What Are Addition Reactions?
Addition reactions add atoms or groups to a C=C or C≡C bond, breaking the π-bond and forming two new σ-bonds. Electrophilic addition is most common with alkenes and alkynes. Understanding carbocation intermediates and Markovnikov's rule is essential for predicting products.
In electrophilic addition, an electrophile attacks the π-electrons of an alkene, forming a carbocation intermediate. A nucleophile then adds to the carbocation. Markovnikov's rule states the electrophile adds to the carbon with more hydrogens, placing the nucleophile (or negative part) on the more substituted carbon.
- 1↓Step 1: Electrophilic attackH⁺ (from HBr) attacks the π-electrons forming a carbocation (more stable one per Markovnikov).
- 2↓Step 2: Nucleophilic additionBr⁻ (nucleophile) attacks the carbocation from either face.
- 3Product: Alkyl halideC-H and C-Br bonds formed; π-bond replaced with two σ-bonds.
Step-by-step worked examples
Draw the mechanism for HBr adding to propene (CH₃CH=CH₂). What is the major product?
Step 1: H⁺ attacks the π-bond. Two possible carbocations: - Primary carbocation (less stable) - Secondary carbocation from CH₃-CH⁺-CH₃ (more stable → formed) Step 2: Br⁻ attacks the secondary carbocation Product: CH₃CHBrCH₃ (2-bromopropane, major) Minor: CH₃CH₂CH₂Br (1-bromopropane, very minor)
Predict the product of HBr + 2-methylpropene. Does rearrangement occur?
Substrate: (CH₃)₂C=CH₂ (2-methylpropene) H⁺ attack forms a carbocation. Initially secondary (CH₃-C⁺-CH₂-CH₃ from β). No rearrangement needed — secondary is already favorable. Br⁻ attacks → product: (CH₃)₃C-Br (tert-butyl bromide, 2-bromo-2-methylpropane) Follows Markovnikov.
What is the product of H₂O adding to 1-methylcyclohexene? Does carbocation rearrangement occur?
Substrate: cyclohexene ring with CH₃ at position 1 H⁺ attacks C=C forming a secondary benzylic-like carbocation. H₂O attacks → product: major is the secondary alcohol (Markovnikov). No rearrangement; secondary carbocation is stable enough. Minor: primary alcohol from alternative attack.
Flashcards
Quick quiz
Q1.HCl + CH₃CH=CH₂ → major product?
Q2.Rank carbocation stability: 1°, 2°, 3°
Q3.HBr + cyclohexene → major product's position of Br?
Q4.Why does HBr + peroxides give anti-Markovnikov?
The full card deck, worked steps and AI-tutor support for “What Are Addition Reactions?” are in Notek — study by hand before your exam.
Common mistakes
Markovnikov's rule applies to all addition reactions. — Correct: Markovnikov applies to electrophilic additions. Free-radical additions (HBr + peroxides) are anti-Markovnikov.
Primary carbocations are more stable than secondary. — Correct: Secondary carbocations are more stable due to hyperconjugation from more alkyl groups.
Carbocation rearrangement always occurs. — Correct: Rearrangement occurs only if it forms a more stable carbocation; many secondary/tertiary carbocations don't rearrange.
H₂O is a strong nucleophile in addition. — Correct: H₂O is a weak nucleophile; it still attacks carbocations, but HBr and HCl react faster.
FAQ
What is an addition reaction?
Atoms or groups add to a C=C or C≡C bond, converting π-bonds into σ-bonds and increasing saturation.
Why do electrophilic additions occur?
The π-electrons of C=C are polarizable and attract electrophiles (electron-poor species).
What happens in carbocation rearrangement?
A hydride or alkyl group shifts from an adjacent carbon to form a more stable carbocation before nucleophile attack.
Is the product always Markovnikov?
For ionic electrophilic additions, yes. But free-radical or other mechanisms can give anti-Markovnikov or different regioselectivity.