5. Alkenes
Alkenes are the workhorses of organic synthesis: the C=C double bond is a nucleophile that adds electrophiles across it, and the regiochemistry of that addition is governed by one rule — Markovnikov's.
Electrophilic Addition
The π bond attacks an electrophile, forming the more stable carbocation; the nucleophile then captures it.
Example (Markovnikov): HBr adds to propene — H+ goes to the terminal carbon, forming the secondary carbocation, and Br− follows:
$$\text{CH}_3\text{–CH=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{–CHBr–CH}_3$$
The proton adds to the carbon with more hydrogens, so the halogen ends up on the more substituted carbon.
Key Addition Reactions
- Acid-catalyzed hydration: $\text{H}_2\text{O}$/H2SO4 → alcohol (Markovnikov). 1-Butene → 2-butanol.
- Halogenation: Br2 → vicinal dibromide; the orange colour disappears — a classic alkene test.
- Hydroboration–oxidation: BH3, then H2O2/NaOH → alcohol with anti-Markovnikov regiochemistry. 1-Butene → 1-butanol.
- Ozonolysis: O3, then Zn/H2O cleaves the double bond into two carbonyls.
- Hydrogenation: H2/Pt → alkane.
Example (ozonolysis): 2-Butene cleaves into two molecules of acetaldehyde: $$\text{CH}_3\text{–CH=CH–CH}_3 \xrightarrow{\text{O}_3,\ \text{Zn/H}_2\text{O}} 2\,\text{CH}_3\text{CHO}$$
Practice Problems
- Predict the product of HBr addition to 2-methylpropene.
- What alcohol forms from acid-catalyzed hydration of 1-butene?
- What are the ozonolysis products of cyclohexene?
- What does catalytic hydrogenation of cis-2-butene give?
Answers: 1. 2-bromo-2-methylpropane 2. 2-butanol (Markovnikov) 3. Hexanedial, OHC–(CH2)4–CHO 4. Butane