Compounds that have the same molecular formula but different structures are called isomers. Butane and 2-methylpropane are both \mathrm{C_4H_{10}}: in butane, the 4 carbon atoms form a straight chain, while in 2-methylpropane, the chain is branched.
Isomers like these, which differ in how their atoms are joined, are called structural isomers. Their properties differ: butane boils at -0.5 °C, and 2-methylpropane at about -12 °C.
The number of structural isomers increases quickly with the number of carbon atoms: \mathrm{C_5H_{12}} has 3, and \mathrm{C_{10}H_{22}} has 75.
Ethanol and dimethyl ether are both \mathrm{C_2H_6O}, but ethanol has a hydroxy group, while in dimethyl ether an oxygen atom joins two carbon atoms.
Their properties are very different. Ethanol is a liquid that boils at 78 °C and reacts with sodium to give off hydrogen. Dimethyl ether is a gas that boils at about -25 °C and does not react with sodium.
Ethanol has the higher boiling point because its molecules are held together by hydrogen bonds between their hydroxy groups.
Because the carbon atoms of a double bond cannot rotate about it, but-2-ene, \mathrm{CH_3CH{=}CHCH_3}, exists in two forms. In the cis form, the two methyl groups are on the same side of the double bond, and in the trans form, they are on opposite sides.
Isomers like these are called cis-trans isomers, or geometric isomers. The cis form of but-2-ene boils at about 4 °C, and the trans form at about 1 °C.
Maleic acid and fumaric acid are another pair of cis-trans isomers. When maleic acid, the cis form, is heated, its two carboxy groups, which are close together, lose a molecule of water to form maleic anhydride. Fumaric acid, the trans form, does not change in this way.
In lactic acid, \mathrm{CH_3CH(OH)COOH}, the central carbon atom is bonded to 4 different atoms or groups of atoms: a hydrogen atom, a hydroxy group, a methyl group and a carboxy group. A carbon atom like this is called an asymmetric carbon atom.
The 4 groups around an asymmetric carbon atom can be arranged in two ways that are mirror images of each other. However they are turned, the two arrangements cannot be made to coincide.
Isomers like these are called enantiomers, or optical isomers. Their melting points, boiling points and most other properties are the same, but they rotate the plane of polarized light in opposite directions.
Almost all the amino acids that make up the proteins of living things are one of the two enantiomers, called the L-form. Enzymes and receptors, which are made of proteins, are therefore different from their own mirror images, and an enantiomer and its mirror image fit them differently.
As a result, the two enantiomers of a compound can differ in smell, taste and effect as a medicine. For example, one enantiomer of carvone smells of spearmint, and the other smells of caraway.
For this reason, many medicines are now made so that they contain only the enantiomer with the desired effect.