Structural isomerism
Compounds that differ in the attachment of atoms without reagard to the spatial arrangement of atoms and have the same molecular formula are called structural (or constitutional) isomers.
Hydrocarbons with more than three carbon atoms can be arranged in various branched chains. In the presence of heteroatoms the situation is even more complicated. This is structural isomerism. On the contrary, in case of stereoisomerism the skeleton of the molecules is the same but the have a different three-dimensional arrangement of atoms:
As you see in the figure above, the butane molecule has only two isomers. The next homologue pentane has 3, hexane 6 isomers and so forth, the number of isomers increases exponentially. Decane has 75, eicosane (C20H42) has more than 366 thousand isomers.
The same applies to the compounds with heteroatoms, but in this case we usually take into account the types (families) of the compounds, and only those belonging to the same type are called isomers – in the next figure we have two isomer ethers and two isomer alcohols:
Excercises
How many isomers has trihydoxy-benzene?
How many isomers has dimethyl-naphthalene?
Which of these compounds are structural isomers?
