What is the essential difference between these two compounds that means one is super imposable on its mirror image and one is not? The answer is symmetry. Acetone cyano hydrin has a plane of symmetry running through the molecule. This plane cuts the central carbon and the OH and CN groups in half, and has one methyl group on each side. All planar molecules (such as our simple aldehyde) cannot be chiral as the plane of the molecule must be a plane of symmetry. Cyclic molecules may have a plane of symmetry passing through two atoms of the ring, as in the cyclohexanone below. The plane passes through both atoms of the carbonyl group and bisects the methyl group as well as the hydrogen atom (not shown) on the same carbon atom. The bicyclic acetal looks more complicated but a plane of symmetry passes between the two oxygen atoms and the two ring-junction carbon atoms while bisecting the two methyl groups. None of these molecules is chiral.

On the other hand, the aldehyde cyanohydrin has no plane of symmetry: the plane of the paper has OH on one side and CN on the other while the plane at right angles to the paper has H on one side and RCH₂ on the other. This compound has no plane of symmetry (has asymmetry) and has two enantiomers.

● Planes of symmetry and chirality

• Any structure that has no plane of symmetry is chiral and can exist as two mirror-image forms (enantiomers).

• Any structure with a plane of symmetry is not chiral and cannot exist as two enantiomers.

مواضيع ذات صلة

Absolute and relative stereochemistry

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

The rotation of plane-polarized light is known as optical activity

R and S can be used to describe the configuration of a chiral centre

Many chiral molecules are present in nature as single enantiomers

Stereogenic centres

Some compounds can exist as a pair of mirror-image forms

Coupling in the proton NMR spectrum Nearby hydrogen nuclei interact and give multiple peaks

Exchange of acidic protons is revealed in proton NMR spectra

Protons on heteroatoms have more variable shifts than protons on carbon

Non-aldehyde protons in the aldehyde region: pyridines