CHAPTER 15: Unit 4. Physical Property: Detection and optical property

Physical Property: Detection and optical property

Two enantiomers have exactly same physical properties, melting point, boiling point and solubility. Only one property can make difference, is the way they interact with plane polarized light.

A plane polarized light looks like the diagram below. Light wave moves in a single plane. A polarimeter is an instrument that allows plane polarized light to travel through a sample tube containing the organic compound. After the light exits the sample tube, analyzer slit is rotated to determine the direction of the plane of the polarized light exiting the sample tube.

An achiral compound does not change the direction of the plane -polarized light and it is said to be an optically inactive.

A chiral compound rotates the plane of polarized light though an angle measured in degree. A compound that rotates the plane polarized light is said to be optically inactive. The rotation of the polarized light can be in the clockwise or counterclockwise direction. Two enantiomers rotate the plane polarized light to an equal extent but in the opposite direction.

If the rotation is clockwise, the compound is called dextrorotatory(R) and is labeled as the (+) enantiomer.

If the rotation is counterclockwise, the compound is called levorotatory(S) is labeled as the (-) enantiomer

Specific rotation: mesured in enatiomeric mixtures, always they have same number but of opposite sign.

Enantiomers: have same chemical and physical properties in an achiral environment but they differ on the sign of rotation of plane polarized light. Same melting/boiling point, same rate of reaction with achiral reagents, same degree of rotation of plane polarized light………thus difficult to separate

For example: Enantiomers of Epinephrine (Adrenaline)

Another example:

No relationship exists between the S/R configuration and the sign or the magnitude of rotation of plane polarizedA 1:1 mixture of enantiomers (racemic mixture) has always no optical activity (rotation equal to zero) because the rotation of 50% of one enantiomer is cancelled out by the rotation (equal but opposite) of 50% of the other