Question

How can you determine whether a molecule is achiral or chiral?

Answer 1

A molecule is chiral if it cannot be superimposed on its mirror image.

Explanation:

How does this help us? Well, first we should look at macroscopic examples. Can you superimpose your left hand on the right hand of your identical twin? The answer is no. Why not? Because hands are handed, they have chirality (our word `"chirality"` literally means `"of the hands"`).

So how does this pertain to molecules? For carbon atoms, it turns out that any tetrahedral carbon that has 4 distinct substituents, i.e. `CR_1R_2R_3R_4`, has in principle 2 optical isomers, which cannot be superimposed but are equivalent structurally (because the connectivity is the same). Given the one optical isomer, the interchange of any 2 groups gives the mirror image, the enantiomer. You have to be able to determine the chirality of a carbon centre using predetermined rules.

I don't know at which level your are (presumably you are an undergraduate), but what I have said here underlies carbon stereochemistry. Most biological molecules, i.e. sugars, and proteins), exhibit chirality.