Lesson 1: WHAT ALKENES?

· Belong to the homologous series of unsaturated hydrocarbons

i) Contain double covalent bonds between carbon atoms in molecules

ii) Contain only hydrogen and carbon atoms

· Ends with –ene

· General Formula: C₂H_2n

· The carbon-carbon double bond is formed by the sharing of two pairs of electrons between two carbon atoms

· The carbon-carbon double bond determines most of the chemical properties of alkenes

An unsaturated compound contains a carbon-carbon double (or triple) covalent bonds

First 3 alkenes:

C₂H_{4 Ethene}

_{C3H6 Propene}

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Isomerism in alkenes

· It is possible for alkenes with more than 3 carbon atoms to possess isomers

· C₄H₈ has 3 isomers from same homologous series

i) The carbon chain can be branched, as in alkanes

ii) The double bond can be at different positions along the carbon chain.

Manufacture of alkenes

1) Industrial method- by the cracking process

Cracking is the process of breaking down long-chain hydrocarbon molecules into small molecules

- Conditions of high temperature

- Pressure

- Catalyst

Importance of cracking process

i. To provide more of the smaller alkane molecules which are in higher demand, like those molecules found in petrol.

ii. As a source of alkenes and hydrogen

- E.g. Cracking of long-chain alkane molecules found in the naptha fraction can produce short-chain alkenes such as ethane and propene.

- Chemicals that are obtained from petrol are known as petrochemicals.

2) Laboratory preparation of ethane – involving dehydration of alcohols

a) Use concentrated sulfuric acid

b) Using a catalyst like aluminium oxide or unglazed porcelain

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Physical properties

- First 3 members are gases at r.t.p

Ø Made up of covalent molecules held by weak intermolecular forces

- All members are insoluble in water but soluble in organic solvent

Ø Like dissolve like

Chemical properties

- Similar for all alkenes

Ø Contains the same C=C covalent bond

- More reactive than alkanes

Ø One of the bonds in the C=C bond (pi bond) can break easily, allowing other atoms to join to the carbon atoms resulting in an addition reaction.

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Combustion

a) Alkenes burn in an excess of air to form CO₂ and H₂O

b) Reaction is exothermic

c) If the combustion is not complete, products formed are CO, C(soot) and H₂O

d) More soot is produced compared to the corresponding alkane because of a higher percentage by mass of carbon.

Reactions of alkenes

Ø As alkenes are unsaturated, they undergo addition reaction, rather than substitution.

An addition reaction is one in which two molecules combine to form a single molecule as the only product.

Ø Simple molecules like hydrogen, water and bromine can be added across carbon-carbon double bond to form only a single product.

Ø No other by-product is formed

Ø Addition reactions are much faster than substitution.

Example: Hydrogenation

v Alkenes react with hydrogen at 200 degrees celcius in the presence of a nickel catalyst to form alkanes.

Importance of hydrogenation

- Hydrogenation is used to make margarine from vegetable oil

- Natural oils are usually liquid containing unsaturated molecules but is hardened to form solids by saturating the molecules with hydrogen at temperature of 140 degrees celcius and the presence of a nickel catalyst.

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Example: Hydration

-Ethene reacts with water to form ethanol.

· Conditions of reaction

· Temperature of 300 degrees celcius

· Pressure 60 atmosphere

· With phosphoric (V) acid as a catalyst

Example: With halogens

-Chlorine or bromine will add readily to the double bond in the alkenes, but not the less reactive halogen, iodine.

Lesson 4: What are alkenes for?

Uses of alkenes

1) To make margarine to vegetable oils

2) To make alcohols; used as antifreeze and solvents

3) To make plastic (polymers) like polyethene, PVC

4) Used in agriculture (in low concentration) to help hasten the ripening of fruits like bananas.

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