Types of Reaction Mechanisms — Overview

What a Reaction Mechanism Is

A chemical reaction occurs when one substance is converted into another, and every such conversion is accompanied by the breaking of some bonds and the making of others. In organic chemistry this can happen in more than one way, and the precise route — the sequence of elementary steps connecting reactants to products — is what we call the reaction mechanism. NIOS Section 23.3 defines it as the detailed knowledge of the steps in which reactant molecules change into products.

Memorising products is not enough for NEET. The examiner tests whether you can predict the major product, explain regioselectivity, or identify an intermediate — and all three demand that you reason through the mechanism. The good news is that the variety is finite. Once you have internalised how bonds break, what species result, and which of four reaction classes applies, the vast majority of organic problems become pattern recognition.

This page is the central map. Each major idea links onward to a dedicated sibling page where the mechanism is worked through in full detail.

Bond Fission: Homolytic and Heterolytic

The breaking of a covalent bond is called bond fission. Since a covalent bond is a shared pair of electrons, the only question is how those two electrons are distributed when the bond breaks. There are exactly two possibilities, and they lead to entirely different chemistry.

In homolytic fission, the bond breaks with equal sharing — each atom retains one electron. The neutral products are free radicals, reactive species carrying an unpaired electron. This mode is favoured by heat or light in non-polar conditions, as in the thermal or photochemical cleavage of ethane:

$$\ce{H3C-CH3 ->[\text{heat / light}] {}^{\bullet}CH3 + {}^{\bullet}CH3}$$

In heterolytic fission, the bond breaks with unequal sharing — one atom takes both electrons. The result is a pair of ions. The fragment whose carbon bears a positive charge is a carbocation; the fragment whose carbon bears a negative charge is a carbanion:

$$\ce{A:B -> A+ + {:}B-}$$

The Reactive Intermediates

Bond fission produces three short-lived but decisive species — free radicals, carbocations and carbanions. They never appear as isolable products, yet the stability of the intermediate often decides which mechanism operates and which product dominates. A carbocation is classified as primary, secondary or tertiary by the number of carbon groups attached to the charged carbon, and stability rises in the same order because electron-releasing alkyl groups disperse the positive charge.

NIOS attributes carbocation stability both to the inductive (+I) electron release of alkyl groups and to hyperconjugation: a tertiary carbocation has nine C–H bonds available for hyperconjugation, a secondary six and a primary three. More overlapping bonds means greater charge delocalisation and a lower-energy ion. This is precisely why tertiary halides ionise readily and favour the SN1 pathway. The stability ordering for carbanions runs the opposite way, since alkyl groups destabilise an already electron-rich centre.

Electrophiles and Nucleophiles

The charged or polarised species produced by heterolytic fission initiate reactions, and NIOS classifies the attacking reagents into two complementary families.

An electrophile is an electron-deficient species — positively charged or neutral with an incomplete octet — that seeks electrons and attacks regions of high electron density. A nucleophile is electron-rich — negatively charged or a neutral species bearing a lone pair — and attacks regions of low electron density. In Lewis terms, electrophiles are acids and nucleophiles are bases.

Electron-Displacement Effects

Before any reagent can attack, the substrate must develop polarity at the relevant carbon. NIOS Section 23.3.2 explains that this polarity arises from the displacement — partial or complete — of bonding electrons. Four effects matter for NEET, and you should know whether each is permanent or temporary.

The inductive effect is the permanent polarisation of a σ bond caused by an electronegativity difference, as in a C–X bond where the carbon carries δ+ and the halogen δ−. NIOS gives the standard −I ordering $\ce{(CH3)3N+} > \ce{-NO2} > \ce{-CN} > \ce{-F} > \ce{-Cl} > \ce{-Br} > \ce{-I}$ and the +I ordering $\ce{(CH3)3C-} > \ce{(CH3)2CH-} > \ce{CH3CH2-} > \ce{-CH3}$. It explains, for instance, why electron-withdrawing groups strengthen carboxylic acids.

Resonance describes molecules best represented by two or more canonical structures whose hybrid is the real species — benzene, the ethanoate ion and nitromethane being NIOS examples. Hyperconjugation, called "no-bond resonance," is the conjugation of a σ(C–H) bond with an adjacent π system. The electromeric effect is the temporary, complete transfer of a π-electron pair toward the more electronegative atom at the moment a reagent approaches, as when the carbonyl π pair shifts in $\ce{C=O}$.

The Four Classes of Reaction

NIOS Section 23.3 organises all organic reactions into four classes. Recognising the class is the first move in any mechanism question, because the class fixes the family of mechanisms available.

Substitution replaces one atom or group with another. Aliphatic haloalkanes undergo nucleophilic substitution, $\ce{R-X + Nu^- -> R-Nu + X^-}$, while aromatic rings undergo electrophilic substitution, losing a ring hydrogen — for example nitration of benzene to nitrobenzene under $\ce{HNO3/H2SO4}$.

Addition adds a reagent across a multiple bond, consuming unsaturation. The weaker π bond of an alkene breaks readily, so $\ce{CH2=CH2 + Br2 -> CH2Br-CH2Br}$ and the bromine colour fades. Elimination is the reverse: a small molecule is removed from adjacent carbons to create a double bond, as in the acid-catalysed dehydration of ethanol, $\ce{CH3CH2OH ->[H2SO4][403\,K] CH2=CH2 + H2O}$. Rearrangement changes the carbon skeleton itself when an atom or group migrates, as when 1-chlorobutane rearranges to 2-chlorobutane over a Lewis acid.

Mapping Mechanisms to Classes

Each class contains named mechanisms distinguished by their reagent type and their kinetics. This is the table to commit to memory; every linked sibling page expands one row.

Notice the symmetry. Electron-poor sites (carbonyl carbon, polarised C–X) attract nucleophiles; electron-rich sites (alkene π bonds, aromatic rings) attract electrophiles. A homolytic initiation, by contrast, launches the free-radical chain that governs alkane halogenation and the anti-Markovnikov addition of HBr in the presence of peroxides.

Reading a Curved-Arrow Mechanism

Mechanisms are communicated with curved arrows, and reading them fluently is a NEET skill in itself. The conventions are simple and absolute.

An arrow always begins at the source of electrons — a lone pair or a bond — and points toward where the new bond is forming or the charge is going. It never starts at a positive centre and points outward. A nucleophile's lone pair, for example, is drawn attacking an electrophilic carbon, while the displaced bonding pair flows onto the leaving group. Trace the arrows and you can reconstruct every intermediate and predict the product without rote memorisation.