Passage
Ethanol is an important source of energy that is frequently used as a supplement to gasoline. It can be obtained from renewable resources such as the cellulose found in plant matter. When subjected to a process called gasification, the plant mass is converted to a mixture called syngas, composed predominantly of carbon monoxide and molecular hydrogen. Syngas can be passed over a rhodium catalyst to generate ethanol and other products. After using this method, a group of researchers analyzed the resulting products by gas chromatography using helium as a carrier gas. The results are shown in Figure 1.
Figure 1 Gas chromatograph of syngas catalysis productsMass spectrometry revealed that the process yielded similar amounts of ethanol, acetic acid, and acetaldehyde, with trace amounts of ethane. In addition, the ethanol peak was found to be contaminated with ethyl acetate. Another group of researchers in the same laboratory had recently produced Compound 1 as a by-product in a separate reaction. They combined Compound 1 with purified ethyl acetate to form megestrol acetate, a drug used in the treatment of certain cancers. This reaction also produced ethanol, as shown in Scheme 1.
Scheme 1
Adapted from Lopez L, Velasco J, Montes V, Marinas A, Cabrera S, Boutonnet M, Järås S. Catalysts 2015.
-Acetic acid most likely eluted from the gas chromatograph at what time point?

Question

Passage
Ethanol is an important source of energy that is frequently used as a supplement to gasoline. It can be obtained from renewable resources such as the cellulose found in plant matter. When subjected to a process called gasification, the plant mass is converted to a mixture called syngas, composed predominantly of carbon monoxide and molecular hydrogen. Syngas can be passed over a rhodium catalyst to generate ethanol and other products. After using this method, a group of researchers analyzed the resulting products by gas chromatography using helium as a carrier gas. The results are shown in Figure 1.

Figure 1 Gas chromatograph of syngas catalysis productsMass spectrometry revealed that the process yielded similar amounts of ethanol, acetic acid, and acetaldehyde, with trace amounts of ethane. In addition, the ethanol peak was found to be contaminated with ethyl acetate. Another group of researchers in the same laboratory had recently produced Compound 1 as a by-product in a separate reaction. They combined Compound 1 with purified ethyl acetate to form megestrol acetate, a drug used in the treatment of certain cancers. This reaction also produced ethanol, as shown in Scheme 1.

Scheme 1
Adapted from Lopez L, Velasco J, Montes V, Marinas A, Cabrera S, Boutonnet M, Järås S. Catalysts 2015.
-Acetic acid most likely eluted from the gas chromatograph at what time point?

Quizplus · Accepted Answer

11ecba2d_126d_4bf9_a3bf_a9f1d1c09f1a_MD0008_00 Gas chromatography (GC) separates molecules primarily on the basis of boiling point. The injection chamber is heated to a temperature above the boiling point of all compounds being analyzed (usually 200°C-300°C), causing all components to vaporize. The walls of the GC column are coated with a thin layer of liquid that acts as the stationary phase. An inert gas flows through the column, carrying other gases with it. Compounds with a lower boiling point tend to stay in the gas phase and move through the column quickly, whereas compounds with higher boiling points have a greater tendency to interact with the liquid phase and a longer retention time. The chromatogram shown in Figure 1 has four peaks. To determine which one corresponds to acetic acid, the boiling point of acetic acid relative to the other compounds must be determined. Boiling point is a function of intermolecular forces, with fewer forces corresponding to lower boiling points. Organic molecules with the same number of carbon atoms follow a pattern of increasing intermolecular forces and boiling points: Alkanes have the weakest intermolecular forces and the lowest boiling points, followed by aldehydes and ketones, then alcohols, and carboxylic acids, which have the highest boiling points. Acetic acid is a carboxylic acid and has the highest boiling point of the compounds produced; therefore, it has the longest retention time and must elute at 15 min. (Choice A) The first peak (1 min) corresponds to ethane, which has the lowest boiling point. (Choice B) The second peak (2 min) corresponds to acetaldehyde. (Choice C) The third peak (5 min) corresponds to ethanol. The contaminating ethyl acetate, which has a similar boiling point to ethanol, is also in this peak. Educational objective: Gas chromatography separates compounds primarily by boiling point. Molecules with low boiling points elute before those with high boiling points. For compounds with the same number of carbon atoms, alkanes have the lowest boiling point, followed by aldehydes and ketones, alcohols, and carboxylic acids.

Passage Ethanol Is an Important Source of Energy That Is Frequently