---
title: "Electrophilic Addition to Alkenes"
description: "The double bond is a region of high electron density, so it attracts electrophiles and opens up. Curly arrows track the pair of electrons through a carbocation to the product."
canonical: https://lightmysky.com/learn/science/electrophilic-addition-to-alkenes-mt_1RB5nj8zR8
source: https://lightmysky.com/learn/science/electrophilic-addition-to-alkenes-mt_1RB5nj8zR8.md
retrieved: 2026-09-12
---

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# Electrophilic Addition to Alkenes

The double bond is a region of high electron density, so it attracts electrophiles and opens up. Curly arrows track the pair of electrons through a carbocation to the product.

Subject: Science · Area: Chemistry · Ages 16 to 17
Page: https://lightmysky.com/learn/science/electrophilic-addition-to-alkenes-mt_1RB5nj8zR8

## Ready when they can

- Draws the mechanism with curly arrows starting at the double bond and ending on the electrophile
- Explains how a non-polar bromine molecule becomes polarised as it approaches
- Compares primary, secondary and tertiary carbocations and uses their stability to predict the major product
- Applies the same mechanism to hydrogen bromide, to bromine and to concentrated sulfuric acid

## Lesson: How alkenes invite attack

A double bond is a region of high electron density, so it attracts electrophiles, species that are short of electrons and looking to grab a pair. That pull opens the double bond up. When you draw the mechanism, start the first curly arrow at the double bond. The arrow shows the pi electron pair moving onto the electrophile to form the new bond.

A bromine molecule is non-polar on its own, but it becomes polarised as it approaches the double bond. The double bond repels the bromine electrons, so the nearer bromine turns slightly positive. That end then acts as the electrophile that the pi pair attacks.

**Example.** Take hydrogen bromide adding to propene. The hydrogen adds first and leaves a carbocation: adding to the end carbon gives a secondary carbocation, while adding to the middle carbon gives a primary one. Carbocation stability runs tertiary above secondary above primary, because alkyl groups push electron density toward the positive carbon and steady it. Bromide then attacks the carbocation, so the major product is 2-bromopropane through the secondary route.

The same mechanism covers bromine and concentrated sulfuric acid. With the acid, hydrogen and hydrogen sulfate add across the double bond to give an alkyl hydrogen sulfate, and water then hydrolyses it to an alcohol. Whenever an electrophile meets a double bond, look for the most stable carbocation first.

**Recap.** Find the most stable carbocation and the major product follows.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Alkanes and Alkenes: The First Homologous Series](https://lightmysky.com/learn/science/alkanes-and-alkenes-the-first-homologous-series-mt_mkLJDtBanL)
- [Free-Radical Substitution in Alkanes](https://lightmysky.com/learn/science/free-radical-substitution-in-alkanes-mt_QkreYC9e2d)

## Opens up

- [Benzene: Delocalisation and Electrophilic Substitution](https://lightmysky.com/learn/science/benzene-delocalisation-and-electrophilic-substitution-mt_jFqF7cZ0Ij)
- [Nucleophilic Substitution in Haloalkanes](https://lightmysky.com/learn/science/nucleophilic-substitution-in-haloalkanes-mt_QG-nKlnVvW)
- [Electrophilic Addition Revisited: Bridged Ions and Stereospecificity](https://lightmysky.com/learn/science/electrophilic-addition-revisited-bridged-ions-and-stereospecificity-mt_sbsBxFJyZy)