Carbon and Its Compounds: Bonding, Allotropes and PYQs

Carbon has atomic number six, so its atom carries four electrons in the outer shell and needs four more to complete an octet. It neither gives up four electrons nor takes four, because either would leave the small nucleus holding a very large charge; instead it shares, forming four covalent bonds. That single fact explains almost everything asked about carbon in an examination, from the hardness of diamond to the number of compounds it forms.

Why carbon forms so many compounds

Two properties account for the enormous number of carbon compounds. The first is catenation, the ability of carbon atoms to link to one another in long chains, branched chains and rings; carbon shows it more strongly than any other element because the carbon to carbon bond is small and very strong. The second is tetravalency: having four bonds, a carbon atom can hold four other atoms of carbon or of elements such as hydrogen, oxygen, nitrogen, sulphur or a halogen. Compounds that differ in structure while sharing the same molecular formula are called isomers; butane with the formula C4H10 has two, pentane has three and hexane, C6H14, has five.

Allotropes of carbon

Diamond and graphite are made of the same element, and the difference in their properties comes only from the way the atoms are joined. In graphite one valence electron of each atom stays free to move, so the solid conducts, while in diamond every electron is locked in a bond.

Hydrocarbons

Compounds of carbon and hydrogen alone are hydrocarbons. Saturated hydrocarbons, or alkanes, have only single bonds and the general formula CnH2n+2, beginning with methane. Unsaturated hydrocarbons have a multiple bond: alkenes carry a carbon to carbon double bond with the formula CnH2n and begin with ethene, while alkynes carry a triple bond with the formula CnH2n-2 and begin with ethyne, also called acetylene. A compound with exactly two carbon to carbon double bonds is called a diene. A series in which each member differs from the next by one CH2 unit, and therefore by fourteen units of mass, is a homologous series; its members share chemical properties and show a gradual change in physical properties.

Functional groups

Reactions worth remembering

Combustion of a hydrocarbon in plenty of air gives carbon dioxide, water and heat, and the flame is clean and blue; when the air supply is short the combustion is incomplete and the flame is yellow and sooty, which is why the air holes of a gas burner are kept clear. Compounds with a high proportion of carbon, such as aromatic hydrocarbons, smoke readily for the same reason. In an addition reaction an unsaturated compound takes on hydrogen in the presence of nickel or palladium and becomes saturated, the industrial example being the hardening of vegetable oil into vanaspati. In a substitution reaction an atom of a saturated compound is replaced by another, as when methane reacts with chlorine in sunlight. Ethanol reacts with sodium to release hydrogen and, heated with concentrated sulphuric acid, loses water to give ethene. Ethanoic acid, a five to eight per cent solution of which is vinegar, reacts with an alcohol in the presence of an acid catalyst to give a sweet-smelling ester, and with sodium hydroxide to give a salt in the reaction used to make soap, called saponification.

Fuels and everyday carbon compounds

Liquefied petroleum gas used for cooking is mainly propane and butane, while compressed natural gas is mainly methane. Soap is the sodium or potassium salt of a long chain fatty acid; its molecules gather into a micelle with the oily tails inside and the charged heads in the water, which is how dirt is lifted off cloth. In hard water soap reacts with calcium and magnesium salts and forms an insoluble scum, which is why detergents, being salts of sulphonic acids, work where soap fails.