Dextrorotatory isomer

Dextrorotatory isomer is a term used in chemistry to describe a type of isomer that rotates plane-polarized light to the right, or clockwise. This property is also known as dextrorotation and is denoted by the prefix (+) or d- before the name of the compound.

Overview

Dextrorotatory isomers are one of two possible spatial configurations, or enantiomers, that can occur in molecules that are chiral. Chiral molecules are those that cannot be superimposed on their mirror image. The other type of enantiomer is the levorotatory isomer, which rotates plane-polarized light to the left, or counterclockwise.

The direction in which a compound rotates plane-polarized light is determined by its stereochemistry, specifically the arrangement of its atoms in space. This property is important in many areas of chemistry and biochemistry, as the two enantiomers of a chiral molecule can have different chemical and biological properties.

Determination of Dextrorotation

The property of dextrorotation is determined using a polarimeter, an instrument that measures the angle by which a substance rotates the plane of polarization of light. The observed rotation is dependent on the path length of the light through the substance and the concentration of the substance, and is given by the formula:

α = [α]l.c

where α is the observed rotation, [α] is the specific rotation (a property of the substance), l is the path length, and c is the concentration.

Biological Significance

Many biologically active molecules are chiral, including amino acids and sugars. In nature, these molecules usually exist in only one of the two possible enantiomeric forms. For example, most amino acids are levorotatory, while most sugars are dextrorotatory. The different enantiomers can have different biological activities. For example, the dextrorotatory form of the drug thalidomide is effective against morning sickness, while the levorotary form is teratogenic, causing serious birth defects.

See Also

• Chirality (chemistry)
• Enantiomer
• Levorotatory isomer
• Polarimeter
• Stereochemistry