Carbon and Its Compounds

One element stands apart in all of chemistry: carbon. Though it makes up only a tiny fraction of the Earth's crust, carbon forms millions of compounds, far more than all other elements combined. These compounds are the stuff of life itself, and also of fuels, plastics, medicines and dyes. Understanding why carbon is so special is the gateway to organic chemistry.

Carbon has four electrons in its outer shell. It needs four more to be stable. Gaining or losing four electrons would take too much energy. So instead, carbon shares its electrons with other atoms and forms covalent bonds.

These shared-electron bonds are strong and stable. That is why carbon compounds are so numerous and durable. Carbon can bond with hydrogen, oxygen, nitrogen and many other elements. This builds an endless variety of molecules.

Carbon has two remarkable abilities:

• Catenation: carbon atoms can bond with one another to form long chains, branches and rings. No other element does this nearly as well. This is why carbon can build such huge and complex molecules.
• Allotropy: carbon exists in different physical forms called allotropes. These include diamond (the hardest natural substance), graphite (soft, conducts electricity, used in pencils), and fullerenes (ball-shaped molecules). All are pure carbon, yet utterly different. Their atoms are simply arranged in different ways.

Modern industry exploits carbon's strength in carbon fibres. These are thin strands of carbon that are extremely strong yet very light. They are embedded in resins to make composites. Two uses stand out:

• Automobiles: carbon fibre composites make lightweight car components, which cuts fuel use.
• Aircraft: the same lightness and strength suit aircraft parts, where every kilogram matters.

Used carbon fibre composites are not waste. They can be recycled: the fibres are recovered from the resin and reused in new products.

Compounds made of only carbon and hydrogen are called hydrocarbons, the simplest carbon compounds and the basis of fuels. They may be saturated (single bonds, like methane) or unsaturated (double or triple bonds).

When other atoms or groups replace some hydrogen, they form functional groups. These are reactive parts that give a compound its character. Examples include the alcohol group and the acid group. Each functional group gives the molecule particular properties. That is what creates the immense variety of organic chemistry.

Hydrocarbon fuels do not have to come from petroleum. Waste-to-energy technologies turn municipal solid waste into useful energy. Two methods are tested often:

• Pyrolysis: heating waste at high temperature without oxygen. The waste does not burn. It decomposes thermally into fuel gases, oils and char.
• Plasma gasification: an extremely hot plasma arc breaks waste down into a fuel gas (mainly carbon monoxide and hydrogen) and a glassy solid residue.

The fuel gases from both routes can be burned to generate electricity. Waste thus becomes a source of fuel and power instead of landfill.

Two everyday carbon compounds are especially important:

• Ethanol (alcohol): used in drinks, as a solvent and increasingly as a fuel.
• Ethanoic acid (acetic acid): the acid in vinegar.

Soaps and detergents are also carbon compounds. Their molecules have a water-loving end and an oil-loving end. This lets them trap grease and wash it away. Carbon compounds shape daily life, from the food we eat to the fuel we burn.

Important carbon compounds can also be built from coal. Coal gasification reacts coal with steam and limited oxygen. The product is syngas, a mixture of carbon monoxide (CO) and hydrogen (H2). Syngas is not just a fuel. It is a versatile chemical feedstock. From syngas, industry can synthesise:

• Ethanol: made directly from syngas or via intermediates.
• Urea: the hydrogen yields ammonia, and ammonia plus carbon dioxide gives urea, the key nitrogen fertiliser.
• Nitroglycerine: syngas-derived chemistry yields glycerol, which is nitrated to nitroglycerine, an explosive also used in heart medicine.

So one gasification stream can feed fuels, fertilisers and explosives alike.

Catenation reaches its peak in polymers. A polymer is a giant molecule built by joining many small repeating units called monomers into a long chain. Plastics are synthetic polymers, and each type comes from a different monomer. Common examples include polyethylene (carry bags, in low-density and high-density forms), polyvinyl chloride (PVC) (pipes and cables) and polycarbonate (hard, transparent items such as water bottles and eyewear).

The most widely used packaging plastic is polyethylene terephthalate (PET), a polyester made from an acid and an alcohol monomer. Its key properties are tested often:

• Textile use: PET fibres (polyester) can be blended with wool and cotton to make the fabric stronger and more durable.
• Recyclable: used PET bottles can be recycled into fibres, sheets and other products. The recycling code for PET is 1.
• Not for all liquids: PET containers are not suitable for storing high-alcohol beverages, because alcohol can interact with the plastic over long contact.
• Incineration is not clean: burning PET releases carbon dioxide, a greenhouse gas, so incineration is not an emission-free disposal route.

A separate health concern is Bisphenol A (BPA), an industrial chemical used as a monomer in polycarbonate plastics and in epoxy resins that line food cans. BPA can leach into food and drink. It acts as an endocrine disruptor, a substance that interferes with hormones. This is why "BPA-free" bottles are marketed. Note the distinction: BPA goes into polycarbonate, not into polyethylene, PET or PVC.