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Offline lolgiro

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ACS Study Guide Problems Help
« on: December 28, 2013, 07:09:51 AM »
Hi guys,

I'm getting ready to take an ACS Organic Exam and I'm going over their official study guide. The Stereochemistry sections are giving me a lot of difficulty, particularly in determining stereochemical relationships. I'm actually using my model kit for the first time.

I guess first of all, is it worth using the model kit on the exam or will I run out of time?

Second, I'm having trouble with determining if molecules are enantiomers, identical, etc. Does anyone have any resources that can help with building up this skill or practicing?


Lastly, I'm attaching a photo of the problems that gave me the most trouble. I worked through them and have what I think might be the reasoning behind the answer, but was wondering if someone could tell me if my reasoning is on the right path or way off?



For #13, is II the best conformation because of the Hs attached to the methyl? Would that mean that CH3 is a bigger molecule/causes more steric strain than OH? Because my first inclination was to say that III is the most stable conformation.

for #14, I'm stuck entirely. My first choice was B, but apparently A is correct. I thought steroisomers = 2^N where N is the # of chiral centers?



For this problem, The way I solved this was by assigning configuration to the chiral center, then going through each molecule in the answers and assigning those too until I found the opposite, meaning that its the enantiomer. I tried redrawing the molecule at first and found it too time consuming and confusing, and then tried building the molecule with the same results. Is it just easier to assign configuration to the chiral center in this case, or should I be working with the model or redrawing for problems like this?



For this problem,  I was completely stuck. Finally I just build the model of the anti conformation of the chair on the left, rotated it on the vertical axis, and found it matched the chair on the right. Was this the right/most efficient way to solve the problem? Redrawing chairs just became confusing for me and was too time consuming.


Any additional advice for the exam would be great! I'm pretty freaked out and need to do well to keep my grade in the class..

Offline Vidya

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Re: ACS Study Guide Problems Help
« Reply #1 on: January 02, 2014, 08:27:04 PM »
For #13, is II the best conformation because of the Hs attached to the methyl? Would that mean that CH3 is a bigger molecule/causes more steric strain than OH? Because my first inclination was to say that III is the most stable conformation.

II is the best conformation because methyl bulky group is between two least hindered groups (H's) and OH groups are forming H -bonding.

for #14, I'm stuck entirely. My first choice was B, but apparently A is correct. I thought steroisomers = 2^N where N is the # of chiral centers?

R,R and S,S is one pair of enantiomers and the other one is R,S and S,R .But R,S and S,R are identical compounds so counted as one only.Identical compounds have identical IUPAC names in all respects.

For this problem, The way I solved this was by assigning configuration to the chiral center, then going through each molecule in the answers and assigning those too until I found the opposite, meaning that its the enantiomer. I tried redrawing the molecule at first and found it too time consuming and confusing, and then tried building the molecule with the same results. Is it just easier to assign configuration to the chiral center in this case, or should I be working with the model or redrawing for problems like this?


For this problem I advise you to convert your newman projection into fischer projection and then draw the enantiomer.Then compare this enantiomer with the given answers

For the last problem -check the positions of the OH groups -they are 1,2 in both the structures so constitutional isomers are ruled out.Now check ,in both the cases ,one OH is pointing up and one OH is pointing down .So anti in both the cases and hence they are identical as they are not even stereoisomers.

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