium ion).
Loss of H⁺ from oxygen gives the ketone (CH₃)₃C–CO–CH₃.
Trihydric alcohol: glycerol
Formation: by hydrolysis (saponification) of oils and fats; by-product of soap industry. Fat + 3NaOH → glycerol + 3RCOONa. Synthetically from propene via allyl chloride.
Reactions:
With Na: forms monosodium glycerolate at room temperature.
With conc. HNO₃ + conc. H₂SO₄: glyceryl trinitrate (nitroglycerine), used in dynamite.
With KHSO₄ on heating: dehydration to acrolein (CH₂=CH–CHO, pungent smell; test for glycerol).
With oxalic acid: at 383 K gives formic acid; at 503 K gives allyl alcohol.
With HI: small amount gives allyl iodide; excess gives 2-iodopropane.
Oxidation: dil. HNO₃ gives glyceric acid; Fenton's reagent gives glyceraldehyde + dihydroxyacetone.
Phenols: structure, preparation and acidity
Structure: –OH on an sp² ring carbon. The O lone pair is in resonance with the ring, so the C–O bond is shorter (136 pm) than in methanol (142 pm).
From cumene: cumene (isopropylbenzene) + O₂ → cumene hydroperoxide; with dilute acid → phenol + acetone (major industrial route).
Physical properties: colourless crystals (turn pink in air), m.p. 316 K, moderately soluble in water due to H-bonding, high b.p.
Acidity: phenol (pKa ≈ 10) is more acidic than ethanol (pKa ≈ 16) but less than carbonic acid. The phenoxide ion is resonance stabilised: the negative charge spreads to ortho and para carbons (three ring structures plus one on O). Ethoxide has no such resonance. Phenol dissolves in NaOH but not in NaHCO₃. –NO₂ at o/p increases acidity (picric acid is strongly acidic); –CH₃ decreases it.
Reactions of phenols
Electrophilic substitution (–OH is strongly o/p directing): with Br₂ water → 2,4,6-tribromophenol (white ppt); with Br₂ in CS₂ at low temperature → o- and p-bromophenol; with dil. HNO₃ → o- and p-nitrophenol (separated by steam distillation, ortho is volatile due to intramolecular H-bond); conc. HNO₃ → picric acid.
Claisen rearrangement: allyl phenyl ether on heating (~473 K) gives o-allylphenol through a cyclic six-membered transition state ([3,3] sigmatropic shift).
Gattermann synthesis: phenol + HCN + HCl with anhydrous ZnCl₂ (or AlCl₃), then hydrolysis → p-hydroxybenzaldehyde (formylation).
Mechanism: Fries rearrangement
Phenyl acetate is heated with anhydrous AlCl₃.
AlCl₃ complexes with the ester oxygen; the acyl group separates as an acylium ion CH₃C≡O⁺.
The acylium ion attacks the ring at ortho or para (electrophilic substitution).
Loss of H⁺ and hydrolysis give o- and p-hydroxyacetophenone. Low temperature (below 333 K) favours para; high temperature (above 433 K) favours ortho.
Nomenclature and structure of the carbonyl group, synthesis of aldehydes and ketones with particular reference to the synthesis of aldehydes from acid chlorides and ketones from nitriles.
Mechanism of nucleophilic additions to carbonyl group with particular emphasis on benzoin, aldol, Perkin and Knoevenagel condensation, Wittig reaction, Mannich reaction, oxidation of aldehydes (by KMnO4, Tollen’s reagent and Fehling solution), Baeyer-Villiger oxidation of Ketones, Cannizzaro reaction with mechanism, MPV, Clemmensen, Wolf-Kishner, LiAlH4 and NaBH4 reductions,
Structure: carbonyl carbon is sp² hybridised; the three σ bonds lie in one plane at about 120°. A π bond forms by sideways overlap of p orbitals of C and O.
Oxygen is more electronegative, so the C=O bond is polar: Cᵟ⁺=Oᵟ⁻. The carbon is electrophilic and is attacked by nucleophiles; the oxygen is basic.
Aldehydes are more reactive than ketones: less steric crowding and only one electron-releasing alkyl group.
Aldehydes from acid chlorides (Rosenmund reduction):
RCOCl + H₂ —(Pd/BaSO₄, poisoned with quinoline or S)→ RCHO + HCl
The poison stops further reduction of the aldehyde to alcohol. Formaldehyde cannot be made this way (HCOCl is unstable).
Ketones from nitriles: Grignard reagent adds to C≡N, then acid hydrolysis:
CH₃–C≡N + C₂H₅MgBr → CH₃C(=NMgBr)C₂H₅ —(H₃O⁺)→ CH₃COC₂H₅ (butan-2-one)
Stephen reduction (nitrile + SnCl₂/HCl, then hydrolysis) gives aldehydes; Friedel–Crafts acylation gives aromatic ketones.
