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MHT-CET Chemistry · Alkenes

Reactions of Alkenes: Addition and Its Direction, KMnO₄ Oxidation, and Ozonolysis

An alkene adds reagents across its double bond — HX with Markovnikov's rule, HBr with peroxide the other way round, water through hydroboration the other way too — and is oxidised by KMnO₄ to a diol (cold, dilute, alkaline) or cleaved to acids (hot, acidic), or cleaved by ozone to aldehydes and ketones.

All Alkenes notesMHT-CET Chemistry strategy

Why this matters

19 PYQs — half the chapter — none HARD. Seven are additions and their direction, nine are KMnO₄ (and one Wacker) oxidations, three are ozonolysis. Three cards; the conditions decide the product every time.

Concept 1 of 3: Addition to Alkenes: Markovnikov, Peroxide Effect and Hydroboration

An acid HX adds so that H goes to the double-bonded carbon that already has more H — Markovnikov — because that leaves the more stable carbocation. With HBr AND a peroxide the mechanism turns radical and Br lands on the less substituted carbon (anti-Markovnikov). Hydroboration–oxidation also puts the OH on the less substituted carbon, giving a primary alcohol from a terminal alkene.

Definition

  • HX (no peroxide): Markovnikov — X on the more substituted carbon. Propene + HBr → 2-bromopropane; 2-methylbut-2-ene + HCl → 2-chloro-2-methylbutane.
  • HBr + peroxide: anti-Markovnikov (only HBr) — 2-methylbut-2-ene → 2-bromo-3-methylbutane.
  • Br₂: adds across the C=C — 2-methylbut-2-ene → 2,3-dibromo-2-methylbutane.
  • Br₂ at high temperature: substitution at the ALLYLIC carbon — propene → CH₂=CH–CH₂Br (allyl bromide).
  • Hydroboration–oxidation (B₂H₆, then H₂O₂/OH⁻): anti-Markovnikov alcohol — propene → propan-1-ol; but-1-ene → butan-1-ol.

Markovnikov's rule

CH3CH=CH2+HBr→CH3CHBrCH3(with peroxide: CH3CH2CH2Br)\mathrm{CH_3CH{=}CH_2 + HBr \rightarrow CH_3CHBrCH_3}\quad(\text{with peroxide: } \mathrm{CH_3CH_2CH_2Br})

Worked example

2-Methylbut-2-ene reacts with HBr in the presence of peroxide. Major product?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 21 April Shift II · Q95Moderate

Example 1 · Alkenes · Reactions of Alkenes, Addition, Oxidation and Ozonolysis

Which of the following compounds is formed as major product in the following reaction. 2-Methylbut-2-ene → Peroxide HBr\xrightarrow[\text{~Peroxide~}]{HBr} Product

Applying the peroxide effect to HCl

Only HBr reverses with peroxide. 2-Methylbut-2-ene + HCl gives the Markovnikov product, 2-chloro-2-methylbutane, peroxide or not.

'3-Bromopropane' is the paper's name for allyl bromide

The 2024 paper prints '3-Bromopropane' as the answer for propene + Br₂ at high temperature. It means CH₂=CH–CH₂Br (3-bromoprop-1-ene): allylic substitution, not addition — pick it over 1,2-dibromopropane.
Drill 6 more on addition to alkenes: markovnikov, peroxide effect and hydroboration

Concept 2 of 3: Oxidation by KMnO₄: Glycols or Cleavage

Cold, dilute, alkaline KMnO₄ (Baeyer's reagent) only adds two OH groups across the double bond — a glycol. Hot KMnO₄ in dilute H₂SO₄ breaks the C=C completely: a =CH– end becomes –COOH, a =CH₂ end becomes CO₂, and a =CR₂ end becomes a ketone. A ring alkene opens into a chain with two acids — cyclohexene gives adipic acid.

Definition

  • Cold, dilute, alkaline KMnO₄: glycol (vicinal diol) — Baeyer's test.
  • KMnO₄ / dil. H₂SO₄ (hot): cleavage — =CH–R → R–COOH; =CH₂ → CO₂; =CR₂ → ketone.
  • Prop-1-ene → ethanoic acid (+ CO₂); phenylethene → benzoic acid; cyclohexene → adipic acid (hexanedioic acid).
  • Wacker process: ethene + O₂ with Pd catalyst → acetaldehyde.

Ring cleavage

cyclohexene→KMnO4/H+, ΔHOOC(CH2)4COOH\text{cyclohexene} \xrightarrow{\mathrm{KMnO_4/H^+},\ \Delta} \mathrm{HOOC(CH_2)_4COOH}

Worked example

Which alkene gives adipic acid with KMnO₄ in dilute H₂SO₄: hex-1-ene, hex-2-ene, hex-3-ene, cyclohexene?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q60Moderate

Example 2 · Alkenes · Reactions of Alkenes, Addition, Oxidation and Ozonolysis

Cyclohexene on oxidation with KMnO4KMnO_{4} in dil. H2SO4H_{2}SO_{4} forms

Stopping at cyclohexanol or cyclohexanone

Hot acidic KMnO₄ does not stop inside the ring — it cleaves the C=C and opens it. Cyclohexanol and cyclohexanone are offered to catch a student who forgets that.
Drill 8 more on oxidation by kmno₄: glycols or cleavage

Concept 3 of 3: Ozonolysis

Ozone cuts the double bond, and with Zn/H₂O workup each half becomes a carbonyl: put =O where the double bond was. A =CH₂ end gives methanal, a =CH–R end an aldehyde, a =CR₂ end a ketone. A symmetrical alkene gives two molecules of the same product.

Definition

  • Rule: replace C=C with two C=O.
  • Propene → methanal + ethanal; but-2-ene → 2 ethanal (acetaldehyde); 2-methylpropene → acetone + methanal.
  • Products are aldehydes and/or ketones (not alcohols or acids) with reductive workup.

Ozonolysis

R2C=CR2′→O3, Zn/H2OR2C=O+O=CR2′\mathrm{R_2C{=}CR'_2 \xrightarrow{O_3,\ Zn/H_2O} R_2C{=}O + O{=}CR'_2}

Worked example

Products of ozonolysis of propene?
Practice this conceptself-check · 1 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 9th May Shift 2 · Q52Easy

Example 3 · Alkenes · Reactions of Alkenes, Addition, Oxidation and Ozonolysis

Identify the products obtained in the ozonolysis of propene.
Drill 2 more on ozonolysis

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 (3)

  • Addition to Alkenes: Markovnikov, Peroxide Effect and Hydroboration

    Markovnikov's rule

    CH3CH=CH2+HBr→CH3CHBrCH3(with peroxide: CH3CH2CH2Br)\mathrm{CH_3CH{=}CH_2 + HBr \rightarrow CH_3CHBrCH_3}\quad(\text{with peroxide: } \mathrm{CH_3CH_2CH_2Br})
  • Oxidation by KMnO₄: Glycols or Cleavage

    Ring cleavage

    cyclohexene→KMnO4/H+, ΔHOOC(CH2)4COOH\text{cyclohexene} \xrightarrow{\mathrm{KMnO_4/H^+},\ \Delta} \mathrm{HOOC(CH_2)_4COOH}
  • Ozonolysis

    Ozonolysis

    R2C=CR2′→O3, Zn/H2OR2C=O+O=CR2′\mathrm{R_2C{=}CR'_2 \xrightarrow{O_3,\ Zn/H_2O} R_2C{=}O + O{=}CR'_2}

Watch out for (3)

Test yourself on Alkenes

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Alkenes chapter testDrill all 19 questions

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