Describe the 3 types of isomers?

Definition: Isomerism is when 2 molecules have the same molecular structure but a different structural arrangement (structural isomerism) or a different arrangement in space (stereoisomerism). 

Structural isomerism occurs when the structure of the molecules are different but they have the same overall chemical formula. This can be due to branching in a hydrocarbon chain. For example, if we have 2 molecules with formula C4H10,  we can have one structured as CH3CH2CH2CH3 and the other as CH3CH(CH3)CH3.

Stereoisomerism comes in two forms; geometric isomerism and optical isomerism. Both of these occur because of different arrangements of atoms in space. These are easier to visualise with diagrams.

Geometric isomerism (aka cis/trans or E/Z isomerism) occurs when there is a carbon-carbon double bond with a H and a different functional group on each carbon in the double bond. For example, a molecule that would show this kind of isomerism is CH3CH2CH=CHCH3. On the first carbon, we have a H and an ethyl group and on the second carbon we have a H and a methyl group. 

Optical isomerism is when we have a central carbon atom with 4 different functional groups attached, forming tetrahedral bonding from this carbon. These isomers are mirror images of each other and are called enantiomers and the central carbon is known as the asymmetrical carbon.

An example of a molcule that would form enantiomers is CH3CH2CH(OH)CH3. The asymmetrical carbon is underlined, and you can see that is an ethyl group (CH3CH2-), a H, and OH group and a methyl group attached (-CH3).

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