JEE Mains Chemistry · Teaching notes

Organic Reaction Mechanisms — JEE Mains Chemistry

Organic Reaction Mechanisms has 48 past-year questions from 2021 to 2026, and 8 of them ask for a number rather than an option. Most of them need two or three organic chapters at once: an addition from one, a named reaction from another, a laboratory test from a third. Thirteen are match lists, so a clean table of names, reagents and colours earns as many marks as the mechanisms do. For a reaction scheme, count the carbons first, mark the steps that add or remove carbon, and only then follow the functional group through each reagent.

Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.

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Subtopic notes

Formula & revision sheet

4 formulas · 6 reference tables · 32 gotchas across all subtopics — the exam-eve cheat-sheet

Carbocations, Rearrangements and Addition Regiochemistry

Formulas (1)

Carbocation stability and 1,2-shifts · Carbocation stability order $3^\circ > 2^\circ > 1^\circ > \mathrm{CH_3^+} \qquad \text{benzylic and allylic cations: resonance-stabilised}$

Reference tables (1)

Addition regiochemistry by reagent8 rows

Reagent	Where the new group goes	Why	Example
HBr (no peroxide)	Br on the more substituted carbon	The more stable cation forms; it can rearrange	$\mathrm{(CH_3)_2C{=}CH_2 \to (CH_3)_3CBr}$
HBr with a peroxide	Br on the less substituted carbon	Free-radical chain; works for HBr only	$\mathrm{(CH_3)_2C{=}CH_2 \to (CH_3)_2CHCH_2Br}$
$\mathrm{H_2O}$ , dilute $\mathrm{H_2SO_4}$	OH on the more substituted carbon	Through a cation; it can rearrange	$\mathrm{(CH_3)_2C{=}CH_2 \to (CH_3)_3COH}$
$\mathrm{B_2H_6}$ , then $\mathrm{H_2O_2/OH^-}$	OH on the less substituted carbon	No cation: syn addition, no rearrangement	$\mathrm{(CH_3)_2C{=}CH_2 \to (CH_3)_2CHCH_2OH}$
$\mathrm{H_2O}$ , $\mathrm{Hg^{2+}/H_2SO_4}$ on an alkyne	O on the inner carbon; a ketone	Markovnikov enol changes to the keto form	$\mathrm{CH_3CH_2C{\equiv}CH \to CH_3CH_2COCH_3}$ Only ethyne gives an aldehyde (ethanal) this way.
$\mathrm{B_2H_6}$ , then $\mathrm{H_2O_2/OH^-}$ on a terminal alkyne	O on the end carbon; an aldehyde	Anti-Markovnikov enol changes to the aldehyde	$\mathrm{CH_3CH_2C{\equiv}CH \to CH_3CH_2CH_2CHO}$
$\mathrm{Br_2}$ in $\mathrm{CCl_4}$	One Br on each carbon, on opposite faces	Cyclic bromonium ion opened from the back	Cyclohexene gives trans-1,2-dibromocyclohexane
$\mathrm{Br_2}$ , $\mathrm{NaHCO_3}$ on an unsaturated acid	Ring O on the inner carbon, Br outside the ring	The molecule's carboxylate opens the bromonium ion	An alkenoic acid gives a bromo-lactone, a five-membered ring when possible

Through a cation: Markovnikov, and a shift is possible. Through boron or a radical: the other carbon, and no shift.

Watch out for (6)

A primary halide in Friedel-Crafts alkylation gives the branched product
→ Carbocation stability and 1,2-shifts
No shift without a better cation at the end
→ Carbocation stability and 1,2-shifts
Stability and rate go together
→ Carbocation stability and 1,2-shifts
Hydroboration puts OH on the less substituted carbon
→ Addition regiochemistry by reagent
The peroxide effect works for HBr only
→ Addition regiochemistry by reagent
Alkyne hydration ends at a carbonyl
→ Addition regiochemistry by reagent

Carbonyl Reactions: Enolates, Haloform and Cannizzaro

Formulas (2)

