Addition reactions

Addition reactions only occur with unsaturated compounds, that is, compounds containing a carbon to carbon double bond or a carbon to carbon triple bond. In other words, alkenes or alkynes.

Addition to alkenes

The most common addition reactions involve addition of a small molecule like a halogen (1), hydrogen (2), a hydrogen halide (3) or water (4).

1. Propene (unsaturated) reacts with bromine to produce 1,2-dibromopropane (saturated)
2. Propene (unsaturated) reacts with hydrogen to produce propane (saturated)
3. Propene(unsaturated) reacts with hydrogen chloride to produce 1-chloropropane (saturated) and 2-chloropropane (saturated)
4. Propene (unsaturated) reacts with water to produce propan-1-ol (primary alcohol) and propan-2-ol (secondary alcohol)

Examples:

The small molecule always adds across the double bond. For example, in this case of reaction (1) above the double bond is between carbon atoms 1 and 2, so the bromine atoms will be in position 1, 2.

All of the products are now saturated as they contain only single carbon to carbon bonds.

The addition of hydrogen, reaction (2), is also known as hydrogenation.

The addition of a hydrogen halide, reaction (3), to an alkene might produce a mixture of isomers depending on how the hydrogen halide adds across the carbon to carbon double bond.

The addition of water, reaction (4), is a very important reaction to remember because it produces alcohols. Addition of water is also known as hydration.

Addition to alkynes

Addition to an alkyne is a two stage process:

alkyne→ alkene→ alkane

with the possibility of isomers being produced.

Complete addition to an alkyne will require twice the quantity of halogen (1), hydrogen (2) or hydrogen halide (3).

1. Ethyne reacts with bromine to produce 1,2-dibromoethene, which in turn reacts with bromine to produce 1,1,2,2-tetrabromoethane
2. Ethyne reacts with hydrogen to produce ethene, which in turn reacts with hydrogen to produce ethane
3. Ethyne reacts with hydrogen chloride to produce chloroethene, which in turn reacts with hydrogen chloride to produce 1,1-dichloroethane or 1,2-dichloroethane

Examples:

Dehydration

Hydration involves the addition of water to an unsaturated molecule.

Dehydration is the reverse of hydration and involves the removal of water from a molecule.

The dehydration of an alcohol is an important way to make alkenes.