What is the difference between structural isomers and stereoisomers?

1. Structural isomers are compounds which have the the same molecular formula but the atoms are arranged in a different order. This can be due to a change in the carbon skeleton, the type of functional group or the position of a functional group. An example of structual isomers would be 1-chloropropane (CH₂ClCH₂CH₃) and 2-chloropropane (CH₃CHClCH₃)

2. Stereoisomers are compounds which have the same structural formula but the atoms are arranged in a fixed position differently in space. Two types of steroisomers are geometrical and optical isomers which I will explain next:

2a. Geometrical isomers are compounds which have a different arrangements of groups on a double bond. It doesnt occur on single bonds as they can rotate but double bonds are fixed in space. On an alkene each sp² carbon has 2 substituents, if they are not the same this gives rise to 2 isomers, cis and trans. To work out which one, you label the substituents a and b, with a being the highest M_r. If both a's are on the same side then this is a cis isomer, also known as a (Z) isomer. If both a's are on opposite sides then this is a trans isomer (E). You can remember the difference between the two by remembering that trans means across (think trans-atlantic!) An example of this would be trans-propene and cis-propene

2b) Optical isomers happen when there are 4 different groups attached to an atom, often carbon. This carbon atom is labelled as 'chiral'. You label the substituents on the C atom the same way as in a geometrical isomer, in this case it would be from a-d. If the order goes in a clockwise direction this is the (R) isomer and the anticlockwise one would be the (S) isomer. An example would be amino acid: H₂H-CH(CH₃)-COOH, the carbon atom in bold is the chiral centre, the 4 groups are an amine, a hydrogen, a methyl group and a carboxylic acid.