Hydrocarbons: alkanes and alkenes

In a nutshell

Alkanes and alkenes are hydrocarbons. Hydrocarbons contain the atoms carbon and hydrogen only. Hydrocarbons are used as fuels, as a lot of energy is released during complete combustion.

Equations

These are the general equations you need to know:

​$$alkane + oxygen \rightarrow​ carbon \space dioxide + water \space (+ \space energy)\\ \ \\alkene + oxygen \rightarrow​ carbon \space dioxide + water \space (+ \space energy)\\ \ \\alkene + oxygen \rightarrow​ carbon\ particulates + carbon\ monoxide + water (+ small\ amounts\ of\ energy) \\ or \\ alkene + oxygen \rightarrow​ carbon\ monoxide + carbon\ dioxide + water (+ small\ amounts\ of\ energy)$$​​

Properties of hydrocarbons

The shorter the length of the hydrocarbon:

• the less viscous they are
• the more volatile and flammable they are
• the lower the boiling point

Alkanes

Alkanes are the simplest hydrocarbons with the general formula $$C_nH_{2n+2}$$. They are saturated compounds as each carbon atom has four single bonds. In other words, alkanes contain single bonds only. 

Here are the first four alkanes you need to know:

Combustion of alkanes

General equation:

​ $$alkane + oxygen \rightarrow​ carbon \space dioxide + water \space (+ \space energy)$$​

Alkenes

Alkenes are classed as unsaturated hydrocarbons as they contain carbon-to-carbon double bonds ($$C=C$$​). They have the general formula $$C_nH_{2n}.$$ 

​Here are the first four alkenes you need to know:

Note: Alkenes must have carbon-carbon double bonds and methene would only have one carbon, therefore methene can not exist. 

Complete combustion

General equation: 

$$alkene + oxygen \rightarrow​ carbon \space dioxide + water \space (+ \space energy)$$​​

​

Incomplete combustion

​General equation: 

​$$alkene + oxygen \rightarrow​ carbon\ particulates + carbon\ monoxide + water (+ small\ amounts\ of\ energy) \\ or \\ alkene + oxygen \rightarrow​ carbon\ monoxide + carbon\ dioxide + water (+ small\ amounts\ of\ energy)$$​​

​

Carbon particulates are not always formed. It is dependant on the number of oxygen molecules available in the reaction. The lower the availability of oxygen the greater likelihood of carbon particulates being produced over carbon dioxide. Usually some carbon dioxide is produced alongside a greater amount of carbon monoxide.

Carbon monoxide is a poisonous gas.

Reactivity of alkenes 

The presence of $$C=C$$​ bond makes alkenes reactive, going from carbon-to-carbon double bond $$(C=C)$$​ to a carbon-to-carbon single bond ($$C-C$$​). They undergo addition reactions. Another molecule is able to add across the $$C=C$$​ bond, resulting in one product. 

The products formed are saturated molecules because there are no $$C=C$$​ bonds present.

Halogenation

Halogenation is when a halogen, such as chlorine or bromine, is 'added' across the $$C=C$$ bond to give a dihaloalkane. 

Hydrogenation 

Hydrogenation is when hydrogen is  ​'added' across the $$C=C$$​ bond to give an alkane, in the presence of a catalyst such as nickel ($$Ni$$).

Hydration

Hydration is the addition of a gaseous water molecule (steam) across the $$C=C$$ bond to form an alcohol, in the presence of a catalyst. 

Bromine water

Bromine water is orangey-brown. When an alkene is added to bromine water it becomes colourless. This indicates a reaction between bromine and alkene molecules has occurred. As bromine is used up it leads to the solution to become colourless.