Where intermediates come from: bond fission
NIOS Section 23.3.1 defines the breaking of a covalent bond as bond fission, and the way the shared electron pair is divided decides which intermediate appears. There are exactly two modes.
In homolytic fission the two bonding electrons are shared equally, one going to each atom. The neutral fragments produced each carry a single unpaired electron and are called free radicals.
In heterolytic fission the pair is divided unequally — both electrons stay with one atom. This generates ions: the fragment with a positive charge on carbon is a carbocation, and the fragment with a negative charge on carbon is a carbanion.
| Fission | Electron sharing | Intermediate | Net charge on C |
|---|---|---|---|
| Homolytic | Equal (one e⁻ each) | Free radical $\ce{R^.}$ | Neutral, one unpaired e⁻ |
| Heterolytic | Unequal (both e⁻ to one atom) | Carbocation $\ce{R+}$ / carbanion $\ce{R-}$ | +1 (cation) or −1 (carbanion) |
The two NIOS prototypes capture this cleanly. Heating or irradiating ethane drives homolysis to two methyl radicals, $\ce{H3C-CH3 ->[\Delta,\ h\nu] 2 H3C^.}$, while heterolysis of a hypothetical bond gives an ion pair, $\ce{A:B -> A+ + B-}$. The charged species then behave as electrophiles (electron-deficient, e.g. $\ce{H+}$, $\ce{NO2+}$, $\ce{BF3}$) or nucleophiles (electron-rich, e.g. $\ce{OH-}$, $\ce{H2O}$, $\ce{:NH3}$), as catalogued in NIOS 23.3.1.
Electron-displacement effects that govern stability
Why one carbocation outlasts another comes down to how electron density is shifted around the charged centre. NIOS Section 23.3.2 lists four electron-displacement effects; three of them are the levers you pull in every stability argument.
| Effect | Nature | Acts through | Stabilises a cation when… |
|---|---|---|---|
| Inductive (I) | Permanent | σ-bond chain | +I (electron-releasing) groups are attached |
| Resonance / mesomeric | Permanent (delocalised) | π / lone-pair conjugation | The charge is delocalised over several atoms |
| Hyperconjugation | Permanent (no-bond resonance) | σ(C–H)–p overlap | More α C–H bonds are available |
| Electromeric (E) | Temporary | π-bond, on reagent attack | Operates only during reaction, on multiple bonds |
The inductive effect is the permanent polarisation transmitted along a σ-bonded carbon chain; NIOS ranks groups by their −I (electron-withdrawing) and +I (electron-releasing) power:
$$\ce{-I:\ (CH3)3N+ > -NO2 > -CN > -F > -Cl > -Br > -I > -OH > -OCH3 > -C6H5 > -H}$$ $$\ce{+I:\ (CH3)3C- > (CH3)2CH- > CH3CH2- > -CH3 > -H}$$
Resonance spreads charge over a delocalised system — NIOS uses benzene and the ethanoate ion as canonical examples, where the true molecule is the resonance hybrid of contributing structures. Hyperconjugation, expressly called no-bond resonance in NIOS 23.3.2, is the conjugation of a σ(C–H) bond with an adjacent π bond or empty orbital. These three are the toolkit for everything below.
Carbocation stability and its order
A carbocation is classified as primary, secondary or tertiary depending on whether the positively charged carbon is bonded to one, two or three other carbon atoms. The NIOS haloalkanes module states the order plainly: as the number of attached alkyl groups rises, stability rises, because alkyl groups are electron-releasing and disperse the positive charge.
The benchmark order, with methyl added at the lower end, is therefore:
$$\ce{(CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+}$$
that is, 3° > 2° > 1° > methyl.
Bar height schematically tracks relative stability. The same factors that lower energy also lower the activation barrier to forming the cation.
NIOS gives the same order a second, independent justification through hyperconjugation. The positively charged carbon carries a vacant p orbital, and each adjacent (α) C–H bond can overlap with it, delocalising the charge into the C–H framework — the more such bonds, the greater the stabilisation. Counting α C–H bonds makes the trend quantitative:
| Cation | Example | α C–H bonds | Stabilising factors |
|---|---|---|---|
| Methyl | $\ce{CH3+}$ | 0 | None (least stable) |
| Primary (ethyl) | $\ce{CH3CH2+}$ | 3 | +I of one alkyl, 3 hyperconjugative C–H |
| Secondary (isopropyl) | $\ce{(CH3)2CH+}$ | 6 | +I of two alkyls, 6 hyperconjugative C–H |
| Tertiary (tert-butyl) | $\ce{(CH3)3C+}$ | 9 | +I of three alkyls, 9 hyperconjugative C–H |
A 3° cation has nine such α C–H bonds; a methyl cation has none. This is why hyperconjugation and +I push the order in the same direction.
