Chiral and Achiral Molecules

Chiral and Achiral Molecules: Chiral molecules have a non-superimposable mirror image due to a chiral center, while achiral molecules are superimposable on their mirror image and lack chirality.

Chiral Molecules

Definition:

• A molecule is chiral if it:
• Has no element of symmetry
• Is not superimposable on its mirror image
• Can rotate plane-polarized light (optically active)

Features:

• Usually contains at least one chiral center (a carbon with four different groups)
• Lacks a plane or center of symmetry

Examples:

• 2-butanol: also has a carbon attached to –H, –CH₃, –CH₂CH₃, and –OH

Test for Chirality:

• If you try to superimpose the molecule onto its mirror image and they don’t match, it’s chiral.

Achiral Molecules

Definition:

• A molecule is achiral if it:
• Can be superimposed on its mirror image
• Has at least one element of symmetry
• Is optically inactive

Features:

• May or may not have chiral centers
• Meso compounds are good examples of achiral molecules with chiral centers

Examples:

• Glycine: the simplest amino acid, has two H atoms on the alpha-carbon → achiral
• Meso-tartaric acid: has two chiral centers but is overall achiral due to symmetry

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