Organic chemistry basics

Organic chemistry is the study of carbon compounds. The name dates from a time when chemists believed such compounds could only be made by living things. In 1828 Friedrich Wöhler made urea, a compound found in urine, from inorganic starting materials, helping to end that belief.

Today organic chemistry covers millions of known compounds, from fuels and plastics to medicines, dyes and the molecules of life, including proteins, carbohydrates, fats and DNA.

A carbon atom can form four strong covalent bonds. It can bond to other carbon atoms in long chains, branches and rings, and it can form single, double and triple bonds. No other element builds such a vast variety of stable structures.

Carbon also bonds readily with hydrogen, oxygen, nitrogen, sulphur and halogens. Swapping one atom or group for another can change a molecule's properties dramatically, which is why small structural differences matter so much in medicines.

• Alkanes have only single bonds, such as methane, the main component of natural gas, and octane in petrol.
• Alkenes contain at least one carbon to carbon double bond, such as ethene, used to make polyethylene.
• Alkynes contain a triple bond, such as ethyne, also called acetylene, used in welding torches.
• Aromatic compounds contain stable rings of electrons, such as benzene, whose ring structure August Kekulé proposed in 1865.
• Crude oil is a mixture of hydrocarbons separated by fractional distillation.

Chemists group organic compounds by their functional groups, specific clusters of atoms that give molecules characteristic behaviour. An alcohol has an OH group; ethanol is the alcohol in drinks. Carboxylic acids have a COOH group; acetic acid gives vinegar its sour taste.

Esters, formed from an acid and an alcohol, often smell fruity and are used in flavourings and perfumes. Amines contain nitrogen and include many drugs and the building blocks of proteins. Recognising functional groups lets chemists predict how an unfamiliar molecule will react.

Isomers are compounds with the same molecular formula but different structures. Butane and isobutane both have the formula C4H10 but have different shapes and boiling points.

Some molecules exist as mirror-image forms, like left and right hands, called enantiomers. They can behave very differently in the body. The drug thalidomide, given to pregnant women around 1957 to 1961, caused severe birth defects, and the tragedy led to much stricter testing of medicines, including attention to how different molecular forms behave.

Polymers are huge molecules made of repeating units called monomers. Natural polymers include cellulose, starch, proteins and rubber. Synthetic polymers include nylon, first made at DuPont in the 1930s, polyester and the many plastics derived from oil.

Plastics are cheap, light and durable, which is also why they persist in the environment for a very long time. Organic chemists now work on biodegradable polymers, better recycling methods and plastics made from plant-based starting materials.