Aldol, Claisen-Schmidt, Claisen and Michael reactions · Claisen-Schmidt condensation and percentage yield $\mathrm{ArCHO + CH_3COR \xrightarrow{OH^-} ArCH{=}CHCOR + H_2O} \qquad \%\,\text{yield} = \dfrac{\text{actual mass}}{\text{theoretical mass}} \times 100$
Haloform, Cannizzaro and decarboxylation · Haloform and Cannizzaro equations $\mathrm{RCOCH_3 + 3X_2 + 4NaOH \to RCOONa + CHX_3 + 3NaX + 3H_2O} \qquad \mathrm{2\,ArCHO + NaOH \to ArCH_2OH + ArCOONa}$

Watch out for (6)

Acetone condenses on both sides
→ Aldol, Claisen-Schmidt, Claisen and Michael reactions
Benzaldehyde cannot form an enolate
→ Aldol, Claisen-Schmidt, Claisen and Michael reactions
Michael addition goes to the β-carbon
→ Aldol, Claisen-Schmidt, Claisen and Michael reactions
The haloform reaction removes one carbon only
→ Haloform, Cannizzaro and decarboxylation
Cannizzaro needs an aldehyde with no α-hydrogen
→ Haloform, Cannizzaro and decarboxylation
In a crossed Cannizzaro, formaldehyde is oxidised
→ Haloform, Cannizzaro and decarboxylation

Multistep Conversions and Road Maps

Formulas (1)

Aromatic road maps: acylation, reduction and diazonium routes · Straight-chain alkylbenzene by acylation then reduction $\mathrm{C_6H_6 \xrightarrow{RCOCl,\ AlCl_3} C_6H_5COR \xrightarrow{Zn{-}Hg,\ HCl} C_6H_5CH_2R}$

Reference tables (1)

Carbon counting in chain-building steps12 rows

Reagent	Carbon count change	Group produced	Example
KCN on an alkyl halide	+1 for each halogen replaced	Nitrile; $\mathrm{H_3O^+}$ gives COOH, $\mathrm{H_2/Ni}$ gives $\mathrm{CH_2NH_2}$	$\mathrm{CH_3CH_2Br \to CH_3CH_2CN \to CH_3CH_2COOH}$
RMgX, then HCHO and $\mathrm{H_3O^+}$	+1 on R	Primary alcohol $\mathrm{RCH_2OH}$	$\mathrm{CH_3CH_2MgBr \to CH_3CH_2CH_2OH}$
RMgX, then another aldehyde R′CHO	R joins R′CHO	Secondary alcohol	$\mathrm{C_6H_5MgBr + CH_3CHO \to C_6H_5CH(OH)CH_3}$
RMgX, then a ketone	R joins the ketone	Tertiary alcohol	$\mathrm{CH_3MgBr + CH_3COCH_3 \to (CH_3)_3COH}$
Two RMgX on an ester R′COOEt	Two R groups join; OEt leaves	Tertiary alcohol with two identical R groups	$\mathrm{2\,CH_3MgBr + CH_3COOC_2H_5 \to (CH_3)_3COH}$
RMgX, then $\mathrm{CO_2}$ and $\mathrm{H_3O^+}$	+1 on R	Carboxylic acid RCOOH	$\mathrm{CH_3MgBr \to CH_3COOH}$
$\mathrm{NaNH_2}$ on a terminal alkyne, then a primary RX	+ the carbons of R	Longer internal alkyne	$\mathrm{CH_3C{\equiv}CH \to CH_3C{\equiv}CCH_2CH_3}$ with $\mathrm{CH_3CH_2Br}$
Base on $\mathrm{ArCH_2CN}$ , then a ketone	The α-carbon joins the C=O carbon	β-Hydroxy nitrile	$\mathrm{C_6H_5CH_2CN + CH_3COCH_3 \to (CH_3)_2C(OH)CH(C_6H_5)CN}$
Ethylene glycol and $\mathrm{H^+}$ on a C=O	0 (temporary)	Cyclic acetal, stable to base, Grignard reagents and $\mathrm{NaBH_4}$	Aqueous acid gives the C=O back
$\mathrm{X_2/NaOH}$ on a methyl ketone	−1	Carboxylate and $\mathrm{CHX_3}$	$\mathrm{CH_3COCH_2CH_3 \to CH_3CH_2COO^-}$
Soda lime on RCOONa	−1	Alkane RH	$\mathrm{CH_3CH_2COONa \to CH_3CH_3}$
$\mathrm{Br_2/NaOH}$ on an amide $\mathrm{RCONH_2}$	−1	Primary amine $\mathrm{RNH_2}$	$\mathrm{CH_3CH_2CONH_2 \to CH_3CH_2NH_2}$

