Question

How to know if you isolated a cis, trans, or a mixture of both isomers following gc lab.

Answer 1

Consider the longest chain containing the double bond: If two groups (attached to the carbons of the double bond) are on the same side of the double bond, the isomer is a cis alkene. If the two groups lie on opposite sides of the double bond, the isomer is a trans alkene.

let us consider an example, There are important differences in the physical properties of the cis and trans isomers of unsaturated long chain amines. Because of these differences, it is desirable to know the ratio of the cis and trans isomers. This separation can be accomplished by capillary column gas chromatography of the amine derivatives; however, the use of capillary columns is too slow for routine analysis. It was found that the cis and trans isomers of unsaturated amines can be rapidly separated on packed columns containing cyanopropyl-phenylsilicone liquid phases (Apolar 10C). The primary amines are converted to the trifluoroacetamides and separated on 6- to 10-ft columns containing 10% Apolar 10C at 180 C. The technique has been applied to unsaturated long chain primary amines and diamines. Useful information on the isomerization of these compounds during their synthesis can be obtained.