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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: CnH2n+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

C3H

-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.

1.

Butane

+

oxygen

carbon
dioxide

+

water

+

energy

___C4H1o

+

___O2

___CO2

+

___H2O

2.

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

C2H4
Alkenes
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: CnH2n 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 reactionThis 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

 

 

Tags:Reaction, Alkanes, Hydrocarbons, Alkenes, Enzymes, Ethanol, Microbes

 

 

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