Mark the steps that change the carbon count before anything else; every other reagent changes only the group.

Watch out for (6)

A Grignard reagent attacks an ester twice
→ Carbon counting in chain-building steps
H₂/Ni adds hydrogen, not carbon
→ Carbon counting in chain-building steps
An O–H in the molecule destroys the carbanion
→ Carbon counting in chain-building steps
Acylate, then reduce, for a straight chain
→ Aromatic road maps: acylation, reduction and diazonium routes
Clemmensen leaves COOH alone
→ Aromatic road maps: acylation, reduction and diazonium routes
Diazotise cold
→ Aromatic road maps: acylation, reduction and diazonium routes

Named Reactions

Reference tables (3)

Named reactions of alkyl halides, aryl halides and diazonium salts12 rows

Name	Reagent	Change	Example
Wurtz	Na, dry ether	2 RX → R–R	$\mathrm{2\,CH_3CH_2Br \to CH_3CH_2CH_2CH_3}$
Fittig	Na, dry ether	2 ArX → Ar–Ar	$\mathrm{2\,C_6H_5Br \to C_6H_5{-}C_6H_5}$ (biphenyl)
Wurtz-Fittig	Na, dry ether	ArX + RX → Ar–R	$\mathrm{C_6H_5Br + CH_3CH_2Br \to C_6H_5CH_2CH_3}$
Finkelstein	NaI, dry acetone	R–Cl or R–Br → R–I	$\mathrm{CH_3CH_2Br \to CH_3CH_2I}$
Swarts	AgF, $\mathrm{Hg_2F_2}$ , $\mathrm{CoF_2}$ or $\mathrm{SbF_3}$	R–Cl or R–Br → R–F	$\mathrm{CH_3Br \to CH_3F}$
Sandmeyer	CuCl/HCl, CuBr/HBr or CuCN	$\mathrm{ArN_2^+}$ → ArCl, ArBr or ArCN	$\mathrm{C_6H_5N_2^+Cl^- \to C_6H_5CN}$ with CuCN
Gattermann	Cu powder with HCl or HBr	$\mathrm{ArN_2^+}$ → ArCl or ArBr	$\mathrm{C_6H_5N_2^+Cl^- \to C_6H_5Cl}$ with Cu/HCl
Balz-Schiemann	$\mathrm{HBF_4}$ , then heat	$\mathrm{ArN_2^+}$ → ArF	$\mathrm{C_6H_5N_2^+Cl^- \to C_6H_5F}$
Lucas	Conc. HCl, anhydrous $\mathrm{ZnCl_2}$	ROH → RCl; rate 3° > 2° > 1°	$\mathrm{(CH_3)_2CHOH \to (CH_3)_2CHCl}$ , cloudy in about 5 min
Nitrile from KCN	KCN (ionic)	RX → R–CN	$\mathrm{CH_3CH_2Br \to CH_3CH_2CN}$
Isocyanide from AgCN	AgCN (covalent)	RX → R–NC	$\mathrm{CH_3CH_2Br \to CH_3CH_2NC}$
Carbylamine	$\mathrm{CHCl_3}$ , alcoholic KOH	Primary amine $\mathrm{RNH_2}$ → R–NC	$\mathrm{C_6H_5NH_2 \to C_6H_5NC}$ , a foul smell

Sodium couples, halide salts swap, copper replaces a diazonium group, silver cyanide attacks through nitrogen.

