Reaction Mechanism (Organic) — NEET Notes

Types of organic reactions — the four families

Every organic reaction NCERT asks you to recognise belongs to one of four families: substitution, addition, elimination, or rearrangement. The classification is mechanical — you can usually assign a family just by counting atoms before and after the arrow. Substitution swaps one atom or group for another while keeping the skeleton; addition takes two molecules and makes one larger one; elimination breaks one molecule into two (typically generating a π-bond); and rearrangement reshuffles the skeleton without changing the molecular formula.

NEET tests this taxonomy directly. "Hydrohalogenation of propene" is addition; "chlorination of methane" is free-radical substitution; "dehydrohalogenation of 2-bromobutane with alcoholic KOH" is elimination. The four families share an underlying script — bond breaks, intermediate forms, bond makes — but the labels matter because they sort the rest of the syllabus into manageable boxes.

How bonds break — homolytic vs heterolytic

The fastest way to predict a mechanism is to ask how the bond breaks. A covalent bond can break in two fundamentally different ways. In homolytic cleavage, the two bonding electrons split symmetrically — one to each atom. The fragments are neutral but carry an unpaired electron: they are free radicals. Homolysis happens under high temperatures, UV light, or in the presence of peroxide initiators — environments that supply enough energy to break the bond evenly. In heterolytic cleavage, both electrons stay with one atom. One fragment leaves as a cation (electron-deficient), the other as an anion (electron-rich). Heterolysis dominates in polar solvents that can stabilise the resulting ions.

Homolysis makes radicals. Heterolysis makes ions. Almost every NEET mechanism reduces to which kind of break opened the story.

The first decision in any mechanism problem

Reaction conditions tell you which mode operates. Chlorination of methane in sunlight: homolytic — radicals. Hydrolysis of an alkyl halide in water: heterolytic — ions. The peroxide effect is striking precisely because it flips HBr addition from heterolytic to homolytic.

Electrophiles and nucleophiles

If heterolytic cleavage rules a reaction, then the species attacking the substrate must be either electron-rich or electron-poor. Electrophiles ("electron-loving") are species that accept an electron pair — they have an empty orbital or carry a partial positive charge. Nucleophiles ("nucleus-loving") are species that donate an electron pair — they carry a lone pair or a negative charge. The two are not always free-floating ions: they can be polarised molecules whose ends carry partial charges. The trick is to identify which end of each reagent is the active centre, and then ask which atom of the substrate it is going to attack.

A quick discipline that saves marks: never describe a reaction without naming the electrophile and the nucleophile. In nitration of benzene, the electrophile is NO₂⁺, the nucleophile is the benzene π-cloud. In hydrolysis of methyl bromide, the nucleophile is OH⁻, the electrophilic carbon is the one carrying the bromide. Train yourself to label them on the page.

Reactive intermediates — the four short-lived species

Heterolysis of a C–X bond generates either a cation or an anion at carbon — a carbocation or a carbanion. Homolysis of the same bond generates a free radical. A separate species, the carbene, arises from loss of two atoms with their electron pair, leaving a carbon with only six valence electrons but no charge. These four species are the protagonists of organic mechanism. Knowing each one's hybridisation, geometry, and stability ordering is non-negotiable for NEET.

Carbocation stability — the most-asked ordering on NEET

Three structural factors stabilise a carbocation: inductive donation (+I) from alkyl groups, hyperconjugation from adjacent C–H σ bonds, and resonance delocalisation where lone pairs or π-systems are available. A tertiary cation has three alkyl groups around the positive carbon — three +I donors and nine α-hydrogens available for hyperconjugation. A primary cation has only one of each. The result is a clean stability order that NEET has tested in every conceivable disguise.

Three subtleties matter for exam-day. First: resonance beats induction. Benzyl (C₆H₅–CH₂⁺) and allyl (CH₂=CH–CH₂⁺) cations enjoy delocalisation into adjacent π-systems and are therefore unusually stable — often more stable than a "plain" tertiary cation. Second: hyperconjugation requires α-hydrogens. (CH₃)₃C⁺ has nine of them, available for nine "no-bond" canonical forms. A vinyl cation (CH₂=CH⁺) has none — it is highly unstable. Third: 1,2-shifts. If a less-stable carbocation can rearrange to a more-stable one by a 1,2-hydride or 1,2-methyl shift, it will — and the final product reflects the rearranged skeleton. Watch for this in SN1 and E1 problems.

Electronic effects — inductive, electromeric, resonance, hyperconjugation

The stability of intermediates and the polarisation of substrates both trace back to four electronic effects that distort electron density along covalent bonds. NCERT introduces them in Class 11 Unit 8 and they reappear in every reaction in Class 12. Get them straight here.

