Question

How can you identify achiral molecules?

Answer 1

By the presence of a mirror plane in the molecule's structure.

Explanation:

How does this help you? Well, not so much, but you can take a bit of a short cut. Carbon has 4 substituents in a tetrahedral array. On the printed page we write `CH_4` or `CR_4`.

If there are 4 different substuents around the central carbon, i.e. `CR_1R_2R_3R_4`, then the central carbon is chiral, and will give rise to 2 non-superposable enantiomers. Given a model of a particular chirality, the interchange of ANY 2 substituents in the model will give the enantiomer. Interchange again, you get the enantiomer of the enantiomer, i.e. the original stereoisomer.

On the other hand, if at least one of the substituents are the same, i.e. `CR_2R'R''`, i.e `H_2CX_2`, `R_2CH_2`, then the central carbon is achiral, and its mirror image is necessarily superposable on the original.

Given what I have said, you are cutting your own throat if you don't use a model set. This could be as simple as plasticene and toothpicks. It is hard for us to see chirality; it is even harder to represent a three-dimensional chiral structure on the printed page.