Skip to content

Course home

Sign up

11.5.2 Addition reactions of alkenes

Addition reactions

Definition

Addition reaction

A reaction in which atoms join across a carbon-carbon double bond so that the reactants form only one product.

  1. In an addition reaction, two molecules join together to form only one product.
  2. Alkenes take part in addition reactions because of their C=C\text{C}=\text{C}C=C double bond.
    1. One of the two bonds in the double bond breaks, leaving a C−C\text{C}-\text{C}C−C single bond.
    2. Each of the two carbon atoms then forms a new bond to one of the added atoms.
  3. The product is saturated, because the double bond has gone.
  4. No atoms are lost, so the product contains every atom from both reactants.
  5. Alkenes react with three substances in this way.
    1. Bromine, or aqueous bromine, with no catalyst.
    2. Hydrogen, with a nickel catalyst.
    3. Steam, with an acid catalyst.

Adding bromine

  1. Ethene reacts with bromine at room temperature to form 1,2-dibromoethane.

    CH2=CH2+Br2→CH2BrCH2Br \text{CH}_2=\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br} CH2​=CH2​+Br2​→CH2​BrCH2​Br

  2. One bromine atom joins each carbon atom that was part of the double bond, which is why the name contains the numbers 1,2.

  3. The displayed formula of 1,2-dibromoethane shows two bromine atoms on neighbouring carbon atoms joined by a single bond.

    HH∣∣Br−C−C−Br∣∣HH \begin{array}{ccccccc} & & \text{H} & & \text{H} & & \\ & & | & & | & & \\ \text{Br} & - & \text{C} & - & \text{C} & - & \text{Br} \\ & & | & & | & & \\ & & \text{H} & & \text{H} & & \end{array} Br​−​H∣C∣H​−​H∣C∣H​−​Br​

  4. With aqueous bromine, the orange colour turns colourless because the bromine is used up as it adds to the alkene.

    1. This colour change is the reason aqueous bromine is used to test for a C=C\text{C}=\text{C}C=C double bond.

Displayed formulae of ethene reacting with bromine, with the double bond opening so that one bromine atom joins each carbon atom to give 1,2-dibromoethane.

Adding hydrogen

Definition

Hydrogenation

The addition of hydrogen across a carbon-carbon double bond, using a nickel catalyst, which converts an alkene into an alkane.

  1. An alkene reacts with hydrogen in the presence of a nickel catalyst to form an alkane.

    1. The mixture is heated to about 150 ∘C150\ ^\circ\text{C}150 ∘C as the gases pass over the nickel.

  2. Ethene reacts with hydrogen to form ethane.

    CH2=CH2+H2→NiCH3CH3 \text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni}} \text{CH}_3\text{CH}_3 CH2​=CH2​+H2​Ni​CH3​CH3​

  3. One hydrogen atom joins each carbon atom, so the product has six C−H\text{C}-\text{H}C−H bonds and one C−C\text{C}-\text{C}C−C bond.

  4. The nickel is written above the arrow, because it is a catalyst and is not used up.

  5. The same reaction is used to turn liquid vegetable oils, which contain C=C\text{C}=\text{C}C=C bonds, into solid spreads such as margarine.

Displayed formulae of ethene and hydrogen reacting over a nickel catalyst, with one hydrogen atom adding to each carbon atom to give ethane.

Adding steam

  1. An alkene reacts with steam in the presence of an acid catalyst to form an alcohol.

    1. Steam is water in the gas state, so the molecule added is H2O\text{H}_2\text{O}H2​O.

  2. Ethene reacts with steam to form ethanol.

    CH2=CH2+H2O→acid catalystCH3CH2OH \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{acid catalyst}} \text{CH}_3\text{CH}_2\text{OH} CH2​=CH2​+H2​Oacid catalyst​CH3​CH2​OH

  3. The water molecule splits so that an H\text{H}H atom joins one carbon atom and an OH\text{OH}OH group joins the other carbon atom.

