Fundamental concepts in organic reaction mechanism

What is a reaction mechanism?

In plain words: a mechanism is the story of a reaction told electron by electron. Bonds are pairs of electrons; when a reaction happens, those pairs move, and curved arrows record where they go. Knowing which parts of a molecule are rich or poor in electrons tells you where a reagent will attack, and intermediate stability helps compare possible paths. Actual rates and product ratios depend on activation barriers and conditions, not intermediate stability alone.

Fission of a covalent bond

A covalent bond can break in two ways (NCERT Section 8.7.1, pages 271–272).

Heterolytic cleavage. The shared pair stays with one fragment. For cleavage of a neutral bond, the fragments carry opposite charges; the exact electron counts depend on the atoms. Formal heterolysis of bromomethane would give CH₃⁺ and Br⁻.

• A species with a carbon bearing a sextet and a positive charge is a carbocation (earlier, carbonium ion). Carbocations are primary, secondary or tertiary according to whether one, two or three carbons are attached to the positive carbon: CH₃CH₂⁺ (ethyl, primary), (CH₃)₂CH⁺ (isopropyl, secondary), (CH₃)₃C⁺ (tert-butyl, tertiary). For these simple alkyl examples, the usual stability order is CH₃⁺ < CH₃CH₂⁺ < (CH₃)₂CH⁺ < (CH₃)₃C⁺, because attached alkyl groups stabilise the charge by the inductive and hyperconjugation effects (below). A simple three-coordinate carbenium centre is sp² hybridised and locally trigonal planar; its empty p orbital is perpendicular to the plane (NCERT Fig. 8.3(a), pages 271–272).
• If the leaving group takes no electrons, carbon keeps the pair: a species with a negative charge on carbon is a carbanion, such as H₃C:⁻. Its carbon is generally sp³ hybridised and the structure is a distorted tetrahedron, the lone pair taking the fourth position (NCERT Fig. 8.3(b), page 272). Strictly, a carbanion conjugated with a π system can be planar (reasoned).

Simple free alkyl carbocations and carbanions are highly reactive, although suitable substitution, solvation or counterions can stabilise carbon-centred ions. The formal cleavage diagrams specify electron allocation, not proof that isolated ions form spontaneously in every solvent. Reactions that go through heterolytic cleavage are called ionic, heteropolar or polar reactions.

Homolytic cleavage. Each bonded atom takes one electron of the pair, usually on heating or in light. For a neutral R–X molecule the products are neutral free radicals, each with an unpaired electron; charged radicals also exist: R–X → R• + X•. Alkyl radicals, like carbocations, are primary, secondary or tertiary, and their stability rises in the same order: CH₃• < CH₃CH₂• < (CH₃)₂CH• < (CH₃)₃C• (page 272). Reactions that go through homolysis, such as the chlorination of methane, are free radical, homopolar or nonpolar reactions.

Substrate, reagent, nucleophiles and electrophiles

The molecule whose carbon forms the new bond is the substrate and the other reactant is the reagent: in CH₂=CH₂ + Br₂ → BrCH₂CH₂Br, ethene is the substrate. When a carbon–carbon bond forms, both reactants supply carbon and the choice is arbitrary: in C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl, NCERT takes benzene as the substrate (NCERT Section 8.7.2, page 272).

A reagent attacks a reactive site of the substrate. An electron-rich reagent attacks an electron-poor site, such as an atom with an incomplete shell or the positive end of a dipole; an electron-poor reagent attacks an electron-rich site, such as the π electrons of a double bond (page 273). This is the Lewis picture of acids and bases applied to carbon:

• A nucleophile (Nu:, nucleus seeking) brings an electron pair to the reactive site. Examples: negative ions with lone pairs, such as HO⁻, NC⁻ and carbanions R₃C:⁻; neutral molecules with lone pairs, such as H₂O, R₃N and R₂NH. The reaction is nucleophilic.
• An electrophile (E⁺, electron seeking) takes an electron pair from the reactive site. Examples: carbocations such as CH₃⁺, whose carbon has only a sextet, and neutral molecules with a polar group, such as the carbonyl group, >C=O, or alkyl halides, R₃C–X, whose carbon carries a partial positive charge. The reaction is electrophilic.

