JEE Mains Chemistry · Hydrocarbons

Alkene Stability and Addition of HX and Water

More alkyl groups on the C=C make an alkene more stable; HX and water add through the more stable carbocation (Markovnikov), which may rearrange first, while HBr with a peroxide adds the other way through a radical.

All Hydrocarbons notes JEE Mains Chemistry strategy

Why this matters

Twenty-nine PYQs, twenty-six of them multiple choice, and eight from 2026. Seven put alkenes or carbanions in order of stability or count hyperconjugating hydrogens. Sixteen ask for the product of HX addition, most of them with a hydride or methyl shift, a peroxide or a conjugated system to trap the careless. Six compare acid-catalysed hydration, oxymercuration and hydroboration of the same alkene.

Concept 1 of 3: Stability of alkenes and carbanions

Alkyl groups on the doubly bonded carbons feed electron density into the π bond through hyperconjugation: each C–H bond on a carbon next to the C=C (an α-H) takes part. So the more alkyl groups on the C=C, the more stable the alkene, and the less heat it gives out when hydrogenated. A carbanion is the opposite case: it already has a negative charge, so alkyl groups, which push electrons, destabilise it, while more s-character holds the lone pair closer to the nucleus and stabilises it.

Definition

Alkene stability: tetrasubstituted > trisubstituted > disubstituted > monosubstituted > ethene. Among disubstituted, trans > cis (the cis alkyl groups crowd each other).
Number of hyperconjugation structures = number of α-H (H on carbons attached to the C=C, or to a cation's positive carbon).
An alkene is less stable than the alkane (the π bond is weaker than a σ bond), yet the C=C as a whole is stronger and shorter (134 pm) than a C–C single bond (154 pm).
Carbanion stability by hybridisation: $\mathrm{HC{\equiv}C^- > CH_2{=}CH^- > CH_3CH_2^-}$ (50%, 33%, 25% s-character).
Carbanion stability by alkyl groups: $\mathrm{CH_3^- > 1^\circ > 2^\circ > 3^\circ}$ . Electron-withdrawing groups (–CHO, –NO₂, halogen) stabilise a carbanion; +R groups such as –OCH₃ destabilise it.

Alkene	Alkyl groups on C=C	α-H count	Place in stability order
2,3-Dimethylbut-2-ene	4	12	Most stable of this list
2-Methylbut-2-ene	3	9	Second
trans-But-2-ene	2	6	Third
cis-But-2-ene	2	6	Fourth (steric crowding of the cis groups)
Propene	1	3	Fifth
Ethene	0	0	Least stable

Stability follows the number of alkyl groups on the C=C; at equal substitution, trans beats cis.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 5 Apr 2026 Shift 2 · Q38Moderate

Example 1 · Hydrocarbons · Alkene Stability and Addition of HX and Water

IUPAC name of some alkenes are given below : Find out the correct stability order. (A) 2-Methylbut-2-ene (B) cis-But-2-ene (C) 2,3-Dimethylbut-2-ene (D) Prop-1-ene Choose the correct answer from the options given below:

Carbanions run the opposite way to carbocations

Alkyl groups stabilise a carbocation (3° > 2° > 1°) but destabilise a carbanion (1° > 2° > 3°). Resonance stabilises both; more s-character stabilises a carbanion but destabilises a carbocation.

A weaker π bond does not make C=C weaker than C–C

The π bond is weaker than a σ bond, which is why alkenes react and are less stable than alkanes. But the C=C double bond (σ + π) is still stronger than a C–C single bond.

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Concept 2 of 3: Markovnikov and anti-Markovnikov addition of HX

H⁺ adds first, to the carbon that leaves the MORE stable carbocation, and X⁻ then bonds to the positive carbon. That puts H on the carbon that already has more H (Markovnikov). If a hydride or methyl shift from the next carbon would give a more stable cation, it happens before X⁻ arrives. With HBr and a peroxide the order flips: Br· adds first, to the carbon that leaves the more stable RADICAL, so Br ends on the carbon with more H (anti-Markovnikov).

Definition

Ionic addition (HCl, HBr, HI): Markovnikov, through the more stable carbocation. HI > HBr > HCl in rate.
Rearrangement: a 1,2-hydride or 1,2-methyl shift turns a 2° cation into a 3° one; a cyclobutylmethyl cation expands to a cyclopentyl ring.
Peroxide effect (Kharasch): only HBr. HCl's bond is too strong to give Cl·, and I· combines to I₂ instead of adding.
Conjugated dienes add at the ends (1,4-addition) as well as 1,2, through an allylic cation.
A C=C next to an O lone pair (an enol ether) protonates fastest: the cation is stabilised by the oxygen (oxocarbenium ion).

Two orientations

\mathrm{R{-}CH{=}CH_2 \xrightarrow{HBr} R{-}CHBr{-}CH_3}\qquad \mathrm{R{-}CH{=}CH_2 \xrightarrow{HBr,\ (PhCOO)_2} R{-}CH_2{-}CH_2Br}

Worked example

What is the major product when 3-methylbut-1-ene reacts with HBr (a) in the dark with no peroxide and (b) with benzoyl peroxide?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 27 Jan 2024 · Q55Moderate

Example 2 · Hydrocarbons · Alkene Stability and Addition of HX and Water

3-Methylhex-2-ene on reaction with

HBr

in presence of peroxide forms an addition product (A). The number of possible stereoisomers for '

A

' is _____ .