Inductive effect (I)

A permanent polarisation of σ bonds caused by electronegativity difference. The effect travels along the σ framework and decays sharply with distance — it is essentially dead beyond three carbons. −I groups withdraw electrons (NO₂, CN, COOH, halogens, OH, NH₂ when not donating by lone pair). +I groups donate electrons (alkyl groups, especially tertiary). Inductive effect determines the acidity of carboxylic acids (Cl substituents make Cl-CH₂-COOH more acidic than CH₃-COOH because −I stabilises the conjugate base) and is one of the three contributors to carbocation stability.

Electromeric effect (E)

A temporary, complete shift of a π-bond's electron pair in the presence of an attacking reagent. It exists only while the reagent is on its way to the substrate. +E denotes electron-shift towards the attacking species (e.g., addition of H⁺ to an alkene); −E denotes electron-shift away (e.g., nucleophile attacking C=O). Unlike inductive effect, electromeric effect is instantaneous and reversible — it disappears when the attacking reagent is removed.

Resonance (mesomeric) effect

A permanent delocalisation of π or lone-pair electrons through a conjugated system. +M / +R groups push electrons into the system (–NH₂, –OH, –OR, halogens). −M / −R groups pull electrons out (–NO₂, –CN, –COOH, –CHO, –COR). Resonance is the most powerful electronic effect — it can override inductive effects entirely (–NH₂ is +M even though N is electronegative). It is the controlling factor in electrophilic aromatic substitution: +M groups activate the ring and are ortho/para-directing; −M groups deactivate it and are meta-directing.

Hyperconjugation (no-bond resonance)

Delocalisation of σ electrons of a C–H bond into an adjacent empty p-orbital, π-bond, or radical centre. Also called the Baker-Nathan effect. The number of hyperconjugative structures equals the number of α-hydrogens. Three students-must-know consequences: (i) the stability of carbocations and radicals follows 3° > 2° > 1° > methyl precisely because of α-H counts; (ii) alkenes with more α-hydrogens (more substituted) are more stable — the basis of Saytzeff's rule; (iii) the C–H bond length of methyl groups attached to a carbocation is measurably longer than ordinary C–H bonds because of the partial double-bond character.

Nucleophilic substitution — SN1 vs SN2

Replace the leaving group on a saturated carbon with a nucleophile and you have a nucleophilic substitution. The reaction proceeds by one of two mechanisms — SN1 or SN2 — and which one wins depends almost entirely on the substrate structure, the nucleophile strength, and the solvent. NEET asks this comparison directly almost every year, and the haloalkane question on the paper is usually a disguised SN1 vs SN2 puzzle.

SN2 (bimolecular) is a single-step, concerted reaction. The nucleophile attacks the electrophilic carbon from the side opposite the leaving group; as the new C–Nu bond forms, the old C–X bond breaks; the transition state has both partial bonds and a flattened sp²-like carbon with the three other substituents in a plane. Because the nucleophile arrives from the back, the configuration at carbon flips — the famous Walden inversion. Kinetics are second-order: rate = k[substrate][nucleophile]. SN2 thrives on small, less-hindered substrates (methyl > 1° > 2° ≫ 3°), strong nucleophiles, and polar aprotic solvents (acetone, DMSO, DMF) that leave the nucleophile bare and reactive.

SN1 (unimolecular) is a two-step reaction. Step 1: the leaving group departs on its own, giving a carbocation. Step 2: the nucleophile attacks the planar carbocation from either face. Kinetics are first-order: rate = k[substrate] only — the nucleophile is not in the rate-determining step. Because the carbocation is planar, the nucleophile attacks from both faces with roughly equal probability, giving a racemic mixture from an originally chiral substrate. SN1 favours stable carbocations (3° ≫ 2° ≫ 1° > methyl — the reverse of SN2), weak nucleophiles, and polar protic solvents (water, ethanol) that stabilise the ionic intermediates.

Two NEET-flavoured shortcuts. If the substrate is methyl or primary, assume SN2. If it is tertiary, assume SN1. The middle (secondary) is where examiners make you work — and the deciding clue is usually the solvent or the nucleophile strength stated in the question. Allyl and benzyl halides do both readily — the carbocation is resonance-stabilised and the substrate carbon is small.

Elimination — E1 vs E2

If the nucleophile in a substitution problem starts behaving as a base — abstracting a β-proton instead of attacking the α-carbon — you have a competing elimination. The result is an alkene; the mechanism is either E1 or E2 depending on conditions. The parallels with SN1/SN2 are deliberate, and worth committing to memory side-by-side.