Electron movement and curved arrows

The movement of electrons is shown by curved arrows (NCERT Section 8.7.3, pages 273–274). A full-headed arrow shows a pair of electrons: it starts where the pair is (a bond or a lone pair) and ends where the pair goes (a new bond position or an atom). A half-headed fish-hook arrow shows a single electron, as in homolysis. For example, HO⁻ + CH₃–Br → CH₃OH + Br⁻ is drawn with one arrow from a lone pair of HO⁻ to the carbon and another from the C–Br bond to bromine (page 274).

Electron displacement effects

Electron density in a molecule can be displaced in two ways (NCERT Section 8.7.4, page 274):

• permanent displacements in the ground state, caused by an atom or group in the molecule: the inductive and resonance effects (and hyperconjugation);
• temporary displacements that appear only when a reagent approaches: the electromeric effect (polarisability effect).

Inductive effect

In a bond between atoms of different electronegativity, the electron density is pulled towards the more electronegative atom, giving a polar covalent bond (NCERT Section 8.7.5, page 274). In chloroethane, CH₃CH₂Cl, the C–Cl bond makes carbon 1 slightly positive (δ+) and chlorine slightly negative (δ−). Carbon 1 in turn draws electron density from the C1–C2 bond, so carbon 2 gets a smaller positive charge (δδ+). This polarisation of a σ bond by the polarisation of an adjacent σ bond is the inductive effect. It decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds (NCERT page 274); three bonds is a rule of thumb, not an exact cutoff (reasoned).

• Electron-withdrawing groups (−I): halogens, nitro (–NO₂), cyano (–CN), carboxy (–COOH), ester (–COOR) and aryloxy (–OAr, such as –OC₆H₅).
• Electron-donating groups (+I): alkyl groups such as methyl (–CH₃) and ethyl (–C₂H₅).

These are relative to hydrogen. The inductive effect explains, for example, why chloroacetic acid is a stronger acid than acetic acid: the −I chlorine draws electron density from the O–H bond and stabilises the carboxylate ion. Acid strength depends on the free-energy change including conjugate-base stabilisation and solvation, rather than bond polarity alone (see factors affecting acid strength).

Resonance structures

Some molecules cannot be described by one Lewis structure (NCERT Section 8.7.6, pages 275–276; see resonance in Unit 4). Benzene drawn with alternating single and double bonds should have two bond lengths, but all its C–C bonds measure 139 pm, between C–C (154 pm) and C=C (134 pm). Its real structure is a resonance hybrid, commonly represented by the two equivalent Kekulé contributors; they are not the only possible contributors. In nitromethane, CH₃NO₂, the two N–O bonds are also of equal length, between N–O and N=O.

• The resonance (canonical, contributing) structures are hypothetical; none is the real molecule. They contribute in proportion to their stability.
• The hybrid is lower in energy than any contributing structure. The energy difference between the hybrid and the lowest-energy contributing structure is the resonance energy; important contributors of similar energy often give substantial stabilisation, but simply counting drawings does not quantify it.
• Resonance structures have the same positions of nuclei and the same number of unpaired electrons. The more stable ones have more covalent bonds, an octet on every atom (a duplet on H), less separation of opposite charges, a negative charge on the more electronegative atom, a positive charge on the more electropositive atom, and more dispersed charge (page 275).

Resonance effect

The resonance (or mesomeric, R or M) effect is the polarity produced in a molecule by the interaction of two π bonds, or of a π bond with a lone pair on an adjacent atom; it is transmitted through the chain (NCERT Section 8.7.7, page 276).

• +R effect: electrons move away from the group into the conjugated system, raising the electron density at certain positions, as from –NH₂ in aniline. +R groups: halogens, –OH, –OR, –OCOR, –NH₂, –NHR, –NR₂, –NHCOR.
• −R effect: electrons move towards the group, as to –NO₂ in nitrobenzene. −R groups: –COOH, –CHO, >C=O, –CN, –NO₂.

Alternate single and double bonds in an open chain or a ring form a conjugated system, such as buta-1,3-diene, aniline and nitrobenzene; their π electrons are delocalised and the system develops polarity (page 277). In an exam, give NCERT’s sentence. Strictly, substituents produce that polarity: conjugation alone need not give a permanent dipole, as symmetrical buta-1,3-diene shows (reasoned).

Electromeric effect

The electromeric (E) effect is a temporary effect, shown by compounds with a multiple bond only in the presence of an attacking reagent: the complete transfer of a shared pair of π electrons to one of the atoms joined by the multiple bond, on the demand of the reagent. “The effect is annulled as soon as the attacking reagent is removed from the domain of the reaction” (NCERT Section 8.7.8, page 277). Strictly, a reaction that has already given products is not reversed by removing the excess reagent (reasoned).

