Compounds that contain carbon are called organic compounds. A few simple carbon compounds, such as carbon monoxide, carbon dioxide and carbonates, are treated as inorganic compounds instead.
The name comes from the old idea that these compounds could be made only by living things. In 1828, however, Friedrich Wöhler made urea, a compound found in urine, from inorganic substances.
A carbon atom has 4 valence electrons in its L shell, and it shares them with other atoms to form 4 covalent bonds. The 4 bonds point towards the corners of a regular tetrahedron with the carbon atom at its centre, and the angle between any two of them is about 109.5°.
Carbon atoms can bond to each other one after another, forming straight chains, branched chains and rings. This is one reason why there are so many kinds of organic compounds.
Compounds whose carbon atoms are joined only in chains are called open-chain compounds, and those that contain a ring of atoms are called cyclic compounds.
Compounds in which all the bonds between carbon atoms are single bonds are said to be saturated, and those that contain a double or triple bond between carbon atoms are said to be unsaturated.
To find out which elements an organic compound contains, the compound is mixed with copper(II) oxide and heated. The copper(II) oxide oxidizes it, turning the carbon into carbon dioxide and the hydrogen into water.
When the gas given off is passed through white anhydrous copper(II) sulfate, the powder turns blue. This shows that water has formed, so the compound contains hydrogen.
When the gas is then passed into limewater, the limewater turns milky. This shows that carbon dioxide has formed, so the compound contains carbon.
\mathrm{Ca(OH)_2} + \mathrm{CO_2} \longrightarrow \mathrm{CaCO_3} + \mathrm{H_2O}
Nitrogen can be detected by heating the compound with soda lime, which gives off ammonia if nitrogen is present. Chlorine can be detected by touching a hot copper wire with the compound and putting the wire back into a flame, which then turns blue-green.
A group of atoms that gives an organic compound its characteristic properties is called a functional group. Compounds that have the same functional group have similar properties, so organic compounds are classified by their functional groups.
For example, methanol and ethanol both have the hydroxy group, -\mathrm{OH}, and both react with sodium to give off hydrogen. Ethanoic acid (acetic acid), which has the carboxy group, -\mathrm{COOH}, is an acid.
The part of a molecule that is made of carbon and hydrogen atoms and joined to a functional group is called a hydrocarbon group, and it is often written as \mathrm{R}. The alcohols, for example, can be written as \mathrm{ROH}.
The same molecule can be written in several ways. The molecular formula of ethanol, \mathrm{C_2H_6O}, shows only how many atoms of each element the molecule contains.
A condensed formula such as \mathrm{C_2H_5OH} shows the functional group, and a structural formula shows how all the atoms are joined, with each bond drawn as a line.
A structural formula drawn on paper is flat, but in the real molecule the 4 bonds of each carbon atom point towards the corners of a tetrahedron. A ball-and-stick model shows this shape.
Dimethyl ether also has the molecular formula \mathrm{C_2H_6O}, but its atoms are joined differently, so it is a different compound. Compounds like these are called isomers.