Question

7. Given a non-polar stationary phase C18 (reverse phase) and a slightly polar mobile phase (15% water 85% acetonitrile) predict the order of elution for the following compounds (with 1 being the fastest and 4 being the slowest) (6 pts) Methanol CH OH 1-florobutane CH CH CH2CH2F Butanol CH,CH CHCHOH Cyclohexane CoH12 Briefly justify how you made your choice(s) Now increase the polarity of the mobile phase to 85% water and 15 % acetonitrile. Explain the relative shifts in retention times. (for each component determine if the retention time will increase, decrease or stay the same) 1-florobutane CH3CHCH-CH2F Methanol CH3OH Butanol СНСНасњенон- Cyclohexane C&H12 -- Now predict the order of elution for again with the mobile phase 95% water and 5 % acetonitrile. Methanol CH3OH 1-florobutane CH3CH CH CH2F Butanol CH,CH CH2CH OH Cyclohexane CoH12 Briefly justify your choice.

Answer 1

7. *In the organic molecules given, methanol,1-flourobutane, and butanol are polar compounds while cyclohexane is a nonpolar compound.

*The polarity of the 4 compounds are in the order

Methanol>1-Fluorobutane>Butanol>Cyclohexane

*Methanol has a polar hydroxyl(-OH) group and the hydrocarbon chain is small. Hence it is the most polar compound in the list.

*Both 1-Fluorobutane and Butanol have same hydrocarbon chain length but they differ in the functional group present. Since fluorine is more electronegative than oxygen, the former has more polarity.

*Cyclohexane does not have any polar groups present in it. So it is highly nonpolar.

*Since like attracts like, the polar compounds are more attached to other polar compounds and the nonpolar compounds have more affinity for other nonpolar compounds.

*Retention time is the time taken by a compound to move through the column and reach the detector.

a) As the stationary phase is nonpolar, the polar groups move faster through the column and reach the detector while cyclohexane interacts with the stationary phase and moves very slowly. The retention time will increase in the order of nonpolar nature of the compounds. Hence they reach in the same order as polarity.

(1)Methanol; (2) 1-Fluorobutane; (3)Butanol; (4)Cyclohexane

b) Now we have increased the polarity of the mobile phase. Surely the order of compounds reaching the detector will be the same. But it will affect the retention time of all the compounds.

Effect on the retention time of all the compounds

Methanol: As the polarity of the mobile phase increases, the interaction between methanol and mobile phase increases. As a result, the compound is easily carried over by the solvent to the detector. Hence the retention time considerably decreases.

1-Fluorobutane  and Butanol: Same effect as on methanol. The retention time decreases for both the compounds.

Cyclohexane: The nonpolar cyclohexane has more interaction with the nonpolar stationary phase. Since the polar nature of the mobile phase increases, the ability of mobile phase - to interact and carryover cyclohexane - decreases. Hence it stays more time in the column than in the first case. The retention time of cyclohexane increases greatly.

c) The order of elution does not change.

(1)Methanol; (2) 1-Fluorobutane; (3)Butanol; (4)Cyclohexane

since the polarity of the mobile phase again increased, the retention time of the first 3 polar compounds decreases again compared to the above two situations. But the retention time of cyclohexane increases again due to the decrease in the non-polar nature of the mobile phase.