9.2 Alkenes

Introduction

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. They are an important family of organic compounds because they are used to make many everyday products, including plastics, antifreeze, and fuels.

The simplest alkene is ethene (also called ethylene). Ethene is the most important commercial organic chemical. It is used to manufacture polythene, one of the most familiar plastics. It is also converted to ethylene glycol, which is used as antifreeze in automobile radiators.

This section explains the structure, preparation, and properties of alkenes.

Definition

Alkenes: Alkenes are unsaturated hydrocarbons that have one or more double bonds between carbon atoms.

General Formula: C_nH_2n

In simple words: Alkenes are hydrocarbons with at least one carbon-carbon double bond.

Main Idea

• Alkenes have one or more double bonds between carbon atoms.
• They have the general formula C_nH_2n.
• When two carbon atoms share two pairs of electrons, they form a double bond between the carbon atoms.
• Doubly bonded carbon atoms form a single bond with two other atoms.
• Alkenes are prepared by:
• Alkenes undergo addition reactions with halogens, hydrogen, and steam.

Core Concept

Key point: Alkenes contain a carbon-carbon double bond and undergo addition reactions.

Detailed Explanation

Structure of Alkenes

Alkenes have one or more double bonds between carbon atoms. They have the general formula C_nH_2n.

What happens in a double bond:

• When two carbon atoms share two pairs of electrons, they form a double bond between the carbon atoms.
• Doubly bonded carbon atoms form a single bond with two other atoms.

Examples:

• Ethene: CH₂ = CH₂
• Propene: CH₃CH = CH₂

Key point: The double bond is the defining feature of alkenes.