R3.4.5 Electrophilic attack

Reactions of Alkenes: Electrophilic Addition and Mechanism

Imagine you’re tasked with designing a new industrial chemical process.
Your starting material is an alkene—a molecule with a reactive carbon-carbon double bond.
How would you transform this simple molecule into something more complex and functional, like an alcohol or a halogenoalkane?

The answer lies in a fundamental reaction type called electrophilic addition.

Why Are Alkenes So Reactive?

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C).
This double bond consists of two parts: a strong sigma (σ) bond and a weaker pi (π) bond.
The pi bond lies above and below the plane of the molecule, making it a region of high electron density.
This electron-rich area is highly attractive to electrophiles—species that are electron-deficient and seek out electrons to form bonds.
Electrophiles are molecules or ions that accept a pair of electrons to form a covalent bond.

Example

Examples include halogen molecules (e.g., $B r_{2}$ , hydrogen halides (e.g., HBr), and protons $H^{+}$ in acidic solutions.

When alkenes encounter electrophiles, they undergo electrophilic addition reactions, where the double bond is broken, and new covalent bonds are formed.

Tip

Remember: The pi bond is weaker than the sigma bond, making it the reactive part of the double bond in alkenes.

Electrophilic Addition Reactions of Alkenes

1. Reaction with Halogens

When alkenes react with halogens like bromine $B r_{2}$ or chlorine $C l_{2}$ , the halogen atoms add across the double bond to form a dihalogenoalkane.

Example

$C_{2} H_{4} + {Br}_{2} \to C_{2} H_{4} {Br}_{2}$

This reaction is often used as a test for unsaturation because bromine water, which is orange, becomes colorless in the presence of an alkene.
The disappearance of color indicates that the double bond has reacted with bromine.

2. Reaction with Hydrogen Halides

Alkenes also react with hydrogen halides (HX, where X = Cl, Br, I) to form halogenoalkanes.

Example

$C_{3} H_{6} + HBr \to C_{3} H_{7} Br$

If the alkene is unsymmetrical, two possible products can form.
The major product is determined by Markovnikov's Rule, which states that the hydrogen atom from HX will add to the carbon with the greater number of hydrogen atoms already attached.

This results in the formation of the more stable carbocation intermediate.

Example

For propene $C_{3} H_{6}$ , reacting with $H B r$ gives two products:

2-bromopropane (major product) via a secondary carbocation intermediate.
1-bromopropane (minor product) via a primary carbocation intermediate.

3. Reaction with Water (Hydration)

In the presence of an acid catalyst (e.g., $H_{2} S O_{4}$ , alkenes react with water to form alcohols.

Example

$C_{3} H_{6} + H_{2} O \overset{H_{2} {SO}_{4}}{\to} C_{3} H_{8} O$

This reaction involves two steps:

Protonation of the double bond to form a carbocation intermediate.
Addition of water, followed by deprotonation to produce the alcohol.

Note

The hydration of alkenes is an important industrial process for producing alcohols like ethanol, which are used as solvents, fuels, and in the manufacture of other chemicals.

Note

The mechanism of the reaction will be covered by HL students in the later sections.

Reflection and Practice

Theory of Knowledge

How does our ability to manipulate molecules like alkenes reflect the interplay of imagination and reasoning in scientific innovation?
What ethical considerations arise from the large-scale industrial use of these reactions?

You've read 2/2 free chapters this week.

Upgrade to PLUS or PRO to unlock all notes, for every subject.

R3.4.5 Electrophilic attack

Reactions of Alkenes: Electrophilic Addition and Mechanism

Why Are Alkenes So Reactive?

Electrophilic Addition Reactions of Alkenes

1. Reaction with Halogens

2. Reaction with Hydrogen Halides

3. Reaction with Water (Hydration)

Reflection and Practice

You've read 2/2 free chapters this week.

Alkenes and Electrophilic Addition