Carbon & Its Compounds

Carbon is only about 0.02 per cent of the Earth's crust, yet every living cell, every fuel, every plastic and every medicine is built on it. More than ten million carbon compounds are known, far more than for all other elements put together. Why does one small element get so much work? The answer lies in its electrons. Carbon has atomic number 6, so its electrons are arranged 2, 4. It has four electrons in the outer shell, exactly half-way to a full shell. It cannot easily gain four electrons (the nucleus would struggle to hold ten electrons) and it cannot easily lose four (that needs huge energy). So carbon does the sensible thing: it shares electrons and forms covalent bonds.

Why not silicon? Silicon sits just below carbon and also has four outer electrons, but it is a much bigger atom. Its Si–Si bonds are weaker and longer than C–C bonds, and its Si–H bonds are easily attacked by oxygen and water. So silicon builds only short chains (silanes) and prefers to combine with oxygen, making sand and rocks. Carbon is small enough for strong, stable chains, which is why life chose carbon, not silicon.

A hydrocarbon is a compound made of only carbon and hydrogen. It is the simplest family of organic compounds, and almost every other family can be seen as a hydrocarbon in which a hydrogen has been swapped for something else. If the carbon atoms are joined only by single bonds, the hydrocarbon is saturated (every carbon holds as many hydrogens as it can). If there is at least one double or triple bond between carbons, it is unsaturated. Saturated open-chain hydrocarbons are alkanes; those with a double bond are alkenes; those with a triple bond are alkynes.

Why does a candle flame look yellow but a gas stove flame blue? A saturated hydrocarbon like methane has little carbon compared with hydrogen, and with enough air it burns completely, giving a clean blue flame. Unsaturated and larger molecules have a high share of carbon. Even with plenty of air, some carbon does not find enough oxygen and glows as tiny hot particles of soot, which gives the yellow, smoky flame. This is also why a stove with its air holes blocked gives a yellow flame and blackens the vessel.

With millions of carbon compounds, chemists needed a system, like a library with shelves. The shelves are homologous series. A homologous series is a family of compounds that have the same functional group, the same general formula and similar chemical properties, and in which each member differs from the next by one CH₂ group (14 mass units). The physical properties, such as melting and boiling points, change smoothly as the chain grows. A functional group is the atom or group of atoms that decides the chemical behaviour of a compound. Change the functional group and you change the personality of the compound.

Why do homologues behave alike? Chemical behaviour is decided by the functional group, not by the length of the carbon chain. Methanol, ethanol and propanol all carry –OH, so all react with sodium, all turn to acids on oxidation and all form esters. The chain only changes the physical properties: a longer chain means a larger molecule, stronger attraction between molecules and so a higher boiling point. That is why a single reaction learnt for ethanol works for the whole alcohol family.

Two carbon compounds sit in almost every home. One is ethanol (ethyl alcohol, C₂H₅OH), the alcohol in hand sanitisers, cough syrups and tincture of iodine. The other is ethanoic acid (acetic acid, CH₃COOH), the sour part of vinegar in the kitchen. Because they carry different functional groups, –OH in ethanol and –COOH in ethanoic acid, they behave very differently, and exams love to test exactly this difference. Let us first meet ethanol. It is a colourless liquid with a pleasant smell and a boiling point of about 78 °C (351 K). It mixes with water in all proportions. It is neutral to litmus, unlike ethanoic acid.

Ethanoic acid. It is a colourless liquid with a sharp smell. Its melting point is about 16.6 °C (290 K), so in cold weather it freezes into an ice-like solid, and the pure acid is therefore called glacial acetic acid. A 5 to 8 per cent solution of ethanoic acid in water is vinegar, used in pickles as a preservative. Ethanoic acid is a weak acid: it ionises only partly in water, which is why a solution of it is less acidic than the same strength of hydrochloric acid. It turns blue litmus red, reacts with a base (CH₃COOH + NaOH → CH₃COONa + H₂O), and, importantly, reacts with carbonates and hydrogencarbonates to give brisk effervescence of carbon dioxide: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. Alcohols do not do this, and so this is the standard test that tells an acid from an alcohol.

