Optical isomerism What is it? Optical isomerism occurs when substances have the same molecular formula and structural formula, but one cannot be superimposed on the other. Put simply, they are mirror images of each other. Molecules like this are said to be chiral pronounced ky-ral.
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This page explains what stereoisomers are and how you recognise the possibility of optical isomers in a molecule. What is stereoisomerism? What are isomers? Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds.
Where the atoms making up the various isomers are joined up in a different order, this is known as structural isomerism. Structural isomerism is not a form of stereoisomerism, and is dealt with on a separate page. What are stereoisomers? In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different spatial arrangement.
Optical isomerism is one form of stereoisomerism. Optical isomerism Why optical isomers? Optical isomers are named like this because of their effect on plane polarised light. Simple substances which show optical isomerism exist as two isomers known as enantiomers. A solution of one enantiomer rotates the plane of polarisation in a clockwise direction. A solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise direction.
This enantiomer is known as the - form. So the other enantiomer of alanine is known as or - alanine. If the solutions are equally concentrated the amount of rotation caused by the two isomers is exactly the same - but in opposite directions.
This is known as a racemic mixture or racemate. It has no effect on plane polarised light. Bear with it - things are soon going to get more visual!
This involves the use of the lowercase letters d- and l-, standing for dextrorotatory and laevorotatory respectively. Unfortunately, there is another different use of the capital letters D- and L- in this topic. This is totally confusing! Could you get them to align by rotating one of the molecules? The next diagram shows what happens if you rotate molecule B.
These are isomers of each other. Something would always be pointing in the wrong direction. The models made by Molymod are both cheap and easy to use.
An introductory organic set is more than adequate. Google molymod to find a supplier and more about them, or have a look at this set or this set or something similar from Amazon. What happens if two of the groups attached to the central carbon atom are the same? The next diagram shows this possibility. The two models are aligned exactly as before, but the orange group has been replaced by another pink one.
Rotating molecule B this time shows that it is exactly the same as molecule A. You only get optical isomers if all four groups attached to the central carbon are different. If there are two groups the same attached to the central carbon atom, the molecule has a plane of symmetry.
If you imagine slicing through the molecule, the left-hand side is an exact reflection of the right-hand side. Where there are four groups attached, there is no symmetry anywhere in the molecule. A molecule which has no plane of symmetry is described as chiral. The carbon atom with the four different groups attached which causes this lack of symmetry is described as a chiral centre or as an asymmetric carbon atom. The molecule on the left above with a plane of symmetry is described as achiral.
Only chiral molecules have optical isomers. The relationship between the enantiomers One of the enantiomers is simply a non-superimposable mirror image of the other one.
In other words, if one isomer looked in a mirror, what it would see is the other one. If an achiral molecule one with a plane of symmetry looked in a mirror, you would always find that by rotating the image in space, you could make the two look identical. It would be possible to superimpose the original molecule and its mirror image.
Some real examples of optical isomers Butanol The asymmetric carbon atom in a compound the one with four different groups attached is often shown by a star. Draw one of them using standard bond notation to show the 3-dimensional arrangement around the asymmetric carbon atom. Then draw the mirror to show the examiner that you know what you are doing, and then the mirror image.
It is, however, quite useful to reverse large groups - look, for example, at the ethyl group at the top of the diagram. As long as your mirror image is drawn accurately, you will automatically have drawn the two isomers. There is no simple way of telling that. The two enantiomers are: It is important this time to draw the COOH group backwards in the mirror image. It has, however, been possible to work out which of these structures is which.
Naturally occurring alanine is the right-hand structure, and the way the groups are arranged around the central carbon atom is known as an L- configuration. Notice the use of the capital L. The other configuration is known as D-. That means that it has this particular structure and rotates the plane of polarisation clockwise. Because the molecules have different spatial arrangements of their various groups, only one of them is likely to fit properly into the active sites on the enzymes they work with.
In the lab, it is quite common to produce equal amounts of both forms of a compound when it is synthesised. This happens just by chance, and you tend to get racemic mixtures.
Look at the structural formula and skeletal formula for butanol. Notice that in the skeletal formula all of the carbon atoms have been left out, as well as all of the hydrogen atoms attached to carbons. In a skeletal diagram of this sort: there is a carbon atom at each junction between bonds in a chain and at the end of each bond unless there is something else there already - like the -OH group in the example ; there are enough hydrogen atoms attached to each carbon to make the total number of bonds on that carbon up to 4.
We have already discussed the butanol case further up the page, and you know that it has optical isomers. The second carbon atom the one with the -OH attached has four different groups around it, and so is a chiral centre. Is this obvious from the skeletal formula?
Well, it is, provided you remember that each carbon atom has to have 4 bonds going away from it. Since the second carbon here only seems to have 3, there must also be a hydrogen attached to that carbon. So it has a hydrogen, an -OH group, and two different hydrocarbon groups methyl and ethyl. Four different groups around a carbon atom means that it is a chiral centre. A slightly more complicated case: 2,3-dimethylpentane The diagrams show an uncluttered skeletal formula, and a repeat of it with two of the carbons labelled.
Look first at the carbon atom labelled 2. Is this a chiral centre? Two bonds one vertical and one to the left are both attached to methyl groups.
In addition, of course, there is a hydrogen atom and the more complicated hydrocarbon group to the right. What about the number 3 carbon atom? This has a methyl group below it, an ethyl group to the right, and a more complicated hydrocarbon group to the left. Plus, of course, a hydrogen atom to make up the 4 bonds that have to be formed by the carbon.
That means that it is attached to 4 different things, and so is a chiral centre. Introducing rings - further complications At the time of writing, one of the UK-based exam boards Cambridge International - CIE commonly asked about the number of chiral centres in some very complicated molecules involving rings of carbon atoms. The rest of this page is to teach you how to cope with these. In this case, that means that you only need to look at the carbon with the -OH group attached.
It has an -OH group, a hydrogen to make up the total number of bonds to four , and links to two carbon atoms. How does the fact that these carbon atoms are part of a ring affect things? You just need to trace back around the ring from both sides of the carbon you are looking at. Is the arrangement in both directions exactly the same?
Going in one direction, you come immediately to a carbon with a double bond. In the other direction, you meet two singly bonded carbon atoms, and then one with a double bond. It is asymmetric - a chiral centre. What about this near-relative of the last molecule? In this case, everything is as before, except that if you trace around the ring clockwise and anticlockwise from the carbon at the bottom of the ring, there is an identical pattern in both directions.
You can think of the bottom carbon being attached to a hydrogen, an -OH group, and two identical hydrocarbon groups. The other thing which is very noticeable about this molecule is that there is a plane of symmetry through the carbon atom we are interested in.
If you chopped it in half through this carbon, one side of the molecule would be an exact reflection of the other. A seriously complicated example - cholesterol The skeletal diagram shows the structure of cholesterol. Some of the carbon atoms have been numbered for discussion purposes below. These are not part of the normal system for numbering the carbon atoms in cholesterol.
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This page explains what stereoisomers are and how you recognise the possibility of optical isomers in a molecule. What is stereoisomerism? What are isomers? Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds. Where the atoms making up the various isomers are joined up in a different order, this is known as structural isomerism.