Carbon & Its Compounds

Carbon sits at the heart of all living matter, and its position in the periodic table explains why it behaves so uniquely.

• It has four electrons in its outermost shell, so it needs four more to complete its octet.
• Carbon is found in the free state (diamond, graphite) and in the combined state (coal, petroleum, carbonates, all living things).
• It is the basis of all organic compounds and life on Earth.

Because carbon needs four electrons to fill its outer shell, it shares them rather than gaining or losing them.

• It cannot lose 4 electrons (would need huge energy) nor gain 4 electrons (would be hard to hold 10 electrons in a small nucleus), so it shares electrons and forms covalent bonds.
• This tetravalency lets carbon bond with up to four other atoms — of carbon, hydrogen, oxygen, nitrogen, sulphur, chlorine and more.
• Carbon almost always forms covalent (not ionic) bonds.

The single most important reason carbon forms millions of compounds is its ability to link with itself.

• Carbon–carbon bonds are very strong and stable, so very long chains are possible.
• The chains may carry single (C–C), double (C=C) or triple (C≡C) bonds.
• Carbon shows catenation to the maximum extent among all elements, which is why it forms the largest number of compounds.

A few properties together make carbon the champion of compound formation.

• Tetravalency — each carbon can bond with four other atoms.
• Strong, stable covalent bonds because carbon is a small atom and its bonds are not easily broken.
• Ability to form multiple bonds (double and triple) with carbon, oxygen, nitrogen and sulphur.
• Over 10 million carbon compounds are known — far more than the compounds of all other elements combined.

The same element, carbon, can exist in different physical forms with very different properties — these are its allotropes.

• The three main crystalline allotropes are diamond, graphite and fullerene.
• The amorphous forms include coal, coke and charcoal.

Diamond is the showpiece allotrope — the hardest natural substance known.

• Diamond is the hardest natural substance known.
• It is a bad conductor (non-conductor) of electricity because all four electrons of every carbon are locked in bonds, leaving none free.
• It is transparent and has a high refractive index (sparkle), making it a gemstone.
• Uses: jewellery, cutting and drilling tools, glass-cutters and abrasives.

Graphite is the soft, slippery, conducting cousin of diamond — the same atoms arranged very differently.

• Each carbon is bonded to only three other carbons, leaving one free electron per atom.
• Graphite is a good conductor of electricity — it is the only non-metal that conducts electricity.
• It is soft and slippery, so it works as a lubricant and as the "lead" in pencils.
• Graphite is the most stable form of carbon and is used as electrodes and as a moderator in nuclear reactors.

The newest allotrope of carbon is a hollow, football-shaped molecule.

• The most common is Buckminsterfullerene, C₆₀, made of 60 carbon atoms.
• C₆₀ looks like a football (soccer ball) — built of 20 hexagons and 12 pentagons.
• It was named after the architect Buckminster Fuller, whose geodesic domes it resembles.
• Carbon nanotubes are related cylindrical forms.

Besides the crystalline forms, carbon occurs in amorphous (shapeless) forms that are vital fuels.

• Coke is obtained by the destructive distillation of coal — a nearly pure carbon used as a reducing agent in metal extraction.
• Charcoal is formed by heating wood in limited air; activated charcoal is highly porous and used to adsorb gases and purify water/decolourise solutions.
• Lampblack (carbon black) is used in making ink, paint and rubber tyres.

Most carbon compounds are studied under organic chemistry, and the simplest of them contain only carbon and hydrogen.

• Hydrocarbons are compounds made of only carbon and hydrogen.
• Hydrocarbons are classified as alkanes, alkenes and alkynes.
• Urea (NH₂CONH₂) was the first organic compound made artificially, by Friedrich Wöhler in 1828, which ended the old "vital force" theory.

Hydrocarbons split into three families by the type of carbon–carbon bond they carry.

• Alkenes have at least one double bond (C=C); general formula CₙH₂ₙ.
• Alkynes have at least one triple bond (C≡C); general formula CₙH₂ₙ₋₂.

• First four alkanes: Methane CH₄, Ethane C₂H₆, Propane C₃H₈, Butane C₄H₁₀.

Whether a hydrocarbon has only single bonds or also multiple bonds decides how it reacts and burns.

• Unsaturated hydrocarbons have double or triple bonds — the alkenes and alkynes.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones burn with a yellow, sooty (smoky) flame.
• Unsaturated compounds are more reactive and undergo addition reactions (e.g. they decolourise bromine water).

Replacing a hydrogen of a hydrocarbon with a special atom or group gives the molecule its characteristic chemistry.

• Alcohol group = –OH (hydroxyl), as in ethanol.
• Carboxylic acid group = –COOH (carboxyl), as in acetic acid.

A handful of carbon compounds turn up again and again in exams and in daily life.

• It is the main component of natural gas, CNG and biogas.
• It is a greenhouse gas and burns with a blue flame.

• Ethanol is ethyl alcohol — the alcohol in alcoholic drinks.
• It is a liquid at room temperature and is used as a fuel, solvent and antiseptic.
• It contains the –OH (alcohol) functional group.

• Acetic acid is the acid in vinegar; a 5–8% solution in water is vinegar.
• It contains the –COOH (carboxylic acid) functional group.
• Pure acetic acid freezes in cold weather and is called glacial acetic acid.

Most of the energy we use comes from burning carbon compounds extracted from the Earth.

• Natural gas is mainly methane (CH₄), found above petroleum deposits.
• LPG (Liquefied Petroleum Gas) is mainly butane (C₄H₁₀) and propane (C₃H₈) — used as cooking gas in cylinders.
• CNG (Compressed Natural Gas) is mainly methane (CH₄) — a cleaner vehicle fuel.
• An odorant, ethyl mercaptan, is added to LPG/CNG to detect leaks (the gases are odourless).

When small molecules join into very long chains, we get polymers — the basis of plastics, fibres and rubber.

• Natural polymers include rubber, cellulose, starch and proteins; synthetic polymers are man-made (plastics, nylon).
• Thermoplastics soften on heating and can be remoulded (polythene, PVC); thermosetting plastics set permanently and cannot be remoulded (bakelite).

• Natural rubber is a polymer of isoprene, obtained from latex of rubber trees.
• Vulcanisation (heating rubber with sulphur) makes rubber harder and stronger — discovered by Charles Goodyear.
• Synthetic rubbers (e.g. neoprene, Buna-S, Buna-N) are man-made and more resistant to oils and heat.

Soaps are everyday carbon compounds with a special cleaning structure.

• Soap is made by saponification — heating fat or oil with sodium hydroxide (caustic soda).
• A soap molecule has a water-loving (hydrophilic) head and an oil-loving (hydrophobic) tail; the tails surround dirt/oil forming a micelle that washes away.
• Soaps do not work well (lather) in hard water because they form an insoluble scum with calcium/magnesium salts.
• Detergents work even in hard water and are synthetic cleansing agents.