alkenes

Alkenes

unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C).

Hydrocarbon

only carbon and hydrogen

Unsaturated 

contains a multiple bond

General formula:

C_nH_{2n}

Examples: of alkenes

• Ethene = C₂H₄

• Propene = C₃H₆

• But-1-ene = C₄H₈

Structure of the Double Bond

The C=C bond contains:

• 1 sigma (σ) bond

• 1 pi (π) bond

The π bond:

• Has electron density above and below the plane of the molecule

• Is weaker than a sigma bond

• Is exposed and easily attacked

A* Phrase

“The π bond is a region of high electron density.”

Why Alkenes Are More Reactive Than Alkanes

Alkanes only contain strong σ bonds.

Alkenes contain a weaker π bond, which:

• Is easier to break

• Attracts electrophiles

A* Phrase

“The electron-rich π bond attracts electrophiles and undergoes electrophilic addition.”

Types of Isomerism in Alkenes: structural isomerism- positional isomerism

Positional Isomerism

Different position of the double bond.

Example:

• But-1-ene

• But-2-ene

Stereoisomerism - E/Z Isomerism

Different arrangement in space around the C=C

E/Z Isomerism - Conditions Required

1. Must contain a C=C bond.

2. Each carbon in the double bond must be attached to two different groups.

E/Z Isomerism - Priority Rules (CIP)

Higher atomic number = higher priority.

Example order:
Br > Cl > O > N > C > H

Meaning

• Z (zusammen) = highest priority groups on the same side.

• E (entgegen) = highest priority groups on opposite sides.

E/Z Isomerism - Priority Rules (CIP) - Example: But-2-ene

CH₃ has higher priority than H.

• Same side → Z-but-2-ene

• Opposite sides → E-but-2-ene

Memory Trick

• Z = Zip together

• E = Enemy sides

Electrophile

Electron pair acceptor.

Examples:

• H⁺

• Brδ⁺

Nucleophile

Electron pair donor.

Examples:

• Br⁻

• Cl⁻

• H₂O

Electrophilic Addition (Main Reaction Type)

Definition:
Two atoms add across the double bond.

General Steps

1. Electrophile attacks the π bond.

2. Carbocation intermediate forms.

3. Nucleophile attacks the carbocation.

Carbocations

A carbocation is a positively charged carbon atom.

stability order:

Tertiary () > Secondary () > Primary () > Methyl

More alkyl groups donate electron density and stabilise the positive charge

Markovnikov’s Rule

When HX adds to an unsymmetrical alkene:

• H goes to the carbon with more hydrogens.

• X goes to the more substituted carbon.

Reason:
The more stable carbocation is formed.

. Reaction with Bromine (Br₂)

Equation

Ethene + Br₂ → 1,2-dibromoethane

Observation

Bromine water changes from orange to colourless.

Use

Test for unsaturation.

Bromination Mechanism (Step by Step)

Step 1

The π bond polarises Br₂, creating Brδ⁺ and Brδ⁻.

Step 2

Curly arrow from the π bond to Brδ⁺.

Step 3

Curly arrow from the Br–Br bond to the other Br atom, forming Br⁻.

Step 4

Carbocation forms.

Step 5

Curly arrow from lone pair on Br⁻ to the positively charged carbon.

Product

1,2-dibromoalkane.

A* Phrase

“The π bond induces a dipole in the bromine molecule.”

Reaction with Hydrogen Halides (HCl, HBr, HI)

Example

Propene + HBr → mainly 2-bromopropane

Product

Haloalkane

Mechanism

Electrophilic addition via carbocation

HBr Mechanism (Step by Step)

Step 1

Curly arrow from π bond to H in HBr.

Step 2

Curly arrow from H–Br bond to Br, forming Br⁻.

Step 3

Most stable carbocation forms.

Step 4

Br⁻ attacks the carbocation.

Product

Major product predicted by Markovnikov’s rule.

Reaction with Steam (Hydration)

Equation

Ethene + H₂O ⇌ Ethanol

Conditions

• Phosphoric acid catalyst

• ~300°C

• 60–70 atm

Product

Alcohol

Reaction with Hydrogen (Hydrogenation)

Equation

Ethene + H₂ → Ethane

Conditions

• Nickel catalyst

• ~150°C (often quoted around 150–200°C)

Product

Alkane

Uses

Hydrogenation of vegetable oils to make margarine.

Summary Table of Reactions

Reagent	Conditions	Product	Mechanism
Br₂	Room temperature	Dibromoalkane	Electrophilic addition
HBr / HCl / HI	Room temperature	Haloalkane	Electrophilic addition
Steam	Phosphoric acid, 300°C, 60–70 atm	Alcohol	Electrophilic addition
H₂	Nickel, 150–200°C	Alkane	Addition

Addition Polymerisation

Alkenes join together when the π bond breaks.

Example

n ethene → poly(ethene)

How to Draw Repeating Units

1. Open the double bond.

2. Put the repeating unit in square brackets.

3. Draw single bonds extending through the brackets.

4. Add “n”

LDPE vs HDPE

LDPE (Low-Density Poly(ethene))

• Highly branched

• Chains pack poorly

• Lower density

• Soft and flexible

HDPE (High-Density Poly(ethene))

• Little branching

• Chains pack closely

• Higher density

• Stronger and more rigid