A mixture of 1-butanol, 1-hexanol, 1-octanol, and 1-propanol is separated by gas-liquid chromatography. What compound elutes last?

Question

A mixture of 1-butanol, 1-hexanol, 1-octanol, and 1-propanol is separated by gas-liquid chromatography.  What compound elutes last?

Quizplus · Accepted Answer

11ecba2d_1281_7085_a3bf_719da44b3304_MD0008_00 Gas-liquid chromatography (GC) is a technique used to separate molecules in a mixture on the basis of boiling point. Relative boiling points of molecules can be determined based on intermolecular forces, molecular weight (MW), and branching. Volatile molecules (low boiling points) spend more time in the gas phase rather than interacting with the liquid stationary phase of a GC column and travel quickly to the detector. Molecules with higher boiling points, however, condense and spend more time interacting with the liquid stationary phase and slowly make their way through the column as the temperature increases. The molecules in the mixture, 1-propanol, 1-butanol, 1-hexanol, and 1-octanol, are all alcohols with three-, four-, six-, and eight-carbon chains, respectively. The chain length can be determined by the compound's prefix. Because these molecules have the same functional group (alcohol) but different carbon counts, the relative boiling points are based on MW only. A higher MW indicates a higher boiling point and longer retention time because molecules with larger MWs have larger surface areas, giving a greater extent of London forces between the molecules. 1-Octanol has the greatest number of carbons (eight), the highest MW, and the highest boiling point of the four alcohols in the mixture and thus corresponds to the last peak in the GC trace. (Choices A, B, and D) 1-Butanol, 1-hexanol, and 1-propanol have smaller carbon chains, lower MWs, and consequently, lower boiling points than 1-octanol. These three alcohols therefore elute before 1-octanol. Educational objective: Gas-liquid chromatography separates molecules based on boiling point. A number of factors contribute to a molecule's boiling point: Intermolecular forces, molecular weight, and branching. For compounds with the same functional group but different carbon counts, a higher molecular weight indicates a higher boiling point and a longer retention time.

A Mixture of 1-Butanol, 1-Hexanol, 1-Octanol, and 1-Propanol Is Separated