A mixture of heptane (bp 98 c) and 1-propanol (bp 97 c) is not fully resolved on a column of SE-30 but is easily separated on a column of DEGs. Explain?
The Correct Answer and Explanation is:
The correct answer:
The mixture of heptane and 1-propanol is not fully resolved on a column of SE-30 because SE-30 is a non-polar stationary phase, which does not effectively differentiate between compounds with similar boiling points but different polarities. In contrast, DEGs (diethylene glycol succinate) is a polar stationary phase that interacts more strongly with polar compounds like 1-propanol, allowing better separation from non-polar compounds like heptane.
Detailed Explanation (≈300 words):
Gas chromatography (GC) separates compounds based on their volatility and interactions with the stationary phase in the column. Two key factors influencing separation are boiling point and polarity. While heptane and 1-propanol have nearly identical boiling points (98°C and 97°C, respectively), their chemical nature is quite different: heptane is non-polar, while 1-propanol is polar due to its hydroxyl (-OH) group.
SE-30 is a non-polar stationary phase made of polydimethylsiloxane. It interacts weakly with both heptane and 1-propanol, primarily via van der Waals forces. Because both compounds have similar volatilities and experience similarly weak interactions with SE-30, they elute at nearly the same time, making separation poor or unresolved.
In contrast, DEGs (diethylene glycol succinate) is a polar stationary phase. It forms stronger dipole-dipole and hydrogen-bonding interactions with polar compounds like 1-propanol but interacts only weakly with non-polar compounds like heptane. This difference in interaction slows down 1-propanol significantly more than heptane, thereby increasing the retention time difference between the two and allowing for effective separation.
This scenario illustrates a fundamental principle in gas chromatography: to separate components with similar boiling points but differing polarities, a stationary phase that can exploit those polarity differences (e.g., polar vs. non-polar) is crucial. Hence, DEGs provides superior separation in this case by leveraging the selectivity based on polarity, which SE-30 cannot offer.
Thus, the resolution of heptane and 1-propanol is successful on DEGs due to its ability to discriminate based on polarity, not just volatility.
