Addition Reactions of Alkenes

Alkenes

What is an addition reaction?

A reaction in which two molecules combine to form one molecule.

Why are alkenes more reactive than alkanes?

Alkenes contain a C=C double bond with an exposed, weaker pi bond that can be attacked by other species.

What two types of bond make up a C=C double bond?

One sigma bond and one pi bond.

What happens to the C=C double bond during an addition reaction?

The pi bond breaks while the sigma bond remains, allowing new sigma bonds to form.

Why is the pi bond more likely to react than the sigma bond?

The pi electrons are more exposed and therefore more susceptible to attack.

Why are addition reaction products usually more stable?

The weaker pi bond is replaced by stronger sigma bonds.

What is the equation for the reaction between ethene and bromine?

C₂H₄ + Br₂ → C₂H₄Br₂.

What test is used to detect a C=C double bond?

Add bromine water and shake; it changes from orange to colourless if a C=C bond is present.

What is the positive result for the bromine water test for an alkene?

Orange bromine water is decolourised and becomes colourless.

Why does bromine water become colourless when added to an alkene?

Bromine reacts with the C=C double bond in an addition reaction to form a colourless product.

Why should the bromine water test for alkenes be carried out away from sunlight?

Alkanes can react with bromine by free radical substitution in the presence of sunlight, giving a misleading result.

What is hydrogenation?

An addition reaction involving the addition of hydrogen.

What happens during hydrogenation of an alkene?

Hydrogen is added across the C=C double bond, converting the alkene into an alkane.

What catalyst is used for hydrogenation of alkenes?

Nickel.

What conditions are used for hydrogenation of an alkene?

Heat with hydrogen in the presence of a nickel catalyst.

What is the equation for hydrogenation of ethene?

C₂H₄ + H₂ → C₂H₆.

What product forms when ethene is hydrogenated?

Ethane.

How is hydrogenation used in the manufacture of margarine?

Hydrogen is added to some C=C bonds in unsaturated vegetable oils, converting them into more saturated, solid fats.

What happens to C=C bonds when vegetable oils are hydrogenated?

Some C=C double bonds are converted into C–C single bonds.

What are saturated fats?

Fats containing no C=C double bonds.

What are monounsaturated fats?

Fats containing one C=C double bond per molecule.

What are polyunsaturated fats?

Fats containing two or more C=C double bonds per molecule.

What is halogenation?

A reaction involving the addition of a halogen.

What happens when an alkene reacts with a halogen?

The halogen atoms add across the C=C double bond to form a dihalogenoalkane.

What is the product when ethene reacts with bromine?

1,2-dibromoethane.

What is the equation for ethene reacting with bromine?

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br.

What is the product when ethene reacts with chlorine?

1,2-dichloroethane.

What is the equation for ethene reacting with chlorine?

CH₂=CH₂ + Cl₂ → CH₂ClCH₂Cl.

What is hydration?

An addition reaction involving the addition of water or steam.

What happens during hydration of an alkene?

H and OH are added across the C=C double bond to form an alcohol.

What conditions are used for hydration of an alkene?

The alkene is heated with steam and passed over a phosphoric acid catalyst.

What catalyst is used for hydration of alkenes?

Phosphoric acid, H₃PO₄.

What is the equation for hydration of ethene?

CH₂=CH₂ + H₂O → CH₃CH₂OH.

What product is formed by hydration of ethene?

Ethanol.

Why is hydration of ethene industrially useful?

It produces ethanol, which is a useful chemical product.

What is the difference between hydrogenation and hydration?

Hydrogenation adds hydrogen, whereas hydration adds water.

What happens when a hydrogen halide reacts with an alkene?

The hydrogen halide adds across the C=C double bond to form a halogenoalkane.

Give two examples of hydrogen halides that can undergo addition with alkenes.

Hydrogen bromide, HBr, and hydrogen chloride, HCl.

What is the equation for ethene reacting with hydrogen bromide?

CH₂=CH₂ + HBr → CH₃CH₂Br.

What product forms when ethene reacts with hydrogen bromide?

Bromoethane.

Why can't the reaction of ethene with HBr be used as a test for a C=C bond?

Both the reactants and product are colourless, so there is no visible colour change.

What is a diol?

A compound containing two OH alcohol groups.

What reagent is used to oxidise an alkene to a diol?

Acidified potassium manganate(VII).

What happens when an alkene is oxidised by acidified potassium manganate(VII)?

Two OH groups are added across the C=C double bond, forming a diol.

What is the equation for oxidation of ethene to a diol?

CH₂=CH₂ + [O] + H₂O → CH₂OHCH₂OH.

What product forms when ethene is oxidised with acidified potassium manganate(VII)?

Ethane-1,2-diol.

What does [O] represent in an organic reaction equation?

Oxygen supplied by an oxidising agent.

What colour change occurs when potassium manganate(VII) reacts with an alkene?

Purple to colourless.

Why can potassium manganate(VII) be used to distinguish alkenes from alkanes?

Alkenes decolourise potassium manganate(VII), whereas alkanes do not react in this way.

What are the main addition reactions of alkenes you need to know?

Hydrogenation, halogenation, hydration, addition of hydrogen halides and oxidation to form diols.