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.

11:47 PM