dextrorotation

Dextrorotation and levorotation (also spelled laevorotation)^[1] refer, respectively, to the properties of rotating plane polarized light clockwise (for dextrorotation) or counterclockwise (for levorotation). A compound with dextrorotation is called dextrorotary, while a compound with levorotation is called levorotary.^[2]

Compounds with these properties are said to have optical activity and consist of chiral molecules. If a chiral molecule is dextrorotary, its enantiomer will be levorotary, and vice-versa. In fact, the enantiomers will rotate polarized light the same number of degrees, but in opposite directions.

It is not possible to determine whether a given chiral molecule will be levorotatory or dextrorotatory directly from its configuration, except via detailed computer modeling.^[3] In particular, both "R" and "S" stereocenters have the ability to be dextrorotatory or laevorotatory.

Prefixes

The prefixes "(+)-", "(–)-", "d-", "l-", "D-", and "L-"

A dextrorotary compound is often prefixed "(+)-" or "d-". Likewise, a levorotary compound is often prefixed "(–)-" or "l-". These "d-" and "l-" prefixes should not be confused with the "D-" and "L-" prefixes based on the actual configuration of each enantiomer, with the version synthesized from naturally occurring (+)-glyceraldehyde being considered the D- form. For example, nine of the nineteen L-amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and D-fructose is also referred to as levulose because it is levorotatory. See the article: Chirality (chemistry).

The prefixes "(R)-" and "(S)-"

There is no correlation between the (R) and (S) configuration of enantiomers and the direction of rotation of plane-polarized light.

Specific rotation

A standard measure of the degree to which a compound is dextrorotary or levorotary is the quantity called the specific rotation [α]. Dextrorotary compounds have a positive specific rotation, while levorotary compounds have negative. Two enantiomers have equal and opposite specific rotations.

The formula for specific rotation is:

where: [α] = specific rotation
α = observed rotation
c = concentration of the solution of an enantiomer
l = length of the tube (Polarimeter tube)

The degree of rotation of plane-polarized light depends on the number of chiral molecules that it encounters on its way through the tube of polarimeter (thus, the length of the tube and concentration of the enantiomer). In many cases, it also depends on the temperature and the wavelength of light that is employed.

References