a Give two conditions based on the symmetry of a molecule th

a) Give two conditions based on the symmetry of a molecule that make it impossible for a molecule to have a dipole moment.

b) Which symmetry element makes it impossible for a molecule to be chiral?

Solution

a)

Molecules with mirror symmetry like oxygen, nitrogen, carbon dioxide, and carbon tetrachloride have no permanent dipole moments. Even if there is no permanent dipole moment, it is possible to induce a dipole moment by the application of an external electric field. This is called polarization and the magnitude of the dipole moment induced is a measure of the polarizability of the molecular species.

b)

Chirality is a property of individual molecules. If a molecule is chiral then we know that the molecule has two enantiomeric forms (which are almost identical but are actually different molecules). If a molecule is chiral then we know that it has a twin (an enantiomer). If a molecule has two or more chiral centers, it is usually chiral. The exception are meso-molecules, which are not chiral. These are molecules that due to symmetry have chiral centers that ‘cancel’ each other out.

A meso-molecule must satisfy BOTH of the following criteria

        1) Has two or more chiral centers.

        2) Has a plane of symmetry.