Analytical Chemistry

The polarimeter is the instrument this topic is named for, and what it measures is optical activity, the property of a liquid that rotates the plane of polarised light. Optical activity comes from chirality, the handedness of a molecule, so the effect is a direct read-out of molecular shape rather than of concentration or conductance. The questions that sit under this heading are a loose family, and the connecting thread is that they are all about light, beams and the configuration of molecules, so they are worth revising together.

The configuration theme runs straight into substitution chemistry. A bimolecular substitution, SN2, takes place through a single step in which the nucleophile attacks the carbon from the side opposite the leaving group, so the product has the opposite configuration from the reactant, described in the bank as 100 per cent opposite from the configuration of the reactant. The unimolecular route is looser. Its intermediate is a carbonium ion whose carbon is sp2 hybridised and carries an empty p orbital, and the nucleophile can attach on either face, which would give a racemic mixture; because the leaving group partly shields its own side, the attack is somewhat favoured from the opposite side and the product shows partial racemization, retaining some optical activity rather than none.

The remaining optical facts are short but exact. When a ray of light passes through a prism, the extent to which it bends depends on the wavelength of the photons, which is why white light spreads into a spectrum. When a charged particle is deflected by a magnetic field and only the direction of one deflection is seen, the answer can be either an alpha ray or a beta ray, because both are charged and the sign of the charge decides the side. A related question asks which charged particle shows the greatest penetration, and the bank's answer is the beta particle, on the principle that the lightest and fastest of the charged particles travels furthest.