Introduction

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). These molecules readily undergo addition reactions, where atoms or molecules add across the double bond to form saturated products. This article explores the four major types of alkene addition reactions: hydrohalogenation, halogenation, hydrogenation, and hydration.

Why Are Alkenes Reactive?

Alkenes are highly reactive due to their electron-rich double bond. The π-electrons in the C=C bond make alkenes excellent nucleophiles (electron-rich species that can attack electron-poor molecules). The carbon atoms in the double bond are each bonded to only three other atoms, allowing them to form additional bonds.

Types of Addition Reactions

1. Hydrohalogenation

Hydrohalogenation is the addition of a hydrogen halide (HX) across a double bond. The process follows these steps:

The reaction can be represented as:

${R-CH=CH2 + HX -> R-CHX-CH3}$

Markovnikov's Rule

Markovnikov's rule predicts the major product of hydrohalogenation:

This can be explained by carbocation stability:

$\text{Tertiary} (3°) > \text{Secondary} (2°) > \text{Primary } (1°)$

2. Halogenation

Halogenation adds a halogen molecule (${X2}$) across the double bond. This forms a dihalogenated product:

${R-CH=CH2 + X2 -> R-CHX-CH2X}$

3. Hydrogenation

Hydrogenation adds hydrogen (${H2}$) across the double bond in the presence of a metal catalyst:

${R-CH=CH2 + H2 ->[Pd/C] R-CH2-CH3}$

Common catalysts include:

4. Hydration

Hydration adds water across the double bond in the presence of an acid catalyst:

${R-CH=CH2 + H2O ->[H+] R-CHOH-CH3}$

Key features: