Question

How would you tell if a 3D molecule is chiral from a 2D drawing?

Answer 1

With difficulty. Nevertheless, there are conventions to make the geometry clear.

Explanation:

A chiral centre is depicted in the plane of the page; 2 bonds lie in the plane; 1 bond projects out of the plane; the other bond projects into the plane. If there are 4 distinct groups attached to the central carbon, that carbon is chiral, i.e. `CR_1R_2R_3R_4`.

Given such a representation, the interchange of any pair results in the enantiomer; interchange again and we restore the original isomer. Why?

So how do you know how to depict stereoisomers? Get a set of models and practise how to represent them on the page unambiguously. Such models are always permitted in exams. You will also find such models on the desks of distinguished profs of inorganic and organic chemistry.