8.1.2 Structural Formulae for Alkenes

Introduction

In the previous section, we learned how to draw structural formulas for alkanes — hydrocarbons with only single bonds. Now we turn to alkenes, which contain at least one carbon-carbon double bond.

Alkenes are unsaturated hydrocarbons because they contain a double bond. This double bond is the key feature that distinguishes alkenes from alkanes.

This section explains how to draw structural formulas for alkenes.

Definition

Alkenes: Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds.

General Formula: C_nH_2n

Where:

• n = number of carbon atoms
• The number of hydrogen atoms is twice the number of carbon atoms

In simple words: Alkenes are hydrocarbons with at least one double bond between two carbon atoms.

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 covalent bond between the carbon atoms.
• Doubly bonded carbon atoms form a single bond with two other atoms.

Core Concept

Key point: Alkenes contain a carbon-carbon double bond and have the general formula C_nH_2n.

Detailed Explanation

What are 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 covalent bond between the carbon atoms.
• Doubly bonded carbon atoms form a single bond with two other atoms.

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

Displayed Formula for Ethene

Ethene (C₂H₄) is the simplest alkene.

Structure:

• Two carbon atoms are joined by a double bond.
• Each carbon atom is bonded to two hydrogen atoms.

Displayed formula:

\beginarrayc H H \\ | | \\ C = C \\ | | \\ H H \endarray

Condensed formula: CH₂ = CH₂

Key point: Ethene has a double bond between the two carbon atoms and two hydrogen atoms attached to each carbon.

Displayed Formula for Propene

Propene (C₃H₆) is the second member of the alkene family.

Structure:

• Three carbon atoms in a chain.
• A double bond between carbon 1 and carbon 2.
• Each carbon atom has the appropriate number of hydrogen atoms.

Displayed formula:

\beginarrayc H H H \\ | | | \\ C - C = C - H \\ | | \\ H H \endarray

Condensed formula: CH₃CH = CH₂

Key point: Propene has a double bond between carbon 1 and carbon 2.

Drawing Structural Formulas for Alkenes

Steps:

• Write the chain of carbon atoms based on the stem name.
• Place the double bond at the correct position (if the name includes a number).
• Attach the correct number of hydrogen atoms to each carbon atom so that each carbon has four bonds.
• For the condensed formula, group the atoms attached to each carbon.

Key point: Follow the same rules as for alkanes, but include the double bond.

Examples of Alkenes

Key point: The position of the double bond is indicated by a number in the name (e.g., but-1-ene, but-2-ene).

Summary Table

Intermediate Level

Drawing structural formulas for alkenes:

• Write the chain of carbon atoms based on the stem name.
• Place the double bond at the correct position.
• Attach the correct number of hydrogen atoms to each carbon atom so that each carbon has four bonds.
• For the condensed formula, group the atoms attached to each carbon.

Examples:

Ethene: CH₂ = CH₂

Propene: CH₃CH = CH₂

But-1-ene: CH₃CH₂CH = CH₂

But-2-ene: CH₃CH = CHCH₃

Key point: The position of the double bond is indicated by a number in the name.

Important Points

• - Alkenes = hydrocarbons with at least one carbon-carbon double bond.
• - General formula: C_nH_2n.
• - Simplest alkene: Ethene (CH₂ = CH₂).
• - Second member: Propene (CH₃CH = CH₂).
• - Doubly bonded carbon atoms form a single bond with two other atoms.
• - Each carbon atom in a double bond still forms four bonds total.
• - The double bond is the key feature of alkenes.
• - Alkenes are unsaturated hydrocarbons.

Remember

• - Alkenes = hydrocarbons with a double bond.
• - General formula: C_nH_2n.
• - Simplest alkene: Ethene (CH₂ = CH₂).
• - Second member: Propene (CH₃CH = CH₂).
• - Double bond = two pairs of shared electrons.
• - Each carbon in a double bond forms 4 bonds total.
• - Double bond = site of addition reactions.
• - Alkenes are unsaturated.

Common Mistakes

Quick Revision

Structural Formulae for Alkenes:

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 covalent bond between the carbon atoms. Doubly bonded carbon atoms form a single bond with two other atoms.

Examples:

• Two carbon atoms joined by a double bond.
• Each carbon bonded to two hydrogen atoms.

• Three carbon atoms in a chain.
• Double bond between carbon 1 and carbon 2.

Drawing Structural Formulas for Alkenes:

• Write the chain of carbon atoms based on the stem name.
• Place the double bond at the correct position.
• Attach the correct number of hydrogen atoms to each carbon atom so that each carbon has four bonds.
• For the condensed formula, group the atoms attached to each carbon.

• Key Point:
• The double bond is the key feature of alkenes. Each carbon atom in a double bond still forms four bonds total.

60-Second Revision

• Alkenes = hydrocarbons with a double bond.
• General formula: C_nH_2n.
• Simplest alkene: Ethene (CH₂ = CH₂).
• Second member: Propene (CH₃CH = CH₂).
• Double bond = two pairs of shared electrons.
• Each carbon in a double bond forms 4 bonds total.
• Double bond = site of addition reactions.
• Alkenes are unsaturated.
• Examples:
• Ethene: CH₂ = CH₂
• Propene: CH₃CH = CH₂
• But-1-ene: CH₃CH₂CH = CH₂
• But-2-ene: CH₃CH = CHCH₃
• ⚠️ Warning: alkenes are unsaturated hydrocarbons — they contain at least one carbon-carbon double bond.