Mechanism of nucleophilic addition
The nucleophile (Nu⁻) attacks the electrophilic carbonyl carbon from above or below the plane.
The π electrons move onto oxygen; carbon changes from sp² (trigonal) to sp³ (tetrahedral), giving an alkoxide intermediate.
The alkoxide picks up H⁺ from the solvent or acid to give the addition product.
Example: HCN addition gives a cyanohydrin: CH₃CHO + HCN → CH₃CH(OH)CN. In acid medium, protonation of oxygen first makes carbon more electrophilic.
Benzoin, aldol, Perkin and Knoevenagel condensations
A weak base removes a proton from the active methylene (CH₂ between two –COOH or –COOR groups); the carbanion adds to C=O; loss of water gives the α,β-unsaturated product.
Wittig and Mannich reactions
Wittig reaction: a phosphorus ylide converts C=O into C=C.
Ph₃P + CH₃Br → Ph₃P⁺CH₃Br⁻ —(BuLi)→ Ph₃P=CH₂ (ylide)
(CH₃)₂C=O + Ph₃P=CH₂ → (CH₃)₂C=CH₂ + Ph₃P=O
Mechanism: ylide carbon attacks carbonyl carbon → betaine → four-membered oxaphosphetane ring → breaks to alkene and triphenylphosphine oxide (strong P=O bond is the driving force).
Mannich reaction: a ketone with α-H, formaldehyde and a secondary amine (as its hydrochloride) give a β-amino ketone (Mannich base).
CH₃COCH₃ + HCHO + (CH₃)₂NH·HCl → CH₃COCH₂CH₂N(CH₃)₂·HCl
HCHO and the amine first form an iminium ion CH₂=N⁺(CH₃)₂; the enol of the ketone attacks it.
Oxidation of aldehydes and Baeyer–Villiger oxidation
Baeyer–Villiger oxidation: a ketone with a peroxy acid (e.g. m-CPBA, CH₃CO₃H) gives an ester. The peracid adds to C=O; one alkyl or aryl group migrates from carbon to oxygen as the O–O bond breaks. Migratory order: 3° alkyl > 2° alkyl ≈ phenyl > 1° alkyl > methyl. C₆H₅COCH₃ → CH₃COOC₆H₅ (phenyl acetate).
Cannizzaro reaction with mechanism
Aldehydes without α-H (HCHO, C₆H₅CHO) in conc. NaOH undergo self oxidation-reduction (disproportionation).
2C₆H₅CHO + NaOH → C₆H₅CH₂OH + C₆H₅COONa
OH⁻ adds to the carbonyl carbon of one aldehyde molecule, giving a tetrahedral anion.
This anion transfers a hydride ion (H⁻) to the carbonyl carbon of a second molecule (rate-determining step).
The first molecule becomes the acid, the second becomes an alkoxide.
Proton exchange gives the carboxylate ion and the alcohol.
MPV, Clemmensen, Wolff–Kishner, LiAlH₄ and NaBH₄ reductions
Reduction
Reagent
C=O becomes
Meerwein–Ponndorf–Verley (MPV)
Aluminium isopropoxide in isopropanol
CH–OH (alcohol); acetone is formed and distilled off
Clemmensen
Zn–Hg + conc. HCl
CH₂
Wolff–Kishner
NH₂NH₂, then KOH in ethylene glycol, heat
CH₂ (via hydrazone, N₂ lost)
LiAlH₄
In dry ether, then H₃O⁺
CH–OH (also reduces acids, esters)
NaBH₄
In water or alcohol
CH–OH (milder, aldehydes/ketones only)
MPV is selective: C=C and –NO₂ are not touched. It is the reverse of Oppenauer oxidation; hydride moves through a six-membered cyclic transition state.
Example: CH₃COCH₃ —(Zn–Hg/HCl)→ CH₃CH₂CH₃. Use Clemmensen for acid-stable compounds, Wolff–Kishner for acid-sensitive compounds.
LiAlH₄ and NaBH₄ act as sources of hydride ion (H⁻), which adds to the carbonyl carbon.
Likely exam questions
Long answer
Explain the mechanism of nucleophilic addition to the carbonyl group. Give the mechanism of aldol condensation and benzoin condensation.
What is the Cannizzaro reaction? Explain its mechanism. Write notes on Clemmensen and Wolff–Kishner reductions.
Short answer
How are aldehydes prepared from acid chlorides?
Write a note on the Wittig reaction.
How does Tollens' reagent distinguish aldehydes from ketones?