Named reactions of carbonyls, acids, amides and phenols14 rows

Name	Reagent	Change	Example
Rosenmund	$\mathrm{H_2}$ , Pd on $\mathrm{BaSO_4}$	RCOCl → RCHO	$\mathrm{CH_3COCl \to CH_3CHO}$
Stephen	$\mathrm{SnCl_2/HCl}$ , then $\mathrm{H_3O^+}$	RCN → RCHO	$\mathrm{CH_3CN \to CH_3CHO}$
Etard	$\mathrm{CrO_2Cl_2}$ in $\mathrm{CS_2}$ , then $\mathrm{H_3O^+}$	$\mathrm{ArCH_3}$ → ArCHO	Toluene → benzaldehyde
Gattermann-Koch	CO, HCl, anhydrous $\mathrm{AlCl_3}$ (CuCl)	ArH → ArCHO	Benzene → benzaldehyde
Clemmensen	Zn–Hg, conc. HCl	C=O → $\mathrm{CH_2}$ , in acid	$\mathrm{CH_3COCH_3 \to CH_3CH_2CH_3}$
Wolff-Kishner	$\mathrm{NH_2NH_2}$ , then KOH in ethylene glycol, heat	C=O → $\mathrm{CH_2}$ , in base	$\mathrm{C_6H_5COCH_2CH_3 \to C_6H_5CH_2CH_2CH_3}$
Cannizzaro	Concentrated NaOH or KOH	2 RCHO (no α-H) → $\mathrm{RCH_2OH + RCOO^-}$	$\mathrm{2\,HCHO \to CH_3OH + HCOO^-}$
Aldol	Dilute NaOH	Two carbonyls with α-H → β-hydroxy carbonyl	$\mathrm{2\,CH_3CHO \to CH_3CH(OH)CH_2CHO}$
Hell-Volhard-Zelinsky (HVZ)	$\mathrm{Cl_2}$ or $\mathrm{Br_2}$ , red P, then $\mathrm{H_2O}$	$\mathrm{RCH_2COOH \to RCHXCOOH}$	$\mathrm{CH_3COOH \to ClCH_2COOH}$
Hofmann bromamide	$\mathrm{Br_2}$ , NaOH	$\mathrm{RCONH_2 \to RNH_2}$ , one carbon fewer	$\mathrm{C_6H_5CONH_2 \to C_6H_5NH_2}$
Reimer-Tiemann	$\mathrm{CHCl_3}$ , aqueous NaOH, then $\mathrm{H_3O^+}$	Phenol → 2-hydroxybenzaldehyde	$\mathrm{C_6H_5OH \to HOC_6H_4CHO}$ (salicylaldehyde)
Kolbe	NaOH, $\mathrm{CO_2}$ at 4–7 atm and 400 K, then $\mathrm{H^+}$	Phenol → 2-hydroxybenzoic acid	$\mathrm{C_6H_5OH \to HOC_6H_4COOH}$ (salicylic acid)
Haloform	$\mathrm{X_2}$ , NaOH	$\mathrm{CH_3COR \to CHX_3 + RCOO^-}$	$\mathrm{CH_3COCH_3 \to CHI_3 + CH_3COO^-}$
Decarboxylation	Soda lime (NaOH and CaO), heat	RCOONa → RH	$\mathrm{CH_3COONa \to CH_4}$

Learn each row as reagent plus change; a match list gives you one and asks for the other.

