Chemistry - STEREO ISOMERISM Question with Solution | TestHub

Ans. (1), (2) and (6) are chiral; (3), (4) and (5) are not chiral.

Sol. Purpose of the problem : revision of the criterion for chirality with examples of the main classes of chiral molecules.

Molecules are chiral if they have no plane of symmetry. This may arise from a tetrahedral atom with four different substituents or from a molecule that is forced to adopt a shape that lacks a plane of symmetry. Examples include spiro compounds, axial chirality in allenes, and chiral C, P, S centres.

(1) CHIRAL. The phosphorus atom itself is not a chiral centre (it carries two pairs of identical nitrogen substituents), but the molecule is chiral because it is a spiro compound: the two five-membered rings are orthogonal, like the second molecule in the previous question.

(2) CHIRAL. The molecule is nearly planar, but the combination of the C=N double bond at one ring position and the tetrahedral centre at the other ring junction removes every possibility of a plane of symmetry.

(3) NOT chiral. It tries to look chiral but is almost planar because of conjugation: the hydrogen atom above the plane reflects the hydrogen atom below it. The plane of the ring is a plane of symmetry.

(4) NOT chiral. This is an allene with the two pi-bonds orthogonal to each other, but one end is part of a symmetric cyclohexane ring (two identical ring paths): there is a plane of symmetry cutting the cyclohexane ring at right angles and passing through the allene axis and the ethyl group on the other end.

(5) NOT chiral. The tricyclic compound has a plane of symmetry vertically down the middle of the molecule.

(6) CHIRAL. The sulfoxide is a simple example of a stereogenic atom other than carbon: sulfur is tetrahedral, carrying =O, phenyl, the alkenyl ring carbon and a lone pair, so it is a stable chiral centre.