Stability and rate of formation point the same way
Students often treat "most stable cation" and "cation that forms fastest" as separate facts. For these intermediates they coincide: a more stable carbocation sits at lower energy, so the transition state leading to it is also lower, and it forms faster. That is exactly why a tertiary halide ionises readily under SN1 conditions.
Lower energy intermediate → lower activation barrier → faster formation → that pathway wins.
Allyl and benzyl cations: resonance
Hyperconjugation and +I are powerful, but resonance is decisive. When the positive carbon is adjacent to a π system, the charge is delocalised over more than one atom, and such cations frequently exceed even a tertiary cation in stability. The two NEET-relevant cases are the allyl cation ($\ce{CH2=CH-CH2+}$) and the benzyl cation ($\ce{C6H5-CH2+}$).
Following the NIOS resonance principle (23.3.2), the real ion is the hybrid of the two contributors, so no single carbon bears the full charge — a strongly stabilising arrangement.
The benzyl cation does the same thing on a larger stage: its positive charge delocalises into the benzene ring, generating contributing structures with the charge at the ortho and para ring positions. Because the charge is dispersed over four carbons, the benzyl cation is exceptionally stable, which is why benzylic substrates are textbook SN1/E1 candidates. A practical ranking that NEET items lean on is:
$$\text{benzyl} \approx \text{allyl} > 3° > 2° > 1° > \text{methyl}$$
See exactly how a stable cation steers the substitution pathway in SN1 vs SN2 comparison.
Carbocation rearrangement
Because a less stable carbocation will convert to a more stable one whenever it can, reactions passing through a free cation are prone to molecular rearrangement. NIOS Section 23.3.8 defines this as the migration of an atom or group from one position to another, with a fundamental change in the carbon skeleton, and gives the canonical example:
$$\ce{CH3CH2CH2CH2Cl ->[AlCl3] CH3CH2CHClCH3}$$
1-chlorobutane rearranges to 2-chlorobutane. Mechanistically, the Lewis acid assists ionisation to a primary cation, which undergoes a 1,2-hydride shift to the more stable secondary cation before chloride returns. The two common shifts are the hydride (1,2-H) shift and the alkyl (1,2-methyl) shift; both move the positive charge to a more substituted, more stable carbon.
Rearrangement only when a free cation exists
Rearrangement is a signature of carbocation pathways — SN1, E1, and acid-catalysed additions. It does not occur in SN2 or E2, which never form a discrete cation, nor in concerted radical chains. If a product looks "shifted" from the expected position, suspect a carbocation intermediate.
No free carbocation → no skeletal rearrangement.
Carbanion stability: the reverse order
A carbanion carries a lone pair and a negative charge on carbon — it is electron-rich. The factors that stabilise a carbocation therefore destabilise a carbanion, and vice versa. Electron-releasing alkyl groups (+I) pump still more electron density onto an already negative centre, raising its energy, so increasing alkyl substitution lowers stability. The order is exactly the reverse of carbocations:
$$\ce{CH3- > CH3CH2- > (CH3)2CH- > (CH3)3C-}$$
that is, methyl > 1° > 2° > 3°.
| Factor | Effect on carbocation | Effect on carbanion |
|---|---|---|
| +I (alkyl, electron-releasing) | Stabilises (disperses +) | Destabilises (adds e⁻ density) |
| −I (EWG: $\ce{-NO2}$, $\ce{-CN}$, halogen) | Destabilises | Stabilises (withdraws e⁻ density) |
| Resonance with adjacent π / EWG | Stabilises | Stabilises (delocalises −) |
| Stability order | 3° > 2° > 1° > methyl | methyl > 1° > 2° > 3° |
Read directly off the NIOS −I series, the strongest electron-withdrawing groups — $\ce{(CH3)3N+}$, $\ce{-NO2}$, $\ce{-CN}$ — are the best carbanion stabilisers, especially when positioned to pull the negative charge through both inductive withdrawal and resonance. This is the structural reason α-hydrogens flanked by carbonyl or nitro groups are acidic.
