Question

How do geometric isomers arise?

Answer 1

In organic chemistry, these arise as a consequence of the structural chemistry of carbon bound to hydrogen, nitrogen, and oxygen in a tetrahedral array.

Explanation:

A carbon centre bound to 4 DIFFERENT groups, i.e. `CR_1R_2R_3R_4` can exist as 2 non-superposable stereoisomers, such that given a pair of stereoisomers, one isomer is the mirror image of the other, and these so-called optical antipodes cannot be superimposed on the other.

The stereoisomers have identical chemistry in all respects, except when in a chiral environment. Should a second chiral centre occur in the molecule, 4 different stereoisomers are possible: `"RR; SS; SR; and RS"`. `"RR"` is enatiomeric to `"SS"` but DIASTEREOMERIC with respect to `"RS"` and `"SR"`.

Biological chemistry and especially the chemistry of enzymes are replete with examples of chirality, such that typically an enzyme, of a definite chirality, will only work on a particular biological substrate. Sugar, which most of us take for granted, is an example of these substrates. It has a definite stereochemistry, upon which only a particular handed enzymatic catalyst, a particular diastereomer, will operate.

A simpler example of diastereoisomerism occurs in the `"2-butylene"` molecule. Of course, you know that we could have `"trans-2-butylene"` versus `"cis-2-butylene"`. These are geometric isomers that are not mirror images, and are thus a simple example of `"diastereoisomerism"`. I urge you to read the relevant chapter in your text in order to consolidate your understanding.