E2 (bimolecular) is concerted, like SN2. The base removes a β-hydrogen while the C–X bond breaks, all in one step. The H and the leaving group depart anti-periplanar to each other (180° dihedral) so the developing π-bond can form cleanly. Kinetics are second-order: rate = k[substrate][base]. E2 needs a strong base — alcoholic KOH being the textbook reagent — and is the dominant elimination mechanism on primary and secondary halides.

E1 (unimolecular) is two-step, like SN1. The leaving group departs first, giving a carbocation; the base then removes a β-proton from the cation. Kinetics are first-order: rate = k[substrate]. E1 needs a stable carbocation — so tertiary and resonance-stabilised substrates dominate — and it competes with SN1 on the same intermediate. The same conditions (heat, polar protic solvent, weak base) push both reactions, and the relative amounts depend on temperature and the basicity of the medium.

When β-elimination can give two alkenes, the more substituted (more stable) alkene is the major product.

Saytzeff's rule (1875)

Saytzeff's rule reflects the same hyperconjugation logic that ordered the carbocations: an alkene with more alkyl groups on its double-bonded carbons has more α-hydrogens for hyperconjugative stabilisation, and is therefore the thermodynamic product. The Hofmann counter-rule — less-substituted alkene as major product — operates only with bulky bases (potassium tert-butoxide) and is beyond NEET's regular scope.

Electrophilic addition to alkenes

The π-electrons of an alkene are electron-rich and attract electrophiles. The mechanism is two-step: the electrophile attacks the π-bond to form a carbocation, then the counter-nucleophile attacks the carbocation. The key prediction is which way the alkene polarises — that is, which carbon takes the positive charge and which takes the X.

The answer, codified by Markovnikov in 1869, is that the more-substituted carbon becomes positive. When HX adds across CH₃–CH=CH₂, protonation gives either (CH₃)₂CH⁺ (secondary) or CH₃–CH₂–CH₂⁺ (primary). The secondary cation is more stable, so it forms preferentially, and the bromide ion then captures it — giving 2-bromopropane as the major product. The popular statement of Markovnikov's rule — "H goes to the carbon with more H, X goes to the carbon with fewer H" — is just the surface description; the mechanism is "the more stable carbocation wins."

Other electrophilic additions worth remembering: halogen addition (Br₂, Cl₂) gives a vicinal dihalide via a bridged halonium intermediate; hydration (H₂O, H⁺) gives an alcohol following Markovnikov; hydroboration-oxidation (BH₃, then H₂O₂/OH⁻) gives an anti-Markovnikov alcohol via concerted syn-addition. Each is a variation on the same electron-flow grammar: alkene π-bond donates to an electrophile, an intermediate forms, a nucleophile closes the addition.

Electrophilic aromatic substitution

Benzene's π-cloud is electron-rich enough to attract electrophiles but, unlike an alkene, addition would destroy its aromatic stabilisation. So benzene undergoes substitution: the electrophile attaches, a proton leaves, the aromatic ring is preserved. The mechanism is the same three-step skeleton for every electrophilic aromatic substitution NCERT lists — nitration, halogenation, sulphonation, Friedel-Crafts alkylation, Friedel-Crafts acylation.

When a substituent already sits on the ring, it controls where the next electrophile attaches. +M / activating groups (–OH, –OR, –NH₂, alkyl) push electron density into the ortho and para positions, so they are ortho/para-directing and accelerate the reaction. −M / deactivating groups (–NO₂, –CN, –COOH, –CHO, –SO₃H) pull electron density out, especially from ortho/para — so they are meta-directing and slow the reaction. Halogens are the textbook exception: deactivating but ortho/para-directing, because their −I effect dominates the rate while their +M lone pair still favours ortho/para regiochemistry.

Free-radical chain mechanism

Alkanes have no π-bond and no good leaving group, so they cannot do heterolytic chemistry under ordinary conditions. They can however undergo homolytic substitution — the textbook example being the chlorination of methane in sunlight. Every free-radical chain reaction obeys the same three-stage script: initiation, propagation, termination.

Two mechanistic facts are worth memorising. First, the rate-determining step is the abstraction of H by Cl• — and the activation energy depends on which C–H bond is being broken. Tertiary C–H bonds are weaker than primary, so radical halogenation is regioselective: bromination strongly favours 3° > 2° > 1°, chlorination is less selective but follows the same order. Second, the same chain mechanism — initiation, propagation, termination — is the one responsible for the peroxide-effect addition of HBr to alkenes. NCERT and NEET test these as variations of one core idea.