Esters. When an alcohol and a carboxylic acid are warmed together with a few drops of concentrated sulphuric acid, they form an ester and water. This is esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. Concentrated sulphuric acid acts as a catalyst and also removes the water that forms. Esters are sweet, fruity-smelling liquids, so they are used in making perfumes and in flavouring. Ethyl ethanoate is also a common solvent. Methyl salicylate, the ester formed from salicylic acid and methanol, is the 'oil of wintergreen' used in pain-relieving balms and sprays; this is the ester used as a medicine. When an ester is heated with sodium hydroxide, it splits back into the alcohol and the sodium salt of the acid, and this is called saponification because it is how soap is made (next topic). Vegetable oil changed by an alcohol with a catalyst into esters is how biodiesel is made (see the fuels topic).

Halogen compounds and the ozone layer. In a haloalkane one or more hydrogen atoms of an alkane are replaced by halogens. The CFCs (chlorofluorocarbons) such as Freon-12 (CCl₂F₂) are made by the Swarts reaction, in which chlorine atoms of a chloro compound are swapped for fluorine using a metallic fluoride such as antimony trifluoride (for example CCl₄ gives CCl₂F₂). CFCs are unreactive, non-toxic and easily liquefied, which is why they were used as refrigerants and aerosol propellants. But when they rise to the stratosphere, ultraviolet light breaks them to release chlorine atoms, which destroy ozone. This is why the Montreal Protocol (1987) phased them out. Their replacements are HFCs, which do not harm the ozone layer but are strong greenhouse gases, and the Kigali Amendment (2016) aims to reduce those as well.

Why does water alone not wash a greasy plate? Because grease and oil do not mix with water. A cleaning agent must therefore be a molecule with two personalities: one end that loves water and another end that loves oil. Soap is exactly such a molecule. Chemically, soap is the sodium or potassium salt of a long-chain fatty acid, for example sodium stearate (C₁₇H₃₅COONa), sodium palmitate (C₁₅H₃₁COONa) and sodium oleate (C₁₇H₃₃COONa). Fats and oils are the raw material. A fat or oil is an ester of glycerol with fatty acids. When it is boiled with sodium hydroxide, the ester splits into the sodium salts of the fatty acids (soap) and glycerol. This alkaline hydrolysis of an ester is saponification: fat or oil + 3NaOH → 3 soap + glycerol.

Why are oils liquid but animal fats solid? The carbon chains of saturated fatty acids are straight, so the molecules pack tightly like pencils in a box and are held together by van der Waals forces; that needs a lot of energy to loosen, so the fat is solid at room temperature. In unsaturated fatty acids the double bond is a cis bond, and it puts a bend or kink in the chain. The kinked molecules cannot pack closely, the van der Waals attraction between them is weaker, and the substance stays liquid. Hydrogenation straightens the chains by removing the double bonds, and that is how liquid vegetable oil is converted into semi-solid vanaspati.

How soap cleans: the micelle. A soap molecule has two parts: a long hydrocarbon tail that is hydrophobic (water-hating, oil-loving), and a charged head (the –COO⁻ Na⁺ end) that is hydrophilic (water-loving). Dirt on cloth is mostly oily and holds the dust. In soapy water the soap molecules surround the oily dirt with their tails buried in the oil and their heads facing out in the water. This ball-shaped cluster is a micelle. Because the outer surface of every micelle carries the same charge, micelles repel each other and do not clump, and so the oily dirt stays dispersed in the water as an emulsion. Rubbing or beating the cloth helps lift the dirt, and rinsing washes the micelles away. Soap solution looks cloudy because micelles are big enough to scatter light (the Tyndall effect).

Hard water and the trouble with soap. Water containing dissolved calcium and magnesium salts is hard water. In hard water, soap reacts with the Ca²⁺ and Mg²⁺ ions to form an insoluble, sticky, whitish solid called scum (calcium stearate and magnesium stearate). The scum wastes soap, does not lather well and leaves a grey film on clothes. Temporary hardness is due to the hydrogencarbonates of calcium and magnesium and is removed by boiling. Permanent hardness is due to the chlorides and sulphates of calcium and magnesium and is removed by adding washing soda (Na₂CO₃), which precipitates the ions as carbonates, or by an ion-exchange method or by Calgon (sodium hexametaphosphate). Soap-free detergents were invented to overcome exactly this problem.