Nomenclature, structure and bonding, Physical properties, acidity of carboxylic acids, effect of substituents on acid strengths preparation of carboxylic acids(from G.R. and cyanides), Reactions of carboxylic acids, Hell-Volhard-Zelinsky reactions.
Reduction of carboxylic acids, Mechanism of decarboxylation.
Methods of formation and chemical reactions of unsaturated monocarboxylic acids (crotonic acid and cinnamic acid).
Dicarboxylic acids : Methods of formation and effect of heat and dehydrating agents.
(Succinic acid, Phthalic acid).
(B) Carboxylic acid derivatives
Structure and nomenclature of acid chlorides, esters, amides and acid anhydrides.
Interconversion of acid derivatives by nucleophilic acyl substitution.
Preparation of carboxylic acid derivatives, Chemical reactions, Mechanism of esterification and hydrolysis (acidic and basic).
Carboxylic acids: nomenclature, structure and physical properties
Structure: the carboxyl carbon is sp², planar, bond angle about 120°. The –OH lone pair is in resonance with C=O, so the carbonyl carbon is less electrophilic than in aldehydes.
Physical properties: lower acids are liquids with sharp smell; they form H-bonded dimers (two –COOH groups joined by two H-bonds), so b.p. is higher than alcohols of similar mass. Lower members (C₁–C₄) are miscible with water; solubility falls as the chain grows.
Acidity and effect of substituents
RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺. Acids are stronger than alcohols and phenols because the carboxylate ion is stabilised by resonance: the negative charge is shared equally by two oxygen atoms (both C–O bonds have the same length, 127 pm).
Electron-withdrawing groups (–I) such as –Cl, –NO₂, –CN stabilise the anion and increase acidity. Order: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.
Effect falls with distance: 2-chlorobutanoic > 3-chlorobutanoic > 4-chlorobutanoic acid.
Electron-releasing groups (+I) such as alkyl decrease acidity: HCOOH (pKa 3.75) > CH₃COOH (pKa 4.76) > CH₃CH₂COOH.
Benzoic acid (pKa 4.2) is stronger than acetic acid; p-nitrobenzoic acid is stronger than benzoic acid.
Preparation from Grignard reagents and cyanides
From Grignard reagent (G.R.): dry CO₂ (dry ice) is added, then acid hydrolysis.
RMgX + CO₂ → RCOOMgX —(H₃O⁺)→ RCOOH + Mg(OH)X
The acid has one carbon more than the alkyl halide.
From cyanides (nitriles): hydrolysis with dilute acid or alkali.
R–C≡N + 2H₂O —(H⁺)→ RCOOH + NH₃ (via amide RCONH₂)
CH₃Br + KCN → CH₃CN → CH₃COOH (ascent of series).
Reactions of carboxylic acids
With metals and bases: 2RCOOH + 2Na → 2RCOONa + H₂; RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂↑ (brisk effervescence; test for –COOH).
With PCl₅ or SOCl₂: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
With alcohol (acid catalyst): ester. With NH₃ then heat: amide.
With P₂O₅ on heating: acid anhydride.
Hell–Volhard–Zelinsky (HVZ) reaction
Acids having α-H react with Cl₂ or Br₂ in the presence of red phosphorus to give α-halo acids.
CH₃COOH + Cl₂ —(red P)→ ClCH₂COOH + HCl
P and Br₂ form PBr₃, which converts the acid to acyl bromide.
The acyl bromide enolises easily; the enol reacts with Br₂ at the α-carbon.
The α-bromoacyl bromide exchanges with more acid, giving the α-bromo acid and regenerating acyl bromide.
Reduction
LiAlH₄ (dry ether), then H₃O⁺: RCOOH → RCH₂OH (1° alcohol). NaBH₄ does not reduce acids. Diborane (B₂H₆) also reduces –COOH.
With HI and red P at high temperature, the acid is reduced to the alkane.
Mechanism of decarboxylation
Sodium salt + soda lime (NaOH + CaO), heat: RCOONa + NaOH → R–H + Na₂CO₃.
The C–C bond between R and COO⁻ breaks; CO₂ is lost and a carbanion R⁻ forms (the slow step).
R⁻ takes a proton from water/NaOH to give R–H.
Acids with an electron-withdrawing group at the α or β position (e.g. malonic acid, β-keto acids) lose CO₂ easily on heating through a cyclic six-membered transition state.
Kolbe electrolysis is another decarboxylation: 2RCOO⁻ → R–R + 2CO₂ + 2e⁻ (at the anode).
Unsaturated monocarboxylic acids: crotonic and cinnamic acid
Crotonic acid (CH₃CH=CHCOOH, but-2-enoic acid):
Formation: oxidation of crotonaldehyde (from aldol) with Tollens' reagent; or Knoevenagel reaction of acetaldehyde with malonic acid in pyridine, followed by loss of CO₂.
