Reactions of carbon compounds

(i) Addition

Alkanes and cycloalkanes are saturated compounds and this means that they contain only carbon to carbon single bonds.

Saturated hydrocarbons such as these cannot undergo addition reactions.

Alkenes are unsaturated compounds and these contain at least one carbon to carbon double bond.

Alkenes undergo addition reactions with molecules such as hydrogen, H₂, the Halogens and Water.

The double bond is broken and replaced by a single bond causing the unsaturated alkene to become a saturated product.

• The rapid decolourisation reaction with bromine is used as the test for an alkene and distinguishes it from an alkane.

Water can also add to an alkene and the addition is called hydration.

The reaction of ethene with water in the presence of phosphoric acid as a catalyst (catalytic hydration) is an industrial method of making alcohol, and supplements the alcohol made by fermentation.

(ii) - Cracking

 
Cracking of long-chain alkanes

• Crude oil contains more long chain fractions than are needed and not enough short chain fractions to meet the demand for fuels.
• A process called cracking is used to break long chains into smaller molecules which are more useful.
• High temperatures cause bonds to break in the long chain alkane and a mixture of compounds, some of which are unsaturated, is formed.
• The presence of unsaturated hydrocarbons can be shown by testing with bromine water.

The equation shows how an alkane molecule can be cracked into a shorter alkane and an alkene.

• As cracking occurs using heat, the process is known as thermal cracking.
• Cracking cannot give two saturated products as there are not enough atoms of hydrogen to allow both products to be saturated.

The cracking can be carried out at a lower temperature if a catalyst made from aluminium oxide or aluminium silicate is used.

The process is called catalytic cracking and is preferred in the oil industry because it saves energy (and money).

 
You can quickly test your knowledge of the above information.

 
(iii) Ethanol

Ethanol is the alcohol in alcoholic drinks, and it is made by a reaction called fermentation of glucose using an enzyme in yeast to catalyse the reaction.

As the ethanol concentration rises during fermentation, the yeast is killed and this limits the alcohol concentration from fermentation to about 15%.

Higher concentrations can be obtained by distillation, and the drink obtained is called a 'spirit' drink.

Whisky, vodka, brandy and gin are examples of spirits and have alcohol concentrations of up to 40%.

 
Effect of Alcohol on the Body and Mind

Alcohol affects the brain and increases reaction time. This means that it will take longer to react to situations making accidents more likely.
Alcohol can affect speech and steadiness.
Excessive alcohol consumption can cause liver damage and cancer of the mouth/throat.

 
Other Ways of making Alcohol

Hydration of ethene produces ethanol as described in 'Addition reactions' earlier.

It is possible to convert the ethanol back into ethene by removing the water in a dehydration reaction.

The dehydration reaction can be achieved by passing alkanol vapour over heated aluminium oxide.
The double bond is produced by the loss of the hydroxyl group and a hydrogen atom from an adjacent carbon atom.

 
Dehydration of ethanol will produce ethene.

Ethanol as a Fuel

Ethanol is a fuel because it burns releasing energy.

 
Ethanol can be mixed with petrol and used as a fuel for cars.
As ethanol can me made by fermenting sugar cane, and sugar cane can be grown annually, ethanol is regarded as a renewable fuel.

 
(iv) - Making and Breaking Esters

Making Esters

Esters are made by a condensation reaction between carboxylic acid and an alcohol in which a molecule of water is eliminated from the functional groups of the carboxylic acid and alcohol.
The -COO- group in the ester is called the ester link and it is formed between the hydroxyl group of the alcohol and the carboxyl group of the carboxylic acid.

Esters are made by warming the carboxylic acid and alcohol in a test tube containing a few drops of concentrated sulphuric acid and heated by a water bath for about 10 minutes.
This prevents the reaction mixture catching fire. However, to prevent the reactants and products being lost during heating, a wet paper towel is wrapped around the outer, upper part of the test tube.
This causes volatile reagents to condense and run back into the test tube in a technique called refluxing

The process is reversible i.e it operates in both directions.
Sulphuric acid is a catalyst for the reaction and as a dehydrating agent, it removes the water that is formed.
By removing the water from the reaction, the reverse reaction is prevented, so that more ester is made.

The ester is obtained by pouring the mixture into a beaker containing an aqueous solution of sodium hydrogencarbonate to neutralise the sulphuric acid.

Evidence that an ester is formed is its typical smell, and that is appears as a solid/oily liquid on the water.

 
Breaking Down Esters

When an ester is heated with water it begin to break down into a carboxylic acid and alcohol.
This breaking down reaction with water is called hydrolysis and it is the exact opposite of condensation.

It is not possible by this method to totally break down all of the ester and the reaction mixture will contain some ester, water, carboxylic acid and alcohol.

The reaction by be speeded up using H⁺ ions or OH^- ions.
If an alkali is used, this further helps by reacting with the carboxylic formed in the hydrolysis and removing the carboxylic acid from the reaction.
This prevents the joining of the alcohol with the carboxylic acid to remake the ester.

New words and their meanings

saturated - compounds that contain only carbon to carbon single bonds eg alkanes and cycloalkanes.

unsaturated - compounds that contain at least one carbon-carbon double bond eg alkenes.

addition - a reaction where a small molecule adds onto an unsaturated substance causing it to become more saturated.

hydration - an addition reaction where water is added to an unsaturated substance.

dehydration - a reaction in which water is removed from a substance.

condensation reaction - a reaction where two or more molecules join together and a small molecule, usually water is eliminated.

refluxing - where liquid evaporate to form gases which are then re-condensed to continue reaction.

hydrolysis - a reaction in which a molecule is broken into smaller pieces using water.