Alkanes and alkenes

CH₄
Alkanes
C₂H₆

Amongst the different hydrocarbons found in crude oil, alkanes are the most abundant. Natural gas which is found with crude oil, is mostly methane. This is he smallest alkane.

Alkanes are saturated hydrocarbons, because the carbon atoms have no spare bonds left. This is why they don't form polymers and will not decolourise bromine water. They are joined by single covalent bonds to the maximum number of hydrogen atoms. Remember, single carbon atom can only have maximum of 4 bonds. See diagram above.

The alkanes form an homologous series where each member differs by a -CH₂- group from the successive member.

The general formula for alkanes is: C_nH_2n+2 where the "n" represents the number of carbon atoms.

All hydrocarbons are named according to the number of carbon atoms present in their molecules. The table below shows the prefixes used in the naming method. The first ten members are given in the table below.

Number of carbons	Prefix
1	Meth -
2	Eth -
3	Prop -
4	But -
5	Pent -
6	Hex -
7	Sept -
8	Oct -
9	Non -
10	Dec -

The bigger the molecule the higher the melting and boiling points, because of the stronger forces of attraction between the molecules (intra-molecular forces). These forces increase as the size of the molecule increases. As a result, these molecules are more viscose and less volatile.

Here are some examples of alkanes.

Name	No. of C atoms in chain	Molecular formula	Structural formula	Boiling point(°C)	Melting point(°C)	State at room temp
Methane	1	CH₄		-162	-183	gas
Ethane	2	C₂H₆		-89	-172	gas
Propane	3	C₃H₈		-42	-187	gas
Butane	4	C₄H₁₀		-1	-135	gas
Pentane	5	C₅H₁₂		36	-130	liquid
Hexane	6	C₆H₁₄		69	-94	liquid
Septane	7	C₇H₁₆	Predict	?	?	?
Octane	8	C₈H₁₈	Predict	?	?	?
Nonane	9	C₉H₂₀	Predict	?	?	?
Decane	10	C₁₀H₂₂	Predict	?	?	?

Reactions of alkanes:

Alkanes are not very reactive hydrocarbons however, they make excellent fuels and burn well in oxygen, giving out lots of energy. Burning is called combustion and is exothermic (gives out heat). In excess oxygen (complete combustion) the products of this reaction are carbon dioxide and water (see below).

Ethane	+	oxygen		carbon dioxide	+	water	+	energy
2 C2H₆	+	7 O₂		4 CO₂	+	6 H₂O

Try to balance the following reaction by filling in the correct number in front.

Butane	+	oxygen		carbon dioxide	+	water	+	energy
___C₄H_1o	+	___O₂		___CO₂	+	___H₂O

Hexane	+	oxygen		carbon dioxide	+	water	+	energy
___C6H₁₄	+	___O₂		___CO₂	+	___H₂O

When ever there is a limited supply of oxygen incomplete combustion takes place, leading to the formation of carbon monoxide (CO). For example, it forms when petrol burns in car engines and cigarettes are smoked. This is an odourless and colourless gas which if inhaled can lead to suffocation and eventually death. It is a poisonous gas. When inhaled, it reacts with the haemoglobin in your red blood cells. Haemoglobin is the substance that carries oxygen round the body. Carbon monoxide stops it working by forming carboxyhaemoglobin. This is why it is important to have the gas fire and boilers at home serviced regularly so that they burn with a clean blue flame and NOT produced any carbon monoxide.

Burning fossil fuels pollute the environment with carbon dioxide. This makes the greenhouse effect worse. These gases trap heat energy radiating from the surface of the earth. This can cause global warming, leading to the melting of the polar ice caps and rising sea levels.

Plants can reduce the amount of carbon dioxide in the atmosphere by means of photosynthesis (see carbon cycle below).

Carbon Cycle

C₂H₄Alkenes
C₃H₆

Alkenes are unsaturated hydrocarbons, because they have spare bonds. Carbon atoms are joined by a double covalent bond.

This makes them unstable and very reactive.

The alkenes form an homologous series where each member differs by a -CH₂- group from the successive member. The simplest alkene is ethene.

Q. Why methane is the smallest alkane and ethene the smallest alkene?

The general formula for alkenes is: C_nH_2n where the "n" represents the number of carbon atoms.

Name	No. of C atoms in chain	Molecular formula	Structural formula	Boiling point°C	Trend	State at room temp
Ethene	2	C₂H₄		-104		Gas
Propene	3	C₃H₆		-47		Gas
Butene	4	C₄H₈		-6		Gas
Pentene	5	C₅H₁₀		30		Liquid
Hexene	6	C₆H₁₂	Predict	Predict		?

As it can be seen from the table that the trend in properties such as boiling point and state change gradually as the number of carbon atoms increases, just like the alkanes, from gas to liquid.

The bigger the molecule the higher the boiling point, because of the stronger forces of attraction between the molecules.

Reactions of alkenes:

When other atoms add across the double bond to form two single bonds, the reaction is called addition reaction.

1. Reaction with bromine is an example of addition reaction. This reaction is used as a test to distinguish between alkanes and alkenes. The alkenes will decolourises the orange bromine water immediately, while an alkane will show no reaction.

2. Reaction with hydrogen is also an addition reaction, but it is known as hydrogenation and is used to saturate vegetable oil in the manufacture of margarine.

3. Reaction with water is also an addition reaction, but is known as hydration. A molecule of water will add across the double bond to produce an alcohol.

Ethanol is produced when ethene reacts with water.

Ethanol is also produced in nature by microbes. This is known as fermentation - the action of yeast enzymes on sugars.

Ethanol has the following uses:

In alcoholic drinks

As a solvent

As a fuel