Carbon and its Compounds

Most of the carbon-containing compounds associated with hydrogen i.e., hydrocarbons are fuels that produce heat on burning. Petroleum products like natural gas, petrol, diesel, kerosene, heavy oils etc., and in a larger sense, wood, biogas, charcoal and coke are all rich source of carbon compounds used as fuels.

Combustion

Combustion means the burning of a substance. It is a process that is highly exothermic i.e., produces a lot of heat. The products of combustion of carbon and its compounds are heat energy, carbon dioxide and water (vapour).

In order that a fuel undergoes combustion, three basic requirements are to be present.

• A combustible substance: All carbon compounds are combustible, but carbon as diamond is not. Petrol is a combustible substance.
• A supporter of combustion: Atmospheric air or oxygen gas is a supporter of combustion. In their absence, combustion will not be supported. Carbon dioxide or nitrogen gases do not support combustion.
• Heating to ignition temperature: A minimum amount of temperature or heat is required to enable a fuel to catch fire. Coal has a high ignition temperature; a matchstick cannot produce enough heat to ignite it. However, a matchstick can ignite paper or LPG gas as it has low ignition temperature.

When the above conditions are present in any combustion process, proper combustion (energy production) takes place with minimum wastage and pollution. For example, if an ideal fuel like LPG (high calorific value and relatively high amounts of branched hydrocarbons) is available, a sufficient and continuous supply of oxygen should be maintained to burn it. If the ignition spark or flame is sufficient then the combustion is smooth and complete as follows.

It produces high heat energy with no wastage of raw material (un-reacted) and no production of undesirable by products (pollutants).

Most of the carbon compounds like the hydrocarbons when burnt in air or oxygen produce large amounts of heat, together with carbon dioxide and water vapour formation. Hence they are used as fuels. For example, methane burns with a blue flame in air.

In a very limited supply of air methane gives carbon black.

Some carbon compounds are very combustible and have an explosive reaction with air e.g., alkenes. They burn with a luminous flame to produce carbon dioxide and water vapour.

Some hydrocarbon compounds undergo cracking or thermal decomposition. In this process, substances are heated to high temperatures of (500 - 800⁰C) in the absence of air, and they decompose into a mixture of saturated and unsaturated hydrocarbons and hydrogen.

Oxidation

Carbon

Carbon undergoes oxidation by combining with oxygen at higher temperature to form to oxides, viz., carbon monoxide (CO) and carbon dioxide (CO₂). Carbon monoxide is formed, when incomplete combustion of carbon or carbon containing fuels takes place

CO is present in automobile exhausts (when there is incomplete combustion), volcanic gases, chimney gases etc.

Carbon dioxide may be prepared by the complete combustion of carbon, hydrocarbons, carbon monoxide etc.

Carbon Containing Compounds

These undergo oxidation reactions when burnt in air or oxygen. For example, when methane is mixed with oxygen and heated in presence of molybdenum oxide, it gets oxidized to methanal or formaldehyde.

Oxidation of carbon compounds is used as for producing other carbon compounds with different functional groups like alcohol, carboxylic acid, ethers etc. Oxidation is achieved by using an oxygen atmosphere or oxidizing agents like alkaline KMnO₄ or acidified K₂Cr₂O₇. Methanol, an industrial alcohol, for instance, is prepared by the oxidation of methane.

Acetic acid is manufactured by the oxidation of fermented liquors (10-15% alcohol) in air along with the presence of mycoderma aceti. A 3-7% solution of acetic acid is obtained and it is called vinegar.

When ethane is passed through an alkaline solution of potassium permanganate, the purple colour of the permanganate solution fades away.

Addition Reaction

The reactions in which two molecules react to form a single product are called addition reactions. For example,

Carbon containing double bonds like the alkenes readily react with certain molecules to form saturated addition products.

The addition of Cl₂, Br₂ or I₂ molecule across the double bond of the alkene is called halogenation.

The addition of a hydrogen molecule across the double bond of the alkene to form saturated products is called hydrogenation. This takes place in the presence of the catalyst, nickel.

Substitution Reaction

The reactions in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or group of atoms are called substitution reaction. For example,

In substitution reactions the hydrogen of the alkane molecule is replaced by another atom or a group of atoms (like alkyl) resulting in the formation of the derivatives of that hydrocarbon. Substitution by halogen atom is generally called halogenation. This type of substitution results in chlorination, bromination or iodination.

Depending upon the type of attacking reagent, the addition or substitution reactions are also classified as free radical, electrophilic or nucleophilic substitution or addition reactions. For example, if the attacking reagent is nucleophile in substitution reaction, the reaction is called nucleophilic substitution.

Chlorination of Methane

Chlorination of methane is carried out by taking a mixture of methane and chlorine in the sunlight or by heating to a temperature of 250^o- 300^oC. If chlorine is in excess, a number of substitution products are obtained.

Like methane, ethane also forms a series of substitution products in the presence of excess chlorine and sunlight.