Free radical stability
A free radical is neutral but carries one unpaired electron in a singly-occupied orbital. Like a carbocation it is electron-deficient at that carbon, so the same stabilising influences apply, and the stability order parallels the carbocation series:
$$\text{3° radical} > \text{2° radical} > \text{1° radical} > \text{methyl radical}$$
Alkyl groups release electron density toward the radical centre, and the half-filled orbital engages in hyperconjugation with adjacent C–H bonds in the same no-bond-resonance manner described for cations — more α C–H bonds mean greater delocalisation of the odd electron and a more stable radical. Allylic and benzylic radicals enjoy genuine resonance delocalisation and are correspondingly very stable.
This order is decisive in the peroxide (anti-Markovnikov) effect. NIOS records that HBr adds to propene against Markovnikov's rule in the presence of benzoyl peroxide:
$$\ce{CH3CH=CH2 + HBr ->[benzoyl\ peroxide] CH3CH2CH2Br}$$
The chain follows initiation, propagation and termination steps (the same skeleton NIOS gives for methane chlorination). In propagation, a bromine atom adds to the terminal carbon so that the more stable secondary carbon radical forms; that radical then abstracts H from HBr, placing bromine on carbon-1 and giving 1-bromopropane. Radical stability — not cation stability — sets the orientation here.
Same orientation logic, two different intermediates
Both Markovnikov and anti-Markovnikov addition obey "form the more stable intermediate." The reversal of the product is purely because the intermediate changes identity: a carbocation in ionic addition (Br on the more substituted carbon) versus a carbon radical in the peroxide route (Br on the terminal carbon).
Peroxide present → radical chain → anti-Markovnikov, but only for HBr.
Carbenes in brief
A carbene ($\ce{:CH2}$, methylene) is a neutral, highly reactive divalent-carbon intermediate in which carbon bears only six valence electrons — two bonding pairs and one non-bonding pair of electrons, leaving it electron-deficient. Carbenes are far more transient than the three intermediates above and appear only at the edge of the NEET scope, but they round out the family of species generated when bonds break in non-standard ways. Their key qualitative trait for examination purposes is extreme reactivity arising from the incomplete octet at carbon; like radicals and cations, they are stabilised by adjacent electron-donating groups.
How stability dictates the mechanism
The payoff of all this ranking is predictive: the stability of the intermediate a substrate can generate decides which mechanism operates. The NIOS haloalkanes module makes the point explicitly — tertiary halides undergo nucleophilic substitution by SN1 precisely because they ionise to a stable tertiary carbocation, whereas primary halides, which would have to pass through an unstable primary cation, instead react by the concerted SN2 route that never forms a free cation.
| Substrate / condition | Intermediate stability | Operative mechanism | Outcome |
|---|---|---|---|
| 1° halide + strong Nu⁻ | 1° cation too unstable | SN2 (concerted) | Inversion of configuration |
| 3° / benzylic / allylic halide | Stable cation | SN1 (via carbocation) | Possible rearrangement, racemisation |
| 1° halide + strong base, heat | No free cation | E2 (concerted) | Alkene, anti-periplanar |
| 3° halide, weak base, heat | Stable cation | E1 (via carbocation) | Most-substituted alkene favoured |
| Alkene + HX (ionic) | More stable cation forms | Electrophilic addition | Markovnikov product |
| Alkene + HBr / peroxide | More stable radical forms | Free-radical addition | Anti-Markovnikov product |
The same logic runs through elimination: E1 shares its rate-determining carbocation with SN1, so it too favours substrates that give stable cations, while E2 is concerted and dominates with primary substrates and strong bases. In every row of the table, the column that actually determines the answer is "intermediate stability." Master that single column and the rest of organic mechanism becomes a matter of reading the substrate.
Stability of Reactive Intermediates — at a glance
- Homolytic fission → free radicals; heterolytic fission → carbocation + carbanion (NIOS 23.3.1).
- Carbocation order: 3° > 2° > 1° > methyl; benzyl ≈ allyl exceed 3° by resonance.
- Three stabilising levers: +I (electron release), resonance (delocalisation), hyperconjugation (α C–H: 0, 3, 6, 9 for methyl→3°).
- Carbanion order is the reverse: methyl > 1° > 2° > 3°; stabilised by −I / EWG, destabilised by +I.
- Free radical order parallels carbocations: 3° > 2° > 1° > methyl; drives the peroxide (anti-Markovnikov) effect.
- Less stable cations rearrange (1,2-H or 1,2-alkyl shift) — only when a free cation exists (SN1/E1).
- Stable cation → SN1/E1 and Markovnikov; no viable cation → SN2/E2.