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Practice Set 3
Divya_4978
#1 Posted : Wednesday, June 10, 2020 9:34:35 PM
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Hello,

I have a couple of questions in some of the passages in the practice set 3.

I do not understand the method that was used to find the answer to passage 2 question 7.

I also do not understand passage 3 question 18 as I find it hard to identify the chiral centers as well as notice that there is an internal plane of symmetry, making it a meso compound.

In passage 3, I also do not understand for question 19, what molecule the question is referring to in order to find the shape of it. I do not know which compound is the Diels Aldur Adduct.



Thank you very much.

INSTR_Katerina_102
#2 Posted : Wednesday, June 10, 2020 10:24:22 PM
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Hello,

Could you be more specific as to where these questions are located and what they entail?

I have tried to find them in the Chemistry practice question packs but the content you have stated have not matched up to the questions numbers I have found.

Thank you for your patience!
Divya_4978
#3 Posted : Thursday, June 11, 2020 12:15:25 AM
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Hi,

Sorry it is located in the Prep 101 portal practice sets. The chemistry practice set for lecture 3


Thank you
INSTR_Katerina_102
#4 Posted : Thursday, June 11, 2020 12:58:46 AM
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Hi,

Thanks for the clarification.

For Passage 2 Q7, we invoke the concepts of crystallization for purification.

1. You would identify that you want to purify amphetamine, which has 1 chiral centre (the C with the N attached). Amphetamine is in a mix of 50/50 R and S, which we want to separate.

2. To purify amphetamine, we would react it with an enantiomerically pure material. In this passage we have used tartaric acid, which has 2 chiral centres.

3. When we form the two salts with tartaric acid (which has stereocentres RS), we will form two diastereomeric salts, which I will represent by their stereocentres

Salt 1: R(RS) and Salt 2: S(RS), where the bold comes from amphetamine and the (RS) comes from tartaric acid.

If we look at these salts, they have 3 stereocentres (eliminating A and B), and only one chiral centre that is opposite in configuration, coming from amphetamine (which eliminates C). This leaves us with D as the best answer.

For Passage 3 Q18

This is a tricky question. I would practice more to improve your assignment of chiral centres and seeing meso planes, but this is generally quite a tough question. What you can also do to help is eliminate answers - if you knew that you could eliminate C and D based on the fact that they are factually incorrect, you could improve your percent of guessing right to 50%, which is greater than other test takers might get on this question.

Unfortunately, there is not much else one can do but practice to identify meso compounds, but I keep in mind that this is a hard question - consider it a victory if you even eliminated to A and B.

Passage 3 Q 19

The Diels Alder Adduct is the product produced in the Diels Alder reaction - but I think this should be revised because it is not clear which adduct, or which carbon and you would normally be given that information in the passage. Thank you for pointing this out, we will fix this when we can.
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