  4. The displayed formula of ethanol shows the O−H\text{O}-\text{H}O−H group bonded to one carbon atom.

    HH∣∣H−C−C−O−H∣∣HH \begin{array}{ccccccccc} & & \text{H} & & \text{H} & & & & \\ & & | & & | & & & & \\ \text{H} & - & \text{C} & - & \text{C} & - & \text{O} & - & \text{H} \\ & & | & & | & & & & \\ & & \text{H} & & \text{H} & & & & \end{array} H​−​H∣C∣H​−​H∣C∣H​−​O​−​H​
    1. This reaction is one of the main ways of manufacturing ethanol, and you will meet its industrial conditions with ethanol manufacture.

Drawing the products

  1. Copy the carbon chain of the alkene and change the C=C\text{C}=\text{C}C=C double bond into a C−C\text{C}-\text{C}C−C single bond.
  2. Add one new atom or group to each of the two carbon atoms that were joined by the double bond.
    1. For bromine, add Br\text{Br}Br to both carbons.
    2. For hydrogen, add H\text{H}H to both carbons.
    3. For steam, add H\text{H}H to one carbon and OH\text{OH}OH to the other.
  3. Leave every other atom exactly where it was, and check that each carbon has four bonds.
Example

Addition reactions of propene

  • Propene, CH2=CHCH3\text{CH}_2=\text{CHCH}_3CH2​=CHCH3​, has its double bond between the first and second carbon atoms, so the new atoms join those two carbons.
  • With bromine, the product is 1,2-dibromopropane.
CH2=CHCH3+Br2→CH2BrCHBrCH3 \text{CH}_2=\text{CHCH}_3 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCHBrCH}_3 CH2​=CHCH3​+Br2​→CH2​BrCHBrCH3​
  • With hydrogen and a nickel catalyst, the product is propane.
CH2=CHCH3+H2→NiCH3CH2CH3 \text{CH}_2=\text{CHCH}_3 + \text{H}_2 \xrightarrow{\text{Ni}} \text{CH}_3\text{CH}_2\text{CH}_3 CH2​=CHCH3​+H2​Ni​CH3​CH2​CH3​
  • With steam and an acid catalyst, the OH\text{OH}OH group can join either carbon of the double bond, so each propene molecule forms one of two possible alcohols.
CH2=CHCH3+H2O→CH3CH2CH2OH or CH3CH(OH)CH3 \text{CH}_2=\text{CHCH}_3 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \ \text{or} \ \text{CH}_3\text{CH(OH)CH}_3 CH2​=CHCH3​+H2​O→CH3​CH2​CH2​OH or CH3​CH(OH)CH3​
  • In every case the third carbon atom, the CH3\text{CH}_3CH3​ group, is unchanged.
Common Mistake
  • Do not leave the double bond in the product, because it becomes a single bond during addition.
  • Do not put both bromine atoms on the same carbon atom, because one bromine atom adds to each carbon of the double bond.
  • Do not write a second product such as HBr\text{HBr}HBr or H2O\text{H}_2\text{O}H2​O, because an addition reaction forms only one product.
  • Do not swap the catalysts, because hydrogen needs nickel and steam needs an acid.
Exam technique

Writing about addition reactions

  • Define an addition reaction by saying that only one product forms, because that is the point that earns the mark.
  • Name the catalyst for each reaction, writing nickel for hydrogen and acid catalyst for steam, while bromine needs none.
  • In a displayed formula, show every bond, including the O−H\text{O}-\text{H}O−H bond in an alcohol, and count four bonds around each carbon.
  • Write structural formulae in full, such as CH2BrCH2Br\text{CH}_2\text{BrCH}_2\text{Br}CH2​BrCH2​Br and CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}CH3​CH2​OH, rather than molecular formulae that do not show the arrangement.
Self review
  • How many products form in an addition reaction?
  • Name the product when ethene reacts with bromine, and give its structural formula.
  • Which catalyst is needed when an alkene reacts with hydrogen?
  • What type of compound forms when an alkene reacts with steam?
  • Draw the displayed formula of ethanol.
PreviousNext

How was this guide?

Teach Genie

Review 11.5.2 Addition reactions of alkenes by teaching Genie

Teach it back in your own words, spot gaps, and remember it better.

Start teaching
Genie and Baby Genie

11.5.2 Addition reactions of alkenes Revision Guide

  1. IGCSE
  2. /Chemistry
  3. /11.5.2 Addition reactions of alkenes

Revision notes for CIE IGCSE Chemistry 11.5.2 Addition reactions of alkenes: explanations and worked examples.

Revision guides