WOW, I just almost every question wrong on this passage and I am not understanding why. (Btw, I normally do really well on Berkeley Review Orgo.)
I have copied and pasted the passage below as well as the questions and the issues I am having with some of the questions even after going over the answer explanations in the back of the book.
"A chemist intended to study the effect of periphery chirality on a nucleophilic substitution reaction. To 50 mL of methanol at 45 °C, the chemist added 0.10 moles of (lS,2R)-2-methylbromocyclopentane in the hopes of carrying out Reaction 1, which is shown below.
The reaction was carried out for one hour at 45°C in an aqueous solution buffered at a pH of 5.0. Excess methanol was removed from the product mixture by fractional distillation under reduced atmospheric pressure. The atmospheric pressure was reduced to lower the boiling point of methanol, in the hopes that additional reactivity would be minimized at a lower temperature. The optical rotation for the crude product mixture was found to be 0°, which is contrary to what the chemist had expected. The chemist had expected that the crude product mixture would exhibit optical activity, although the exact value would be different than that of the reactant. The chemist reevaluated the proposed reaction, Reaction
1, and decided that the temperature and the pH should be changed. Under the reaction conditions used, the proposed reaction proceeds by a mechanism that is susceptible to
rearrangement. The chemist also failed to consider other reactions that compete with nucleophilic substitution at elevated temperatures, such as elimination. Under different
conditions, the chemist found that optical activity could be retained."
36. The chemist attempted to carry out what type of reaction?
A. SnI
B. Sn2
C. E1
D. E2
^According to BR, the answer is SN1. In the passage I can see that he did carry out SN1 given the racemic mixture (as denoted in the passage that the product mixture was 0 degrees in optical rotation). But since that wasn't what he INTENDED wouldn't that have meant he was attempting to carry out SN2 reaction?
40. The small amount of substitution product isolated was found to have both the OCH3 group and the CH3 group both on the same carbon. This can best be explained in what way?
A. First an elimination reaction took place followed by a Markovnikov addition reaction.
B. First a Markovnikov addition reaction took place followed by an elimination reaction.
C. A hydride shift occurred.
D. A methyl shift occurred.
^The answer is a hydride shift occurred, but wouldn't it be a methyl shift to create a tertiary carbocation before the nucleophile attacks?
42. The following distribution of products can best be explained by which of the explanations?
A. The intermediate undergoes a hydride shift.
B. The methyl group on the carbon adjacent to the carbocation influences the attack of methanol.
C. The methyl group on the carbocation influences the attack of methanol.
D. The intermediate undergoes a methyl shift.
The answer is B, but again, I don't understand why it isn't D, with the intermediate undergoing a methyl shift.
Thank you!
I have copied and pasted the passage below as well as the questions and the issues I am having with some of the questions even after going over the answer explanations in the back of the book.
"A chemist intended to study the effect of periphery chirality on a nucleophilic substitution reaction. To 50 mL of methanol at 45 °C, the chemist added 0.10 moles of (lS,2R)-2-methylbromocyclopentane in the hopes of carrying out Reaction 1, which is shown below.
The reaction was carried out for one hour at 45°C in an aqueous solution buffered at a pH of 5.0. Excess methanol was removed from the product mixture by fractional distillation under reduced atmospheric pressure. The atmospheric pressure was reduced to lower the boiling point of methanol, in the hopes that additional reactivity would be minimized at a lower temperature. The optical rotation for the crude product mixture was found to be 0°, which is contrary to what the chemist had expected. The chemist had expected that the crude product mixture would exhibit optical activity, although the exact value would be different than that of the reactant. The chemist reevaluated the proposed reaction, Reaction
1, and decided that the temperature and the pH should be changed. Under the reaction conditions used, the proposed reaction proceeds by a mechanism that is susceptible to
rearrangement. The chemist also failed to consider other reactions that compete with nucleophilic substitution at elevated temperatures, such as elimination. Under different
conditions, the chemist found that optical activity could be retained."
36. The chemist attempted to carry out what type of reaction?
A. SnI
B. Sn2
C. E1
D. E2
^According to BR, the answer is SN1. In the passage I can see that he did carry out SN1 given the racemic mixture (as denoted in the passage that the product mixture was 0 degrees in optical rotation). But since that wasn't what he INTENDED wouldn't that have meant he was attempting to carry out SN2 reaction?
40. The small amount of substitution product isolated was found to have both the OCH3 group and the CH3 group both on the same carbon. This can best be explained in what way?
A. First an elimination reaction took place followed by a Markovnikov addition reaction.
B. First a Markovnikov addition reaction took place followed by an elimination reaction.
C. A hydride shift occurred.
D. A methyl shift occurred.
^The answer is a hydride shift occurred, but wouldn't it be a methyl shift to create a tertiary carbocation before the nucleophile attacks?
42. The following distribution of products can best be explained by which of the explanations?
A. The intermediate undergoes a hydride shift.
B. The methyl group on the carbon adjacent to the carbocation influences the attack of methanol.
C. The methyl group on the carbocation influences the attack of methanol.
D. The intermediate undergoes a methyl shift.
The answer is B, but again, I don't understand why it isn't D, with the intermediate undergoing a methyl shift.
Thank you!
