Chirality & Enantiomers

1. Conditions for chirality

• A stereogenic centre: carbon attached to four different groups.
• Absence of a plane and centre of symmetry overall.
• Molecules with a single stereogenic centre are always chiral.

2. Optical activity

A chiral compound rotates plane-polarized light: a clockwise rotation is labelled (+) or d (dextrorotatory); anticlockwise is (−) or l (laevorotatory). A 1:1 (racemic) mixture shows no rotation. Specific rotation [α] is the standard measured value.

3. R and S configuration

Use CIP sequence rules: rank the four groups by priority (by atomic number). Orient the lowest priority group away from you; trace 1→2→3. Clockwise = R (rectus), anticlockwise = S (sinister). If the viewer keeps the lowest group toward you, invert the answer.

4. Fischer, Newman, Sawhorse, Flying wedge representations

Fischer: horizontal lines come out of the page, vertical lines go into the page. Newman: view down a C–C bond. Sawhorse: oblique view of the bond. Flying-wedge: solid (out) and dashed (in) wedges. Interconversion of Fischer with Newman/wedge diagrams is a standard question.

Chiral = non-superimposable mirror image. Enantiomer = one member of the pair. d/l = rotation sign; R/S = CIP configuration. Racemic mixture has zero net rotation.