• +E effect: the π electrons move to the atom to which the reagent becomes attached, as when H⁺ adds to an alkene.
• −E effect: the π electrons move to the atom to which the reagent does not attach. NCERT draws this on a C=C; the clean case is CN⁻ attacking the carbon of a C=O group, where the π pair moves onto oxygen (as in Q4 (b)).

When the inductive and electromeric effects act in opposite directions, the electromeric effect predominates (page 277).

Hyperconjugation

Hyperconjugation is a general stabilising interaction: the σ electrons of a C–H bond of an alkyl group directly attached to an unsaturated system, or to an atom with an unshared p orbital, are partly delocalised into it (NCERT Section 8.7.9, pages 277–278). It is a permanent effect. In the ethyl cation, CH₃CH₂⁺, one C–H bond of the methyl group can line up with the empty p orbital of the positive carbon and share its electron density, which spreads out the positive charge (NCERT Fig. 8.4(a)).

For the methyl/ethyl/isopropyl/tert-butyl comparison, more adjacent methyl groups supply more C–H donors: (CH₃)₃C⁺ has nine such C–H bonds, (CH₃)₂CH⁺ six, CH₃CH₂⁺ three and CH₃⁺ none. More generally, C–C σ bonds can participate too, so an α hydrogen is not a universal requirement for every kind of hyperconjugation (reasoned). Hyperconjugation also occurs in alkenes such as propene and in alkylarenes; it may be pictured as giving the C–H bond partial ionic character, and so is also called no-bond resonance (page 278).

Types of organic reactions

Organic reactions fall into four broad classes (NCERT Section 8.7.10, page 278), studied in detail in the hydrocarbons unit (halogenation of alkanes, addition to alkenes) and in Class 12:

• substitution: one atom or group replaces another, CH₃Br + HO⁻ → CH₃OH + Br⁻;
• addition: two molecules combine across a multiple bond, CH₂=CH₂ + Br₂ → BrCH₂CH₂Br;
• elimination: atoms are removed from neighbouring carbons to form a multiple bond, CH₃CH₂OH → CH₂=CH₂ + H₂O (concentrated H₂SO₄, heat; see preparation of alkenes);
• rearrangement: the carbon skeleton or the position of a group changes (Example 6 (d)).

Solved examples

Common mistakes to avoid

Quick revision summary

• Heterolysis: ions (carbocations, sp², planar; carbanions, sp³, pyramidal); homolysis: free radicals.
• Stability of carbocations and of free radicals: methyl < primary < secondary < tertiary.
• Nucleophile: electron-pair donor (HO⁻, CN⁻, H₂O, NH₃); electrophile: electron-pair acceptor (carbocations, >C=O carbon, R–X carbon).
• Inductive effect: through σ bonds, permanent, fades after about three bonds; −I halogens, NO₂, CN, COOH; +I alkyl groups.
• Resonance: hybrid of contributing structures, lower in energy; +R (–OH, –NH₂, halogens) and −R (–NO₂, –CN, –CHO, –COOH).
• Electromeric effect: temporary, on demand of the reagent; wins over the inductive effect when they oppose.
• Hyperconjugation: C–H σ electrons delocalised into an empty p orbital or a π system; more C–H bonds on carbons next to C⁺, more stable cation.
• Reaction types: substitution, addition, elimination, rearrangement.

Questions students ask

What is the difference between hyperconjugation and resonance?

Ordinary π conjugation involves p orbitals and lone pairs, while hyperconjugation involves interaction of σ bonds, such as C–H or C–C, with neighbouring acceptor orbitals. The familiar school C–H examples need a suitably oriented adjacent hydrogen, but hyperconjugation in general does not. “No-bond” contributors are bookkeeping pictures; they do not mean the real molecule has released a proton. Both descriptions involve delocalisation, and stabilisation cannot be inferred by counting contributors alone.

Which is stronger, the inductive effect or the resonance effect?

There is no universal strength ranking independent of structure and reaction. In many conjugated aromatic examples, resonance donation outweighs induction, as it moves easily shifted π electrons or lone pairs through a whole conjugated system, while the inductive effect pulls σ electrons and fades quickly with distance. So –OH, –OCH₃ and –NH₂, which are −I but +R, make a benzene ring richer in electrons than benzene: phenol and aniline react with bromine water at room temperature. The halogens are the exception, as their −I effect is stronger: chlorobenzene undergoes electrophilic substitution more slowly than benzene, though their +R effect still directs attack to the ortho and para positions.

