Alkenes and Alkanes

Properties

The alkanes are a homologous series of hydrocarbons. This means that they have similar chemical properties to each other and they have trends in physical properties. For example, as the chain length increases, their boiling point increases.

Homologous series

A 'family' of organic compounds that have the same functional group and similar chemical properties.

Double bond

A covalent bond resulting from the sharing of four electrons (two pairs) between two atoms.

Saturated hydrocarbons

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air - which we call burning or combustion.

Properties

Alkenes are a homologous series of hydrocarbons that contain a carbon-carbon double bond. The number of hydrogen atoms in an alkene is double the number of carbon atoms.

Unsaturated

Alkenes are unsaturated, meaning they contain a double bond. This bond is why the alkenes are more reactive than the alkanes.

Alkenes

An alkene is an organic compound that contains a double bond between two carbon atoms. For this reason an alkene is referred to as an unsaturated hydrocarbon.

Test

Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene. Alkenes can decolourise bromine water, but alkanes cannot.

Ethene

C2H4

Ethanol

C2H5OH

Ethane

C2H6

Propene

C3H6

Propanol

C3H7OH

Propane

C3H8

Butane

C4H10

But-1-ene

C4H8

But-2-ene

C4H8

Pentane

C5H12

Methanol

CH3OH

Methane

CH4

The general formula for a simple alcohol

CnH(2n+1)OH

The general formula for an alkane

CnH(2n+2)

The general formula for an alkene

CnH2n

Functional group

Functional groups are groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of. The functional group for alkenes is the double bond between the carbon atoms C=C. The functional group for alcohols is the -OH group.

Isomerism

Like other homologous series, the alkanes show isomerism. This means that their atoms can be arranged differently to make slightly different compounds with different properties. For example, an isomer of butane is methylpropane.

Name

Molecular Formula

Name -

Molecular formula -

The straight chain alkanes share the same general formula

The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6.

Testing for alkenes

The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot. This allows us to tell alkenes apart from alkanes using a simple chemical test.

Reaction

The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction. The bromine is decolourised because a colourless dibromo compound forms. For example: ethene + bromine → dibromoethane C2H4 + Br2 → C2H4Br2

Alkanes

These are hydrocarbons with single covalent bonds between the carbon atoms. They are referred to as saturated hydrocarbons for this reason.