Something would always be pointing in the wrong direction. They are described as being non-superimposable in the sense that (if you imagine molecule B being turned into a ghostly version of itself) you couldn't slide one molecule exactly over the other one. They still aren't the same - and there is no way that you can rotate them so that they look exactly the same. Could you get them to align by rotating one of the molecules? The next diagram shows what happens if you rotate molecule B. Obviously as they are drawn, the orange and blue groups aren't aligned the same way. These two models each have the same groups joined to the central carbon atom, but still manage to be different: The examples of organic optical isomers required at A' level all contain a carbon atom joined to four different groups. This is totally confusing! Stick with (+) and (-). Unfortunately, there is another different use of the capital letters D- and L- in this topic. This involves the use of the lowercase letters d- and l-, standing for dextrorotatory and laevorotatory respectively. There is an alternative way of describing the (+) and (-) forms which is potentially very confusing. Bear with it - things are soon going to get more visual! Note: One of the worrying things about optical isomerism is the number of obscure words that suddenly get thrown at you. It has no effect on plane polarised light. This is known as a racemic mixture or racemate. When optically active substances are made in the lab, they often occur as a 50/50 mixture of the two enantiomers. If the solutions are equally concentrated the amount of rotation caused by the two isomers is exactly the same - but in opposite directions. So the other enantiomer of alanine is known as or (-)alanine.
This enantiomer is known as the (-) form. This enantiomer is known as the (+) form.įor example, one of the optical isomers (enantiomers) of the amino acid alanine is known as (+)alanine.Ī solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise direction. Simple substances which show optical isomerism exist as two isomers known as enantiomers.Ī solution of one enantiomer rotates the plane of polarisation in a clockwise direction. Help! If you don't understand about plane polarised light, follow this link before you go on with this page. Optical isomers are named like this because of their effect on plane polarised light. Optical isomerism is one form of stereoisomerism. In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different spatial arrangement. Note: If you aren't sure about structural isomerism, it might be worth reading about it before you go on with this page. Structural isomerism is not a form of stereoisomerism, and is dealt with on a separate page.
Where the atoms making up the various isomers are joined up in a different order, this is known as structural isomerism. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds. Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. This page explains what stereoisomers are and how you recognise the possibility of optical isomers in a molecule. Optical isomerism is a form of stereoisomerism.
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