Question 1

Can a compound be optically active without a chiral carbon?

Accepted Answer

Yes. Optical activity needs the molecule as a whole to be chiral — no plane of symmetry or centre of symmetry — and its enantiomers must not interconvert quickly. Allenes, spiranes, alkylidene cycloalkanes, hindered biphenyls, quaternary ammonium salts, phosphines and sulfoxides are all optically active without any chiral carbon.

Question 2

Why is penta-2,3-diene optically active?

Accepted Answer

In an allene the two terminal carbons are sp² and the central carbon is sp, so the substituents on one end lie in a plane perpendicular to those on the other end. With two different groups (H and CH₃) on each end there is no plane of symmetry, so the molecule is axially chiral. If either end carries two identical groups, a plane of symmetry appears and the compound is achiral.

Question 3

What is atropisomerism in biphenyls?

Accepted Answer

Atropisomers are stereoisomers that exist because rotation about a single bond is blocked. In biphenyls, bulky ortho groups such as NO₂, COOH or Br clash when the rings try to become coplanar, locking the twisted shape. If each ring also carries two different ortho groups, the locked molecule has no plane of symmetry and can be resolved into enantiomers — for example 6,6′-dinitrobiphenyl-2,2′-dicarboxylic acid.

Question 4

Why can't amines with three different groups be resolved?

Accepted Answer

The nitrogen is a stereocentre (three groups plus a lone pair), but it undergoes rapid pyramidal inversion — like an umbrella flipping inside out in the wind. The barrier is only about 25 kJ/mol, so at room temperature the two enantiomers interconvert extremely fast and any sample is racemic. Quaternary ammonium salts (no lone pair) and Tröger's base (nitrogen locked in a ring) cannot invert and are resolvable.

Question 5

Why does a cumulene with three double bonds show geometrical rather than optical isomerism?

Accepted Answer

Each extra cumulated double bond rotates the plane of the terminal substituents by 90°. With an even number of C=C (like allene) the end groups sit in perpendicular planes, giving axial chirality. With an odd number (like butatriene) the end groups are coplanar, so the molecule is planar and shows cis–trans isomerism instead of optical isomerism.

Optical Activity Without a Chiral Carbon — Allenes, Biphenyls & N

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