Mechanism class and reagent roles13 rows

Reaction or reagent	Class or role	Key species	Result
Williamson synthesis: RONa + primary RX	Nucleophilic substitution ( $\mathrm{S_N2}$ )	Alkoxide ion	Ether
tert-Butyl bromide in water	Nucleophilic substitution ( $\mathrm{S_N1}$ )	Tertiary carbocation	2-Methylpropan-2-ol
Nitration, sulphonation, Friedel-Crafts	Electrophilic substitution	$\mathrm{NO_2^+}$ , $\mathrm{SO_3}$ , $\mathrm{R^+}$ or $\mathrm{RCO^+}$	Substituted arene
$\mathrm{Br_2}$ on an alkene	Electrophilic addition	Cyclic bromonium ion	Vicinal dibromide
HCN on a ketone	Nucleophilic addition	Cyanide ion	Cyanohydrin
$\mathrm{Cl_2}$ on methane or on toluene's side chain, in light	Free-radical substitution	Cl atom; methyl or benzyl radical	Chloromethane or benzyl chloride
$\mathrm{Cl_2}$ on benzene in UV light, no catalyst	Free-radical addition	Cl atom	Benzene hexachloride, $\mathrm{C_6H_6Cl_6}$
Alcoholic KOH on an alkyl halide	β-Elimination	Strong base in ethanol	Alkene, the more substituted one
Heating a quaternary ammonium hydroxide	Hofmann elimination	Bulky trialkylamine leaving group	The less substituted (anti-Saytzeff) alkene
Lindlar's catalyst with $\mathrm{H_2}$	Partial hydrogenation	Poisoned palladium	Alkyne to cis-alkene
Na in liquid $\mathrm{NH_3}$	Dissolving-metal reduction	Solvated electrons	Alkyne to trans-alkene
Hinsberg reagent, $\mathrm{C_6H_5SO_2Cl}$	Test that sorts amines	Sulphonamide	1° dissolves in alkali, 2° does not, 3° does not react
Na with $\mathrm{H_2}$	Not a reducing system for organic groups	Sodium hydride forms	No organic group is reduced

Name the species that attacks and whether it adds or replaces; the class follows from those two facts.

Watch out for (9)

Sandmeyer uses a copper(I) salt, Gattermann uses copper powder
→ Named reactions of alkyl halides, aryl halides and diazonium salts
AgCN gives the isocyanide, KCN the nitrile
→ Named reactions of alkyl halides, aryl halides and diazonium salts
Wurtz-Fittig needs one aryl and one alkyl halide
→ Named reactions of alkyl halides, aryl halides and diazonium salts
Rosenmund stops at the aldehyde
→ Named reactions of carbonyls, acids, amides and phenols
Hofmann bromamide loses a carbon
→ Named reactions of carbonyls, acids, amides and phenols
Reimer-Tiemann gives an aldehyde, Kolbe gives an acid
→ Named reactions of carbonyls, acids, amides and phenols
Toluene and chlorine: light or catalyst decides where
→ Mechanism class and reagent roles
Benzene hexachloride is an addition product
→ Mechanism class and reagent roles
Alcoholic KOH eliminates, aqueous KOH substitutes
→ Mechanism class and reagent roles

Functional Group Identification Tests

Reference tables (1)