Reactions: adds H₂ (Ni) → butanoic acid; adds HBr → 3-bromobutanoic acid (Br goes to the β-carbon, because the –COOH group makes the β-carbon electron-poor); forms esters and salts.
Cinnamic acid (C₆H₅CH=CHCOOH, 3-phenylprop-2-enoic acid):
Formation: Perkin reaction (benzaldehyde + acetic anhydride + sodium acetate) or Knoevenagel reaction with malonic acid.
Reactions: H₂/Ni → 3-phenylpropanoic acid; Br₂ → 2,3-dibromo-3-phenylpropanoic acid; oxidation with KMnO₄ → benzoic acid; decarboxylation on heating with quinoline/Cu → styrene. Exists as trans (common) and cis isomers.
Dicarboxylic acids: succinic and phthalic acid
Succinic acid (HOOC–CH₂–CH₂–COOH):
Formation: from ethylene dibromide → KCN → ethylene dicyanide → hydrolysis; or reduction of maleic acid (H₂/Ni).
Effect of heat (about 573 K) or dehydrating agents (P₂O₅, acetyl chloride): loses water to form the cyclic succinic anhydride (five-membered ring). With NH₃ and heat it gives succinimide.
Phthalic acid (benzene-1,2-dicarboxylic acid):
Formation: oxidation of o-xylene or naphthalene (V₂O₅, air, ~773 K, naphthalene gives phthalic anhydride directly).
Effect of heat: the two –COOH groups are close (ortho), so on heating it loses water to give phthalic anhydride. With NH₃ it gives phthalimide (used in Gabriel synthesis).
Remember: Oxalic and malonic acids lose CO₂ on heating; succinic and phthalic acids lose H₂O to form cyclic anhydrides.
Carboxylic acid derivatives
Derivative
General formula
Example (IUPAC)
Acid chloride
RCOCl
CH₃COCl, ethanoyl chloride
Acid anhydride
(RCO)₂O
(CH₃CO)₂O, ethanoic anhydride
Ester
RCOOR′
CH₃COOC₂H₅, ethyl ethanoate
Amide
RCONH₂
CH₃CONH₂, ethanamide
Reactivity order (nucleophilic acyl substitution): acid chloride > anhydride > ester > amide. A better leaving group (Cl⁻ best, NH₂⁻ worst) and less resonance donation make the derivative more reactive.
Mechanism (addition–elimination): Nu⁻ adds to C=O → tetrahedral intermediate → the leaving group is expelled and C=O reforms.
Interconversion: a more reactive derivative can be converted to a less reactive one. RCOCl + RCOONa → anhydride; RCOCl + R′OH → ester; RCOCl + 2NH₃ → amide + NH₄Cl; (RCO)₂O + R′OH → ester + RCOOH; RCOOR′ + NH₃ → RCONH₂ + R′OH (ammonolysis).
Preparation: acid chloride from acid + SOCl₂/PCl₅; anhydride from acid + P₂O₅ or acid chloride + carboxylate; ester by esterification; amide by heating ammonium salt (RCOONH₄ → RCONH₂ + H₂O).
Reactions: all hydrolyse to the acid; amide + Br₂/NaOH → amine with one carbon less (Hofmann bromamide); esters reduced by LiAlH₄ to alcohols; esters + RMgX → 3° alcohols.
Mechanism of esterification (acid catalysed)
H⁺ protonates the carbonyl oxygen of the acid, making the carbon more electrophilic.
The alcohol oxygen attacks the carbonyl carbon → tetrahedral intermediate.
A proton shifts to one –OH, which leaves as water.
Loss of H⁺ gives the ester. All steps are reversible; isotope (¹⁸O) studies show the O of the ester comes from the alcohol.
Hydrolysis of esters
Acidic hydrolysis: exact reverse of esterification; reversible.
Basic hydrolysis (saponification): OH⁻ adds to C=O → tetrahedral intermediate → R′O⁻ leaves → the acid formed gives up its proton to R′O⁻, forming RCOO⁻ and R′OH. Irreversible because the carboxylate cannot be attacked by alcohol.
Likely exam questions
Long answer
Explain the acidity of carboxylic acids. Discuss the effect of substituents on acid strength with suitable examples.
What is nucleophilic acyl substitution? Explain the mechanism of acid-catalysed esterification and of basic hydrolysis of esters.
Short answer
Write a note on the Hell–Volhard–Zelinsky reaction.
What is the effect of heat on succinic acid and phthalic acid?
How is cinnamic acid prepared? Give any two reactions.