What is the order of the inductive effect of common groups?

Among common electron-withdrawing groups the −I effect falls in the order –NO₂ > –CN > –F > –Cl > –Br > –I, the halogens following their electronegativity; –COOH and –COOR are also −I, but books rank them differently. Alkyl groups are +I, usually ranked (CH₃)₃C– > (CH₃)₂CH– > CH₃CH₂– > CH₃–, with hydrogen as the zero. The halogen trend is illustrated by aqueous acid strengths near room temperature: fluoroacetic acid, FCH₂COOH (pK_a about 2.6), is stronger than iodoacetic acid, ICH₂COOH (about 3.2), because fluorine, the stronger −I group, spreads out the negative charge of the carboxylate ion more.

What is the difference between the electromeric effect and the mesomeric effect?

The mesomeric (resonance) effect is a permanent shift of π electrons or lone pairs through a conjugated system, while the electromeric effect is a temporary, complete transfer of the π pair of a multiple bond to one of its two atoms, which happens only while an attacking reagent is present. The mesomeric effect needs conjugation, as with the lone pair on nitrogen next to the ring in aniline, and shows up in measured bond lengths and dipole moments; the electromeric effect also works at an isolated multiple bond, such as the C=C of propene attacked by H⁺, and helps describe a reaction step rather than the unperturbed ground state; product formation still depends on the full mechanism.

Is a carbocation the same as a carbonium ion?

A carbocation was called a carbonium ion in older books, as NCERT notes, but the two names now differ: carbocation is IUPAC’s general name for any ion whose positive charge sits mainly on carbon, while carbonium ion is used for nonclassical carbocations with multicentre bonding, including five-coordinate carbon, such as CH₅⁺ (the methanium ion). Ions with a three-bonded, sp² positive carbon, such as CH₃⁺ and (CH₃)₃C⁺, are carbenium ions. These are the carbocations of Class 11, JEE and NEET, so ‘carbocation’ is the safe name to use in answers.

What is a carbocation rearrangement?

A carbocation rearrangement is a 1,2-shift of a hydrogen atom or alkyl group, with its bonding pair, from a carbon next to the positive carbon to it, turning a carbocation into a more stable one, such as secondary into tertiary. When HCl adds to 3-methylbut-1-ene, CH₂=CH–CH(CH₃)₂, H⁺ gives the secondary cation CH₃CH⁺CH(CH₃)₂, and a hydride shift gives the tertiary CH₃CH₂C⁺(CH₃)₂, so rearranged 2-chloro-2-methylbutane forms along with the expected 2-chloro-3-methylbutane. Example 6 (d) is a methyl shift.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which is more stable, O₂NCH₂CH₂O⁻ or CH₃CH₂O⁻, and why (NCERT Exercise 8.9)?

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   O₂NCH₂CH₂O⁻. The –NO₂ group is strongly electron-withdrawing (−I) and spreads out the negative charge on oxygen; in CH₃CH₂O⁻ the ethyl group (+I) pushes electron density towards the oxygen and concentrates the charge.

2. Q2. Which electron displacement effect explains these orders of acidity (NCERT Exercise 8.17)? (a) Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH; (b) CH₃CH₂COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH.

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   The inductive effect, in both. (a) Each chlorine is −I and stabilises the carboxylate ion, so more chlorines mean a stronger acid. (b) Alkyl groups are +I and destabilise the carboxylate ion, so more methyl groups mean a weaker acid. Give that in an exam.

   NCERT slip (page 293): NCERT calls order (b) “correct”, but measured aqueous pK_a values put 2-methylpropanoic acid (4.86) slightly ahead of propanoic acid (4.88), with 2,2-dimethylpropanoic acid (5.03) the weakest (Wikipedia, “Isobutyric acid”, “Propionic acid” and “Pivalic acid”); solvation as well as the +I effect matters (reasoned).

3. Q3. Classify the cleavage CH₃O–OCH₃ → 2CH₃O• and name the intermediate (NCERT Exercise 8.16 (a)).

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   Homolysis: each oxygen keeps one electron of the O–O bond (two fish-hook arrows). The products are free radicals (methoxy radicals).