Observation of the rotation of plane-polarized light is known as polarimetry; it is a straight forward way of finding out if a sample is racemic or if it contains more of one enantiomer than the other. Polarimetric measurements are carried out in a polarimeter, which has a single wavelength (monochromatic) light source with a plane-polarizing filter, a sample holder, where a cell containing a solution of the substance under examination can be placed, and a detector with a read-out that indicates by how much the light is rotated. Rotation to the right is given a positive value, rotation to the left a negative one.

The angle through which a sample of a compound (usually a solution) rotates plane-polarized light depends on a number of factors, the most important ones being the path length (how far the light has to pass through the solution), concentration, temperature, solvent, and wave length. Typically, optical rotations are measured at 20 °C in a solvent such as ethanol or chloro form, and the light used is from a sodium lamp, with a wavelength of 589 nm. The observed angle through which the light is rotated is given the symbol α. By dividing this value by the path length ℓ (in dm) and the concentration c (in g cm⁻³) we get a value, [α], which is specifi c to the compound in question. Indeed, [α] is known as the compound’s specific rotation. The choice of units is eccentric and arbitrary but is universal so we must live with it.

Most [α] values are quoted as [α]D (where the D indicates the wavelength of 589 nm, the ‘D line’ of a sodium lamp) or [α] _D ²⁰, the 20 indicating 20 °C. These defi ne the remaining variables. Here is an example. A simple acid, known as mandelic acid, can be obtained from almonds in an enantiomerically pure form. When 28 mg was dissolved in 1 cm3 of ethanol and the solution placed in a 10-cm-long polarimeter cell, an optical rotation α of –4.35° was measured (that is, 4.35° to the left) at 20 °C with light of wavelength 589 nm. What is the specific rotation of the acid? First, we need to convert the concentration to grammes per cubic centimetre: 28 mg in 1 cm³ is the same as 0.028 g cm⁻³. The path length of 10 cm is 1 dm, so

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Absolute and relative stereochemistry

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

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

Chiral molecules have no plane of symmetry

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