Addition Reactions of Alkenes

Introduction

The double bond in an alkene is a reactive site waiting to be used. In an addition reaction , a small molecule adds across the C=C bond: the double bond becomes a single bond, and each of the two carbons gains a new atom or group. Only one product forms. Addition reactions turn ethene into fuels, solvents and alcohols, and they harden vegetable oils into spreads. This page covers the three key examples at this level: hydrogen, halogens and water.

Core explanation

The general pattern. For any alkene and any adding molecule X–Y:

C=C + X–Y → X–C–C–Y

One part of the double bond breaks, the X–Y bond breaks, and two new single bonds form. The product is saturated .

1. Adding hydrogen (hydrogenation). Alkenes react with hydrogen when heated in the presence of a nickel catalyst . The product is an alkane:

CH₂=CH₂ + H₂ → CH₃CH₃ (ethene → ethane)

CH₂=CHCH₃ + H₂ → CH₃CH₂CH₃ (propene → propane)

Industrially, hydrogenation is used to turn liquid vegetable oils, which are unsaturated, into more solid fats for margarine and baking. Adding hydrogen to some of the C=C bonds raises the melting point.

2. Adding halogens (halogenation). Chlorine and bromine add quickly at room temperature, with no catalyst needed. Each carbon of the old double bond gains one halogen atom:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane)

CH₂=CHCH₃ + Cl₂ → CH₂ClCHClCH₃ (1,2-dichloropropane)

This is the reaction behind the bromine water test for unsaturation.

3. Adding water (hydration). Alkenes react with steam at a high temperature and pressure, using a phosphoric acid catalyst , to form alcohols :

CH₂=CH₂ + H₂O → CH₃CH₂OH (ethene → ethanol)

This is one of the two main ways of making ethanol; the other is fermentation of sugars. Hydration is fast and gives pure ethanol continuously, but it uses ethene from crude oil, a non-renewable resource. The reaction is reversible, so unreacted ethene is recycled.

Atom economy. Because an addition reaction gives just one product, every atom in the reactants ends up in that product. The atom economy is 100% , making addition reactions efficient and low in waste.

Naming products. Count the carbons, name the parent alkane, and show added groups with numbers: the positions are the two carbons that were joined by the double bond.

Step-by-step reasoning

To predict the product of an addition reaction:

1. Draw the alkene and locate the C=C bond. 2. Change the double bond to a single bond. 3. Split the adding molecule into two parts (H and H, Br and Br, or H and OH). 4. Attach one part to each carbon of the former double bond. 5. Check every carbon has four bonds and name the product.

Visual explanation

Picture ethene as two carbons holding hands with both arms. A bromine molecule arrives; each carbon lets go with one arm and grabs one bromine atom. The two carbons are now joined by one arm only, and each holds a bromine. In SIM-ORG-001, the flat ethene model twists into the tetrahedral shape of the saturated product.

Real-world analogy

Picture two rail wagons joined by a double coupling. Release one of the couplings and each wagon now has a free hook to take a new load, while the two wagons stay joined by the coupling that remains. The double bond works the same way in an addition reaction.

Real-world example

Soft margarine is made by partially hydrogenating vegetable oils. Using a nickel catalyst, hydrogen adds to some of the C=C bonds, turning a runny oil into a spreadable solid. Many manufacturers now limit hydrogenation because it can produce trans fats linked to heart disease.

Why?

Why do alkenes add rather than substitute? Breaking the weaker part of the double bond and forming two new single bonds releases energy overall, and nothing needs to be removed from the molecule. Alkanes have no double bond, so the only route for them is the much harder substitution.

Common misconception

"In hydration, water adds as H₂O onto one carbon." Water splits into H and OH: the hydrogen atom attaches to one carbon and the OH group to the other. That is why the product is an alcohol, containing the –OH group.

Worked example

Question: Write an equation for the reaction of propene with steam and name a possible product.

Reasoning: Propene is CH₂=CHCH₃. H adds to one carbon of the double bond and OH to the other. If OH goes to carbon 1, the product is CH₂OHCH₂CH₃, propan-1-ol; if OH goes to carbon 2, it is CH₃CHOHCH₃, propan-2-ol.

Answer: C₃H₆ + H₂O → C₃H₇OH; for example propan-2-ol (the main product) or propan-1-ol.

Quick check

1. What catalyst is used to add hydrogen to an alkene? Answer: Nickel.

Exam focus

Learn the reagent and conditions for each addition: hydrogen with a nickel catalyst; halogens at room temperature; steam with a phosphoric acid catalyst at high temperature and pressure. In displayed-formula answers, make sure the double bond has become single and every carbon has four bonds. State that addition reactions have 100% atom economy.

Advanced insight

With an unsymmetrical alkene such as propene, two products are possible from HBr or water. The major product follows Markovnikov's rule: the hydrogen adds to the carbon that already has more hydrogens. This happens because the reaction goes through a positively charged intermediate, and the more stable, more substituted intermediate forms preferentially.

Summary

In addition reactions, a molecule adds across the C=C bond of an alkene to give a single saturated product. Hydrogen adds with a nickel catalyst to form an alkane; halogens add at room temperature to form dihaloalkanes; steam adds with a phosphoric acid catalyst to form an alcohol. Addition reactions have 100% atom economy.

Practice questions

1. Write an equation for the reaction of ethene with hydrogen and name the product. Answer: CH₂=CH₂ + H₂ → CH₃CH₃; ethane. 2. Name the product formed when ethene reacts with chlorine. Answer: 1,2-dichloroethane, CH₂ClCH₂Cl. 3. State the reagent and catalyst used to make ethanol from ethene. Answer: Steam, with a phosphoric acid catalyst. 4. Explain why addition reactions have an atom economy of 100%. Answer: There is only one product, so all the atoms in the reactants end up in the desired product and none are wasted. 5. Explain why hydrogenation turns vegetable oil into a more solid fat. Answer: Hydrogen adds to C=C bonds, making the molecules more saturated; saturated chains pack together more closely, raising the melting point.