Question

What is the R/S nomenclature?

Answer 1

`"A bloody nuisance...?"`

Explanation:

It is a fact that a carbon in a tetrahedral array with FOUR different substituents, i.e. `CR_1R_2R_3R_4`, can exist as one of TWO geometric isomers, the which cannot be superposed? Before we consider this in detail, let me ask you the question, what is the difference between your left and right hand? They are similar, but try as you might you will not be able to superimpose your right hand on the LEFT hand of your identical twin, or vice versa right on left.

Because such chirality is ubiquitous in biological chemistry, it follows that chemists have had to develop a flexible nomenclature capable of describing chiral carbons and structures.

What you got in the diagram is a so-called Fisher projection.

The horizontal `Cl`, and `CH_3` substituents are conceived to point out towards you, and the vertical substituents, `CO_2H`, and `CH_2OH` are conceived to project into the page away from you. The tetrahedral geometry of the central carbon is assumed.

And another picture...

This last perspective means that the methyl group of the given diagram projects into the page, and methyl is the least-ranked substituent. The priority of the other substituents is `1.` `Cl`, `2.` `CO_2H`, `CH_2OH` (these follow atomic number)...these are positioned clockwise (FROM OUR POINT OF VIEW) around the methyl. And so this is the `R` isomer, as the caption of the diagram specifies.

And how do you learn this stuff? Make models, and consider your text, and old exam questions. It does not matter how clever, or learned you are, if you cannot vizualize, and assign, AND then REPRESENT the geometry you are a shot bird in these questions.