Addition Reactions in Alkenes and Alkynes

Addition Reactions in Alkenes and Alkynes

Overview of Addition Reactions

Alkenes and alkynes are unsaturated hydrocarbons that commonly undergo addition reactions. In these reactions, atoms are added to the carbon atoms of double or triple bonds, converting them into single bonds and increasing the saturation of the molecule.

• Addition Reaction: The process in which atoms are added to π bonds (double or triple bonds), resulting in the formation of new single bonds.

• Bond Breakage: The π bonds are broken, and new σ bonds are formed.

• There are three major types of addition reactions:

  1. Halogenation

  2. Hydrogenation

  3. Hydrohalogenation

Types of Addition Reactions

The three main types of addition reactions differ by the reagents added to the unsaturated hydrocarbon:

• Halogenation: Addition of halogen molecules (e.g., Br2, Cl2) to the double or triple bond, forming a dihalide.

• Hydrogenation: Addition of hydrogen (H2) across the double or triple bond, converting it to an alkane.

• Hydrohalogenation: Addition of a hydrogen halide (e.g., HCl, HBr) to the double or triple bond, forming an alkyl halide.

General Equations

• Halogenation: $\ce{RCH=CH2 + X2 -> RCHX-CH2X}$

• Hydrogenation: $\ce{RCH=CH2 + H2 -> RCH2-CH3}$

• Hydrohalogenation: $\ce{RCH=CH2 + HX -> RCHX-CH2}$

Stoichiometry of Addition Reactions

For each double or triple bond present, a specific number of moles of reagent are required:

• 1 mole of reagent is needed for every double bond.

• Double bond: $1 \text{ mole } = 1 \text{ bond(s)}$

• Triple bond: $1 \text{ mole } = 2 \text{ bond(s)}$

Example: Identifying Addition Reactions

Consider the following reactions. Which one represents an addition reaction?

Reaction	Type
a) Cyclohexene + Br2 (FeBr3) → Cyclohexane + HBr	Substitution (not addition)
b) Cyclohexanol + H3PO4 (Δ) → Cyclohexene + H2O	Elimination (not addition)
c) 1-Butene + HCl → 2-Chlorobutane	Addition

Correct answer: Reaction c) is an addition reaction, where HCl adds across the double bond of 1-butene to form 2-chlorobutane.

Practice Problem

How many moles of reagent are needed for the addition to the following alkyne?

• For a triple bond, 2 moles of reagent are needed to fully saturate the molecule.

Key Terms

• Alkene: Hydrocarbon containing at least one carbon-carbon double bond.

• Alkyne: Hydrocarbon containing at least one carbon-carbon triple bond.

• Halogenation: Addition of halogen atoms to a molecule.

• Hydrogenation: Addition of hydrogen atoms to a molecule.

• Hydrohalogenation: Addition of hydrogen halide to a molecule.

• Alkyl halide: Organic compound containing a halogen atom bonded to an alkyl group.

Summary Table: Types of Addition Reactions

Type	Reagent	Product
Halogenation	X2 (Br2, Cl2)	Dihalide
Hydrogenation	H2	Alkane
Hydrohalogenation	HX (HCl, HBr)	Alkyl halide

Additional info: The notes infer the general mechanism and stoichiometry of addition reactions, as well as the classification of reaction types, based on standard GOB Chemistry curriculum.