Carbon and its Compounds

The atomic number of carbon is 6, so its electronic configuration is 2,4. It has four electrons in its outermost shell and needs four more to complete its octet. Carbon cannot lose 4 electrons to form a C⁴⁺ cation (it would need enormous energy to remove four electrons) nor gain 4 electrons to form a C^4- anion (its nucleus of 6 protons cannot hold 10 electrons). So carbon completes its octet by sharing its four valence electrons with other atoms, forming covalent bonds.

A covalent bond is formed by the mutual sharing of a pair of electrons between two atoms. Because carbon forms four covalent bonds, it is said to be tetravalent. For example, in methane (CH₄) one carbon atom shares one electron each with four hydrogen atoms, forming four single covalent bonds so that both carbon and every hydrogen complete their outermost shell.

Two special features make carbon compounds so numerous. First, catenation is the unique ability of carbon atoms to link with one another through covalent bonds forming long chains, branched chains and rings. Second, tetravalency lets each carbon bond to as many as four other atoms of carbon or of other elements such as hydrogen, oxygen, nitrogen, sulphur or the halogens. Since carbon–carbon bonds are very strong and stable, these compounds are largely unreactive and stable.

Compounds made up of only carbon and hydrogen are called hydrocarbons. When all the carbon atoms are joined by single covalent bonds, the compound is a saturated hydrocarbon called an alkane (general formula C_nH_2n+2). For example, ethane is CH₃–CH₃ (C₂H₆).

When carbon atoms are joined by a double bond (C=C) or a triple bond (C≡C), the compound is an unsaturated hydrocarbon. A double bond gives an alkene (general formula C_nH_2n, e.g. ethene CH₂=CH₂, C₂H₄) and a triple bond gives an alkyne (general formula C_nH_2n-2, e.g. ethyne CH≡CH, C₂H₂).

Saturated and unsaturated hydrocarbons burn differently. Saturated hydrocarbons generally burn in air with a clean blue flame. Unsaturated hydrocarbons, having a higher proportion of carbon, undergo incomplete combustion and burn with a yellow, sooty (smoky) flame because unburnt carbon particles glow. This is why the bottom of a cooking vessel can blacken when the air supply to a burner is limited. Carbon skeletons can also close up into rings, for example cyclohexane C₆H₁₂.

In a carbon compound, hydrogen (or a part of the chain) can be replaced by other atoms or groups of atoms that give the molecule its characteristic properties. Such a reactive group is called a functional group. Examples are the halo group (–Cl, –Br), alcohol (–OH), aldehyde (–CHO), ketone (>C=O), and carboxylic acid (–COOH). Since the four valencies of carbon must always be satisfied, a functional group is bonded to the carbon chain by replacing one or more hydrogen atoms.

A homologous series is a group of organic compounds having the same functional group and the same general formula, in which successive members differ by a –CH₂– unit (a difference of 14 u in molecular mass). For example the alcohols CH₃OH, C₂H₅OH, C₃H₇OH form a homologous series. Members of a homologous series show similar chemical properties (same functional group) but a gradual change in physical properties such as melting point, boiling point and solubility as molecular mass increases.

In IUPAC nomenclature, the name has a root indicating the number of carbon atoms (meth- 1, eth- 2, prop- 3, but- 4) and a suffix or prefix for the functional group. A saturated chain ends in -ane. A double bond changes the suffix to -ene and a triple bond to -yne. For an alcohol the final 'e' of the alkane is replaced by -ol (e.g. propane → propanol), for a carboxylic acid by -oic acid (ethane → ethanoic acid), for an aldehyde by -al, and for a ketone by -one. Thus CH₃COOH is named ethanoic acid and CH₃CH₂OH is ethanol.

Combustion: Carbon and its compounds burn in oxygen (air) to give carbon dioxide, water, heat and light. For example, methane burns as CH₄ + 2O₂ → CO₂ + 2H₂O. This exothermic reaction is why hydrocarbons such as CNG and LPG are used as fuels.

Oxidation: Alcohols can be oxidised to carboxylic acids using oxidising agents such as alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇) on heating. For example, ethanol is oxidised to ethanoic acid. Because these substances add oxygen to another substance, they are called oxidising agents.

Addition reaction: Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. This hydrogenation reaction is used to convert vegetable oils (unsaturated) into solid fats (saturated) such as vanaspati ghee: C₂H₄ + H₂ → C₂H₆ (Ni catalyst).

Substitution reaction: Saturated hydrocarbons are fairly unreactive, but in the presence of sunlight chlorine replaces the hydrogen atoms one at a time. For methane: CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight). Here a chlorine atom substitutes a hydrogen atom, so it is a substitution reaction.

Ethanol (C₂H₅OH): a liquid at room temperature commonly called alcohol. It reacts with sodium metal to give sodium ethoxide and hydrogen gas: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. On heating with excess concentrated sulphuric acid at 443 K, ethanol is dehydrated to ethene: C₂H₅OH → C₂H₄ + H₂O (here conc. H₂SO₄ acts as a dehydrating agent). Ethanol also burns readily in air.

Ethanoic acid (CH₃COOH): commonly called acetic acid; a 5–8% solution in water is vinegar. Pure ethanoic acid freezes at 290 K in cold climates and is therefore called glacial acetic acid. It reacts with a base such as NaOH: CH₃COOH + NaOH → CH₃COONa + H₂O; and with carbonates/hydrogencarbonates releasing CO₂: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. With ethanol and an acid catalyst it forms a sweet-smelling ester (esterification): CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. On treatment with a base, an ester is hydrolysed back to alcohol and the sodium salt of the acid — this reaction is called saponification and is used to make soap.

Soaps and detergents: Soaps are sodium or potassium salts of long-chain carboxylic (fatty) acids. A soap molecule has two parts: a hydrophobic (water-repelling) hydrocarbon tail and a hydrophilic (water-attracting) ionic end. In water the molecules cluster into a micelle, with the hydrocarbon tails pointing inward towards the oily dirt and the ionic ends pointing outward into the water. The oily dirt gets trapped at the centre of the micelle and is washed away with water. Soaps do not work well in hard water (rich in calcium and magnesium ions) because they form an insoluble scum. Detergents are ammonium or sulphonate salts of long-chain hydrocarbons; their charged ends do not form insoluble salts with Ca²⁺ and Mg²⁺, so they clean effectively even in hard water.