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.
-During the conversion of Compound 2 to megestrol, why was the secondary alcohol affected, whereas the tertiary alcohol was not?

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.
-During the conversion of Compound 2 to megestrol, why was the secondary alcohol affected, whereas the tertiary alcohol was not?

Quizplus · Accepted Answer

11ecba2d_126e_d29d_a3bf_9bd022cfc618_MD0008_00 The conversion of Compound 2 to megestrol is an oxidation reaction, with K₂Cr₂O₇ acting as the oxidizing agent. When carbon atoms are oxidized, they lose bonds to hydrogen and/or gain bonds to oxygen. Therefore, oxidizing agents convert primary alcohols to aldehydes and carboxylic acids, and secondary alcohols to ketones. However, tertiary carbon atoms have no bonds to hydrogen. Because no bonds to hydrogen can be lost, additional bonds to oxygen would result in a carbon atom with five bonds. Carbon can participate in only four bonds, so this is not possible. Therefore, tertiary alcohols cannot be oxidized by losing bonds to hydrogen or by gaining bonds to oxygen. (Choice B) The double bonds in Compound 2 have resonance forms, but the alcohol carbon has no lone electrons or pi bonds, which are required to participate in resonance structures. In addition, resonance does not significantly contribute to a molecule's ability to be oxidized. (Choice C) Tertiary alcohols are less acidic than secondary alcohols because tertiary alcohols are more sterically hindered, which makes its conjugate base less stable than that of a secondary alcohol. However, acidity does not affect a molecule's ability to be oxidized. (Choice D) Although secondary alcohols are better nucleophiles than tertiary alcohols, a compound's nucleophilicity is not related to its ability to be oxidized. Educational objective: Oxidation of carbon atoms involves the loss of bonds to hydrogen and is frequently accompanied by the formation of bonds to oxygen. Primary and secondary alcohols can be oxidized to carboxylic acids and ketones, respectively, but tertiary alcohols are in their highest possible oxidation state.

Passage Ethanol Is an Important Source of Energy That Is Frequently