Question

How can enantiomers be distinguished?

Answer 1

How? Well, principally by the use of models.

Explanation:

In any undergrad exam in organic chemistry, a set of models will be permitted. Of course, you have to be able to use the models, and also be able to represent the 3D model on the page.

If you have (correctly) represented a stereocentre on the printed page (or on a model!), say you have `CR_1R_2R_3R_4`, it is a FACT that the interchange of ANY 2 substituents, `R_1` for `R_4`, `R_2` for `R_3` etc., gives the enantiomer. It follows that if another pair of substituents are interchanged (including of course the original pair), I get the enantiomer of the enantiomer, i.e. the original molecule.

So, not only do have to use the models, you also have to be able to represent them graphically. This is a non-trivial exercise. Check past exams for the types of questions you will be asked.

Answer 2

You can distinguish enantiomers by (a) making models, (b) assigning `R` and `S` designations to the chiral centres, and (c) seeing if they are nonsuperimposable mirror images.

Explanation:

A typical homework or exam question goes something like: "Are the following compounds identical, enantiomers, diastereomers, or structural isomers?"

(a) Making models

When to use them — when you have lots of time!

(From biowiki.ucdavis.edu)

Models are great when you are just learning the concepts, but it takes time to build them.

In a test or exam situation, you are typically stressed for time.

Use them only when you have answered the other questions and have time to build the models.

Often, you will not have enough "atoms" to build the models, anyway.

(b) Assigning `R` and `S` designations

This usually requires no manipulations of the structures.

In the example question, the priorities of the groups are:

• `"NH"_2 = bb1`
• `"COOH" = bb2`
• `"CH"_3 = bb3`
• `"H" = bb4`

In the wedge-dash structure, the `"H"` is behind the paper, and the `"NH"_2 → "COOH" → "CH"_3` direction is counterclockwise (`S`).

In the Fischer projection, the horizontal bonds are wedges, and the vertical bonds are dashes.

The `"H"` is behind the paper, and the `"NH"_2 → "COOH" → "CH"_3` direction is clockwise (`R`)

The compounds have opposite configurations, so they are enantiomers.

(c) Looking for mirror images

Here, you will have to convert one structure to the same format as the other.

Let's convert the Fischer projection to wedge-dash.

We rotate it 90 ° counterclockwise to bring the `"COOH"` to the top (wedges stay wedges and dashes stay dashes).

We tilt the `"COOH"` up to bring the `"C-COOH"` and `"C-NH"_2` bonds into the plane of the paper.

We see that the molecules are nonsuperimposable mirror images (enantiomers).