Electrophilic Addition

8 MCQs2 revision cards9-step worked example
Source: NCERT Aldehydes, Ketones and Carboxylic AcidsPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The trap first: NEET questions on electrophilic addition to alkenes frequently test whether you remember the peroxide exception. The same reagent — HBr — gives opposite products depending on whether peroxide is present. Aspirants who apply Markovnikov's rule mechanically without checking the reaction conditions lose marks on what should be a straightforward question.

The mechanism. Electrophilic addition is the characteristic reaction of alkenes. The electron-rich π-bond of the C=C double bond acts as a nucleophile, attacking an incoming electrophile. The general sequence:

  1. Electrophile attacks the π-bond — one carbon bonds to the electrophile, generating a carbocation intermediate on the adjacent carbon.
  2. Nucleophile attacks the carbocation — the remaining fragment (anion or nucleophilic species) bonds to the positively charged carbon.

For HX addition (X = Cl, Br, I) to an unsymmetrical alkene, Markovnikov's rule governs regioselectivity: the hydrogen adds to the carbon already bearing more hydrogens, placing X on the more substituted carbon. The driving force is carbocation stability — the more substituted carbocation is more stable (NCERT Class 11 Chemistry, Chapter 9, page 14).

The peroxide exception (Kharasch effect). When HBr reacts with an alkene in the presence of peroxide (ROOR), the mechanism switches from ionic to radical. The radical intermediate is stabilised at the more substituted carbon, so Br ends up on the less substituted carbon — anti-Markovnikov addition. This reversal is specific to HBr only. HCl's C–Cl bond is too strong for radical initiation; HI's radical chain terminates too readily (NCERT Class 11 Chemistry, Chapter 9, page 16).

Watch-out for NEET: always scan the question stem for "in the presence of peroxide," "ROOR," or "Kharasch conditions." If absent, default to Markovnikov. If present and the acid is HBr, switch to anti-Markovnikov.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

In electrophilic addition to an alkene, the first step involves attack of the electrophile on the:

MCQ 2Easy RecallPractice

Markovnikov's rule states that during addition of HX to an unsymmetrical alkene (without peroxide), hydrogen adds to the carbon that:

MCQ 3Easy RecallPractice

The anti-Markovnikov addition of HBr in the presence of peroxide is also known as:

MCQ 4Direct ApplicationPractice

What is the major product when HBr is added to propene (CH₃–CH=CH₂) in the absence of peroxide?

MCQ 5Direct ApplicationPractice

What is the major product when HBr is added to propene in the presence of benzoyl peroxide?

MCQ 6Direct ApplicationPractice

HCl is added to 2-methylpropene (isobutylene) in the presence of peroxide. The major product is:

MCQ 7Concept TrapPractice

In the electrophilic addition of HBr to propene (without peroxide), the intermediate carbocation is:

MCQ 8CalculationPractice

When HBr is added to 3-methylbut-1-ene (CH₂=CH–CH(CH₃)₂) without peroxide, a secondary carbocation initially forms. However, the major product corresponds to a tertiary halide. The best explanation is:

Quick recall before you leave

Worked Example

  1. 1

    Given

    - Substrate: but-1-ene (CH₃CH₂CH=CH₂) - Reagent: HBr - Condition: in the presence of organic peroxide (ROOR)

  2. 2

    Required

    Identify the major product of the reaction.

  3. 3

    Concept

    HBr addition to an unsymmetrical alkene in the presence of peroxide follows the radical (anti-Markovnikov) mechanism. This is the Kharasch effect, specific to HBr only.

  4. 4

    Formula / Rule

    Anti-Markovnikov addition: Br radical adds to the less substituted carbon (terminal carbon); H ends up on the more substituted carbon. The radical intermediate on the more substituted carbon is more stable (analogous to carbocation stability reasoning, but for radicals).

  5. 5

    Substitution

    - But-1-ene: CH₃CH₂CH=CH₂ - Peroxide initiates radical chain → Br• attacks the terminal C-1 (less substituted) - H adds to C-2

  6. 6

    Calculation

    This is a product-prediction problem, not a numerical calculation. Br• adds to C-1 → radical on C-2 (secondary, more stable than primary on C-1) → H abstracts to C-2 → product: 1-bromobutane (CH₃CH₂CH₂CH₂Br).

  7. 7

    Final answer

    **Major product: 1-bromobutane (CH₃CH₂CH₂CH₂Br)** This is the anti-Markovnikov product. Without peroxide, the product would be 2-bromobutane (Markovnikov product).

