1. Predict qualitatively how the retention times in gas chromatography will vary

Question

1. Predict qualitatively how the retention times in gas chromatography will vary for a. heptane, octane, nonane, and decane b. benzene, toluene, ethyl benzene, and xylene 2. 2-pentanol (an alcohol) and 3-hexanone (a ketone) have nearly identical boiling points. Does this mean that these two compounds cannot be separated by GC? 3. Suppose you find that two compounds have identical retention times. What can you do to try to get a separation? 4. On a carbowax column such as that used in lab, would you expect to get separation of the two isomers shown below? Hint: These compounds represent a class of isomers called diastereomers. Diastereomers have different chemical and physical properties. OH OH and

Explanation / Answer

1)

The retention time in gas chromatogrphy is dependent on the equilibrium between the amount of adsorption of a compound versus the amount of compound in vapour phase. This is dependent on the boiling point of the compound and adsorption based on the functional groups in the stationery phase versus the compound.

In this case the compound sin questions are alkane and aromatic compounds which do not have any specific functional groups like alcohol or ketone

SO the retention time will be dependent of the boiling point.

The boiling point of heptane, octne, nonane and decane increases in this order oly so the retention time will also be in the same order with heptane having the lowest retenbtions time followed by octne, none and decane

The boiling point of benzene, toluene, ethyl benzene and xylene increases in this order. SO the retention times will also have the same order with benzene appearing first with the lowest retention time followed by toluene, ethyl benzene and finally xylene.

2) 2-pentanol and 3-hexanone have similar boiling point but the functional group on them is different. SO there are two possible ways to carry out separation using GC

1) by derivatization of the the compound selectively over the other.

2) Using a stationary phase which interacts more with one of the compounds over the other like if we use a moderately polar stationary phase like trifluoropropylmethyl polysiloxane it will interact with alcohol more than the ketone and thus it will increases chances of separation.

3) If two compound have identical separation we can try and derivatize one of the groups selectively so that the separation will increase like for example derivatives of aldehydes and ketones can be made using 2,4-Dinitrophenylhydrazone.

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