Detergents. A detergent is a synthetic cleansing agent. It is usually the sodium salt of a sulphonic acid (such as sodium alkylbenzene sulphonate) or of an alkyl hydrogen sulphate (such as sodium lauryl sulphate, SLS). Like soap, it has a long hydrocarbon tail and a charged head and forms micelles, but its calcium and magnesium salts are soluble in water, so it does not form scum and works well even in hard water and in acidic solutions. Detergents are of three kinds. Anionic detergents have a negatively charged head: sodium lauryl sulphate in shampoos, toothpastes and washing powders. Cationic detergents have a positively charged head, usually a quaternary ammonium salt (cetyltrimethylammonium bromide); they are germicidal and are used in hair conditioners and fabric softeners. Non-ionic detergents have no charge; they are used in liquid dishwashing products. A detergent with a straight, unbranched hydrocarbon chain is biodegradable (bacteria can break it down), while one with a branched chain is non-biodegradable and builds up as foam in rivers. Phosphates in detergent powders cause eutrophication, an overgrowth of algae that uses up the oxygen of the water.

A polymer is a very large molecule made by joining thousands of small molecules called monomers. The word comes from Greek: poly means many and meros means part. Think of a paper-clip chain: one clip is the monomer and the whole chain is the polymer. Carbon is the only element that can make such long chains, because of catenation, and that is why almost every polymer is a carbon compound.

Why does Bakelite not melt but polythene does? In polythene the long chains lie side by side, held only by weak forces, so heat easily slides them past each other. In Bakelite the chains are tied to each other by strong covalent cross-links, so there is nothing to slide, and that is why Bakelite is used for switches, plugs and pan handles.

Plastic waste is a serious problem because most plastics are non-biodegradable. Bacteria and fungi cannot break their long chains, so a discarded bag stays for hundreds of years, chokes drains and harms animals. Burning plastic is no solution: PVC, in particular, releases poisonous gases. The sensible order of action is reduce, reuse, recycle. Thermoplastics are sorted by the resin codes 1 to 7 stamped inside the recycling triangle: 1 is PET, 2 is HDPE, 3 is PVC, 4 is LDPE, 5 is polypropylene, 6 is polystyrene and 7 is other plastics.

A fuel is a substance that burns in air to give useful heat, and often light, without producing too much smoke or ash. Radhika's gas stove, a bus on the highway and a power plant all do the same thing: they burn a carbon compound and use the heat. A good fuel should have a high calorific value, which is the heat produced by the complete combustion of 1 kg of fuel, measured in kJ/kg. It should also be cheap, easy to store and transport, burn at a moderate rate, and leave little residue.

Petroleum (rock oil) is a dark, thick mixture of hundreds of hydrocarbons trapped in rocks under the earth. It is a fossil fuel, formed over millions of years from the remains of tiny sea organisms buried under mud, heat and pressure. Crude oil is not used directly. At the refinery it is heated and the vapours rise through a tall column in which the temperature falls upwards; each hydrocarbon condenses at a level that matches its boiling point. This is fractional distillation. Lighter, smaller molecules with low boiling points are collected at the top, and heavy, large molecules with high boiling points at the bottom.

Heating coal strongly in the absence of air is called destructive distillation. It gives four products: coke (nearly pure carbon, used in steel making and metal extraction), coal tar (a black liquid from which benzene, naphthalene, phenol and many dyes and medicines are obtained), coal gas (used as a fuel) and ammoniacal liquor.

Why was leaded petrol removed? Tetraethyl lead was added to petrol to stop knocking, but it was released through the exhaust as poisonous lead particles and harms the brain, especially of children. India phased out leaded petrol by 2000, and cleaner emission norms followed: Bharat Stage VI (BS-VI) came into force across the country on 1 April 2020.