INTEC Chemistry Blog

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How to visualise and identify R and S enantiomers.JPG

Posted on: May 3, 2013

Salam. Being able to visualise R and S chiral centres (or stereocentres) is an important skill. There are many ways to do it. I chose the way below because I thought it is the easiest and quickest way to identify/designate the configuration.

The diagram shows an ‘expanded’ Fischer projection, i.e. the molecule is shown with the East & West bonds coming out of the plane and the North & South bonds going into the plane. (The usual Fisher projection usually doesn’t specifically show the bond orientations). I think the ‘expanded’ sketch help make visualising what’s going on easier.\

So, as always, put your eye opposite the lowest priority (in terms of mass) group. In the diagram below the lowers priority group is H  and it goes into the plane, so our eye must be OUT of the plane, towards you.

You can now fix your reference point.: Your might be looking down the molecule from the left hand side, illustrated by the head in position B, or, you may prefer to look at the molecule from the right hand side which is position A, or, you may want to look at it straight-on as shown by the head with the label ‘C‘.

One you inspect the molecule, keep the same reference point. If you choose another ‘vantage(observation) point’ then you mix up the configurations and may get an error.

Hopefully you can see, no matter that vantage point you choose, the answer will always be the same. Of course, because the location of the groups with respect to each other is constant.

How to visualise and identify R and S enantiomers

It would be best if you can do this for all vantage points and for molecules in which the priority group has switched places. as in the molecules below.

Designate R or S to these three molecules.

How to visualise and identify R and S enantiomers - practice

Good luck.

Answers to questions at identify-r-and-s-enantiomers

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