Look for a shift before placing X

Whenever the first cation is 2° and a neighbouring carbon is 3° or quaternary, check for a hydride or methyl shift. The answer that ignores the shift is always among the options.

Peroxide changes only HBr

HCl and HI with a peroxide still give the Markovnikov product. Only HBr adds anti-Markovnikov.

Count stereocentres in the product, not the alkene

A question may ask for the stereoisomers of the ADDITION product. Adding H and Br can create new stereocentres: two stereocentres give up to four stereoisomers.

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Concept 3 of 3: Three ways to add water to an alkene

All three routes put H and OH across the C=C, but they differ in where the OH goes and whether the carbon skeleton can change. Dilute acid goes through a free carbocation, so it is Markovnikov and can rearrange. Oxymercuration goes through a bridged mercurinium ion, so it is Markovnikov but never rearranges. Hydroboration puts boron, and later OH, on the LESS substituted carbon, with H and OH added on the same face.

Definition

Acid-catalysed hydration (dil. H₂SO₄ or H₂O/H⁺): Markovnikov; carbocation shifts possible.
Oxymercuration–demercuration (Hg(OAc)₂, H₂O; then NaBH₄): Markovnikov; no rearrangement.
Hydroboration–oxidation (B₂H₆ or BH₃·THF; then H₂O₂, OH⁻): anti-Markovnikov; syn addition; no rearrangement.

Route	Orientation	Rearrangement	Product from 3,3-dimethylbut-1-ene
$\mathrm{H_2O,\ H^+}$	Markovnikov	Yes (methyl shift here)	2,3-Dimethylbutan-2-ol
$\mathrm{Hg(OAc)_2,\ H_2O}$ ; $\mathrm{NaBH_4}$	Markovnikov	No	3,3-Dimethylbutan-2-ol
$\mathrm{B_2H_6}$ ; $\mathrm{H_2O_2,\ OH^-}$	Anti-Markovnikov, syn	No	3,3-Dimethylbutan-1-ol

One alkene, three different alcohols: the acid route is the only one that can move a methyl group.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 4 April 2024 · Q127Moderate

Example 3 · Hydrocarbons · Alkene Stability and Addition of HX and Water

Consider the above reactions, identify product

B

and product

C

Oxymercuration does not rearrange

The mercurinium ion is bridged, so no free carbocation forms and no group shifts. An option that shows a shifted skeleton from oxymercuration is wrong.

Hydroboration is anti-Markovnikov without any peroxide

Boron is the electrophile and goes to the less crowded carbon. The result, OH on the less substituted carbon, needs no radical and no peroxide.

Drill 5 more on three ways to add water to an alkene

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Markovnikov and anti-Markovnikov addition of HX
Two orientations
$\mathrm{R{-}CH{=}CH_2 \xrightarrow{HBr} R{-}CHBr{-}CH_3}\qquad \mathrm{R{-}CH{=}CH_2 \xrightarrow{HBr,\ (PhCOO)_2} R{-}CH_2{-}CH_2Br}$

Reference tables (2)

Stability of alkenes and carbanions6 rows

Alkene	Alkyl groups on C=C	α-H count	Place in stability order
2,3-Dimethylbut-2-ene	4	12	Most stable of this list
2-Methylbut-2-ene	3	9	Second
trans-But-2-ene	2	6	Third
cis-But-2-ene	2	6	Fourth (steric crowding of the cis groups)
Propene	1	3	Fifth
Ethene	0	0	Least stable

Stability follows the number of alkyl groups on the C=C; at equal substitution, trans beats cis.

Three ways to add water to an alkene3 rows

Route	Orientation	Rearrangement	Product from 3,3-dimethylbut-1-ene
$\mathrm{H_2O,\ H^+}$	Markovnikov	Yes (methyl shift here)	2,3-Dimethylbutan-2-ol
$\mathrm{Hg(OAc)_2,\ H_2O}$ ; $\mathrm{NaBH_4}$	Markovnikov	No	3,3-Dimethylbutan-2-ol
$\mathrm{B_2H_6}$ ; $\mathrm{H_2O_2,\ OH^-}$	Anti-Markovnikov, syn	No	3,3-Dimethylbutan-1-ol

One alkene, three different alcohols: the acid route is the only one that can move a methyl group.

Watch out for (7)

Carbanions run the opposite way to carbocations
→ Stability of alkenes and carbanions
A weaker π bond does not make C=C weaker than C–C
→ Stability of alkenes and carbanions
Look for a shift before placing X
→ Markovnikov and anti-Markovnikov addition of HX
Peroxide changes only HBr
→ Markovnikov and anti-Markovnikov addition of HX
Count stereocentres in the product, not the alkene
→ Markovnikov and anti-Markovnikov addition of HX
Oxymercuration does not rearrange
→ Three ways to add water to an alkene
Hydroboration is anti-Markovnikov without any peroxide
→ Three ways to add water to an alkene

Test yourself on Hydrocarbons

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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