Unit 2: The World of Carbon

(d) Uses of carbon compounds

There are competing demands for the use of crude oil for fuels and the manufacture of petroleum-based consumer products.

Many consumer products are compounds of carbon.

Esters

Esters are used as flavouring (essences), perfumes and as solvents (highly covalent and very volatile)

 

Carboxylic acids

Carboxylic acids are used in a variety of ways.

Ethanoic acid is found in vinegar, benzoic acid is used as a food preservative, terylene is made from terephthalic acid, nylon is made from hexanedioic acid, and salts of carboxylic acids, mainly stearic, oleic and palmitic, are used in soap manufacture.

 

Halogenoalkanes

Halogenoalkanes have properties which make them useful in a variety of consumer products

Chlorofluoro carbons (CFCs), which contain chlorine, fluorine and carbon, are examples of halogenoalkanes. CFCs are very unreactive, have low flammability and low toxicity. Also, the different family members have different boiling points which suit different application.

Everyday uses of CFCs include:

as propellants in an aerosol can

When the valve opens, the pressure falls inside the can causing the propellants to vaporise. The propellant escapes into the atmosphere along with the other contents of the can.

as refrigerants in food refrigerators and air-conditioning units

A total of 55 000 tonnes of CFCs went into the air conditioners of cars in the USA in 1987. Eventually some of these refrigerants end up in the atmosphere, through leakage or when the refrigerator is scrapped.

 

as blowing agents

For making plastic a volatile CFC is incorporated in the plastic when it is made. The heat given off during the polymerisation reaction vaporises the CFC so it `blows' tiny bubbles in the plastic making a foam. Inevitable, some of the CFC escapes into the atmosphere during the blowing process, and more escapes when the plastic is finally disposed of.

as cleaning solvents

CFCs dissolve grease and are used as solvents in dry-cleaning, cleaning electronic circuits, etc. Some of the solvent escapes into the atmosphere during use.

 

Environmental Problems

In the atmosphere, ozone, O3 , forms a protective layer which absorbs ultraviolet radiation from the sun. This protects life on Earth from ultraviolet radiation.

Destroying the ozone layers through the extensive release of certain CFCs into the atmosphere results in increased ultraviolet radiation

Possible consequences.

This could lead to more cases of skin cancer and eye cataracts, could affect species such as plankton in the oceans and in turn, other organisms involved in the food chain. Changes in the level of radiation reaching the Earth will affect the temperature of the Earth and in turn the weather.

Ozone is being produced and destroyed all the time in the atmosphere by reactions involving oxygen atoms. These can be made by splitting dioxygen molecules. This requires quite a lot of energy - the bond enthalpy of the oxygen-oxygen bond in dioxygen is +498 kJ mol-1. In this case the energy can be provided by ultraviolet radiation or by an electric discharge.

light

O2 ® 2O

 

As soon as oxygen atoms have been produced they react with the dioxygen molecules which are always present in the air. You can often smell the sharp odour of ozone near electric motors or photocopiers. The electric discharges happening inside the machine make some of the dioxygen molecules in the air dissociate into atoms. Also, you can often smell ozone near ultraviolet lamps.

O + O2 ® O3

Because CFCs are very unreactive they have a very long life-time in the troposphere. This gives them plenty of time to be transported up into the stratosphere where they can absorb high energy solar radiation and break down to give halogen atoms which react with ozone.

As more became known about the effect if CFCs on ozone in the stratosphere, the chemical industry began to search for compounds that could replace CFCs, but have no significant damaging effect in the ozone layer.

Progress has been very rapid. The replacement compounds are hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs). The difference is that these molecules contain H - C bonds and are broken down in the troposphere.

Sadly, they are not a perfect solution. Both CFCs and their replacements are greenhouse gases and contribute to global warming. There are fears too that some of the decomposition products may be toxic.

Governments have gradually realised that it is not worth risking the global experiment that is needed to find the answers to these questions. In 1987, at an international meeting in Montreal, a procedure was agreed for restricting the production and release of CFCs into the atmosphere. Three years later, a second meeting was held in London and the restrictions were tightened to include a total phasing out of CFCs by the year 2000. More than 60 countries signed this revised Montreal Protocol ad a special fund was set up to help developing countries move away from the use of CFCs.

But CFCs are very stable, and they take a long time to travel to the stratosphere. Even after CFCs are phased out in the year 2000, it will be well into the 2000s before the atmosphere returns to the condition it was in before damage to the ozone layer began.

 

 

Benzene Related Compounds

Benzene and its related compounds are important as feedstocks.

One or more hydrogen atoms of a benzene molecule can be substituted to form a range of consumer products.

 

phenol

xylene

aniline

(phenyl amine)

naphthalene

 

The uses of compounds containing the aromatic ring are varied. They are important in the manufacture of antiseptics, "carbolic acid" (phenol solution) was the very first antiseptic used by Lister.

The common antiseptic TCP (trichlorophenol) is manufactured from benzene.

Aspirin is salicylic acid formed from benzene.

TCP

salicylic acid