Functional-group tests: reagent, group and observation18 rows

Test (reagent)	Detects	Positive result	Example that passes
Bromine water	C=C or C≡C; also phenol and aniline	Reddish-orange colour disappears; white precipitate with phenol or aniline	Cyclohexene
Baeyer's reagent: cold, dilute, alkaline $\mathrm{KMnO_4}$	C=C or C≡C	Pink colour disappears; brown $\mathrm{MnO_2}$	But-2-ene
Ceric ammonium nitrate	Alcoholic –OH	Yellow solution turns red	Ethanol
Neutral $\mathrm{FeCl_3}$	Phenol (and enols)	Violet, blue, green or red colour	Phenol gives violet
Phthalein dye: phthalic anhydride and conc. $\mathrm{H_2SO_4}$ , then NaOH	Phenol	Pink or red dye in alkali	Phenol gives phenolphthalein
Lucas reagent: conc. HCl and $\mathrm{ZnCl_2}$	Class of alcohol	Cloudy at once for 3°, in about 5 min for 2°, not at room temperature for 1°	2-Methylpropan-2-ol, at once
2,4-Dinitrophenylhydrazine (2,4-DNP)	C=O of aldehydes and ketones	Yellow, orange or orange-red precipitate	Propanone
Tollens' reagent: ammoniacal $\mathrm{AgNO_3}$	Aldehyde, aliphatic or aromatic	Silver mirror	Benzaldehyde
Fehling's or Benedict's solution	Aliphatic aldehyde	Red-brown precipitate of $\mathrm{Cu_2O}$	Ethanal
Schiff's reagent	Aldehyde	Pink or magenta colour returns	Methanal
Iodoform: $\mathrm{I_2}$ and NaOH, or KI and NaOCl	$\mathrm{CH_3CO{-}}$ or $\mathrm{CH_3CH(OH){-}}$	Yellow precipitate of $\mathrm{CHI_3}$	Propan-2-ol
$\mathrm{NaHCO_3}$ solution	Carboxylic acid	Brisk effervescence of $\mathrm{CO_2}$	Ethanoic acid
Carbylamine: $\mathrm{CHCl_3}$ and alcoholic KOH	Primary amine, aliphatic or aromatic	Foul-smelling isocyanide	Aniline
Hinsberg's reagent, $\mathrm{C_6H_5SO_2Cl}$	Primary, secondary or tertiary amine	1°: product dissolves in alkali; 2°: insoluble solid; 3°: no reaction	Ethanamine dissolves; N-methylaniline gives an insoluble solid
Azo dye: $\mathrm{NaNO_2/HCl}$ at 0–5 °C, then alkaline 2-naphthol	Aromatic primary amine	Orange-red dye	Aniline
Molisch's test: α-naphthol and conc. $\mathrm{H_2SO_4}$	Carbohydrate	Violet ring where the layers meet	Glucose
Biuret test: alkaline $\mathrm{CuSO_4}$	Peptide bond	Violet colour	Egg albumin
Alcoholic $\mathrm{AgNO_3}$ , warm	Reactive C–X: benzylic, allylic or tertiary	AgCl white, AgBr pale yellow precipitate	Benzyl chloride A halogen on the ring (chlorobenzene) gives no precipitate.

Every row is reagent, group and observation; a question gives one and asks for another.

Watch out for (5)

Fehling's and Benedict's miss aromatic aldehydes
→ Functional-group tests: reagent, group and observation
Iodoform needs CH₃CO or CH₃CH(OH)
→ Functional-group tests: reagent, group and observation
Phenols dissolve in NaOH but not in NaHCO₃
→ Functional-group tests: reagent, group and observation
CAN is for alcohols, FeCl₃ is for phenols
→ Functional-group tests: reagent, group and observation
Decolourised bromine water does not prove a C=C
→ Functional-group tests: reagent, group and observation

PYQ weightage by concept

10 concepts · 48 PYQs — where the marks actually sit, so you know what to drill first

Carbocations, Rearrangements and Addition Regiochemistry11 PYQs · 23%

Concept	PYQs	Share
Addition regiochemistry by reagent	6	13%
Carbocation stability and 1,2-shifts	5	10%

Carbonyl Reactions: Enolates, Haloform and Cannizzaro8 PYQs · 17%

Concept	PYQs	Share
Aldol, Claisen-Schmidt, Claisen and Michael reactions	4	8%
Haloform, Cannizzaro and decarboxylation	4	8%

Multistep Conversions and Road Maps9 PYQs · 19%

Concept	PYQs	Share
Carbon counting in chain-building steps	5	10%
Aromatic road maps: acylation, reduction and diazonium routes	4	8%

Named Reactions12 PYQs · 25%

Concept	PYQs	Share
Named reactions of alkyl halides, aryl halides and diazonium salts	4	8%
Named reactions of carbonyls, acids, amides and phenols	4	8%
Mechanism class and reagent roles	4	8%

Functional Group Identification Tests8 PYQs · 17%

Concept	PYQs	Share
Functional-group tests: reagent, group and observation	8	17%

Test yourself on Organic Reaction Mechanisms

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Carbocations, Rearrangements and Addition Regiochemistry

Carbonyl Reactions: Enolates, Haloform and Cannizzaro

Multistep Conversions and Road Maps

Named Reactions

Functional Group Identification Tests

Formula & revision sheet

PYQ weightage by concept

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