  8. 8

    Common trap

    The high-frequency trap here: if you ignore the "presence of peroxide" condition, you'd predict 2-bromobutane (Markovnikov product) — and select the wrong option. Also, if the question changed the acid to HCl or HI while keeping peroxide, the answer switches back to Markovnikov because the peroxide effect is HBr-specific.

  9. 9

    Similar NEET-style question

    "When HBr reacts with 2-methylbut-2-ene in the presence of peroxide, the major product is ___." (Answer: anti-Markovnikov product — 2-bromo-3-methylbutane, via Br• adding to C-2 and H to C-3.) ---

Before solving, remember these

Law

Markovnikov's rule

In addition of HX to unsymmetrical alkene, H goes to carbon with more H atoms; X to carbon with fewer H atoms. Driven by carbocation stability. Anti-Markovnikov in presence of peroxide (Kharasch effect): radical mechanism.

-- NCERT, p. 14
Key Fact

Electrophilic addition to alkenes

Addition of HBr, H₂O (acid catalyst), Br₂. Mechanism: (1) electrophile attacks C=C, forms cation; (2) nucleophile attacks cation. Regioselectivity by Markovnikov.

-- NCERT, p. 16

Formulas

Markovnikov's rule (and anti-Markovnikov)

Without peroxide: ionic mechanism — H goes to carbon with MORE hydrogens (carbocation stability rule). With peroxide (HBr only, Kharasch): radical mechanism — anti-Markovnikov.

SymbolQuantitySI Unit
H,Xadded atoms-

Valid when

  • Asymmetric alkene
  • H-X with X = Cl, Br, I
  • Without peroxide for Markovnikov

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Markovnikov reverses with peroxide

Category: Organic Reaction Conditions

HBr addition to alkene: WITHOUT peroxide → Markovnikov (H to C with more H). WITH peroxide (Kharasch effect) → anti-Markovnikov (radical mechanism). Specific to HBr only — not HCl, HI.

When it triggers

Question gives HX addition to alkene with explicit peroxide condition or hints (e.g. ROOR, light).

How to avoid

Without peroxide: ionic mechanism, carbocation stability → Markovnikov. With peroxide: radical mechanism, radical stability → anti-Markovnikov. Effect ONLY for HBr (HCl too strong, HI too weak).

Trap

Solvent changes product distribution

Category: Organic Reaction Conditions

Same starting materials give different products depending on solvent. Polar protic (water, alcohols): SN1/E1 favoured. Polar aprotic (DMSO, DMF): SN2 favoured. Affects substitution vs elimination.

When it triggers

Question contrasts product when solvent is changed; or specifies solvent type.

How to avoid

Polar protic stabilises carbocation → SN1/E1 (3° preferred). Polar aprotic doesn't solvate nucleophile → strong SN2 nucleophile (1°/2° preferred). Bulky base (t-BuOK) favours E2 over SN2.

Mistake

Confuses ortho/para directors with meta directors.

Root cause: concept gap

Correction

o,p-directors (activators except halogens): -OH, -OR, -NH₂, -NHR, alkyl. m-directors (deactivators): -NO₂, -CN, -COOH, -CHO. Halogens: o,p-directors but DEACTIVATORS.

Past Year Questions

12 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2022

Given below are two statements Statement I: The acidic strength of monosubstituted nitrophenol is higher than phenol because of electron withdrawing nitro group. Statement II: o-nitrophenol, m-nitrophenol and p-nitrophenol will have same acidic strength as they have one nitro group attached to the phenolic ring. In the light of the above statements, choose the most appropriate answer from the options given below:

1Statement I is incorrect but Statement II is correct.
2Both Statement I and Statement II are correct.
3Both Statement I and Statement II are incorrect.
4Statement I is correct but Statement II is incorrect.
NTA Answer: Option 4(final)
NEET 2022

Given below are two statements : Statement I : The boiling points of aldehydes and ketones are higher than hydrocarbons of comparable molecular masses because of weak molecular association in aldehydes and ketones due to dipole - dipole interactions. Statement II : The boiling points of aldehydes and ketones are lower than the alcohols of similar molecular masses due to the absence of H-bonding. In the light of the above statements, choose the most appropriate answer from the given below

1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 2(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Sources

NCERT refs: Class 11 Chemistry Chapter 9, p.14 | Class 11 Chemistry Chapter 9, p.16

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