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.
-In Scheme 1, conversion of Compound 1 to Compound 2 most likely begins with which of the following steps?

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.
-In Scheme 1, conversion of Compound 1 to Compound 2 most likely begins with which of the following steps?

Quizplus · Accepted Answer

11ecba2d_126c_6197_a3bf_7ddcd76eaf73_MD0008_00 Nucleophilic substitution reactions can occur through one of two mechanisms: S_N1 or S_N2. S_N1 reactions proceed in two steps and involve the formation of a carbocation intermediate. In these reactions, the leaving group leaves before the nucleophile attacks. S_N2 reactions occur in a single step in which the nucleophile attacks and the leaving group leaves simultaneously. Tertiary leaving groups are more likely to undergo S_N1 reactions than S_N2 reactions for two reasons. First, they are sterically hindered by the bulky alkyl groups surrounding them, and these groups impede nucleophilic attack. Second, tertiary carbon atoms can form relatively stable carbocations compared with secondary or primary carbon atoms. The positive charge associated with carbocations enhances their electrophilicity, helping them overcome the steric hindrance that otherwise inhibits nucleophilic attack. The conversion of Compound 1 to Compound 2 entails the removal of an iodide ion from a tertiary carbon and its replacement by a hydroxyl group. Because a tertiary carbon is involved in the reaction, it will most likely proceed by an S_N1 mechanism. Therefore, the first step will be the formation of a carbocation as iodide leaves. (Choice A) Nucleophilic attack is the first step in an S_N2 reaction but not in an S_N1 reaction. (Choice B) Deprotonation of water would enhance its nucleophilicity, but the first step of an S_N1 reaction is always the formation of a carbocation as the bond to a leaving group is broken. (Choice D) Halides such as iodide generally cannot form more than one bond at a time. Therefore, iodide would be unable to be protonated before it leaves the tertiary carbon. Educational objective: Nucleophilic substitution reactions can proceed by S_N1 or S_N2 mechanisms. Tertiary carbons are more likely to participate in S_N1 reactions due to steric hindrance and the ability to form stable carbocations.

Passage Ethanol Is an Important Source of Energy That Is Frequently