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Topic: Practice problem 6  (Read 9382 times)

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Offline Big-Daddy

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Practice problem 6
« on: April 30, 2014, 02:24:10 PM »
The specific rotation [α] for pure (R)-2-bromooctane is [α]25D = +36.0, while the specific rotation [α] for pure (S)-2-octanol is[α]25D = -10.3.

A sample of 2-bromooctane gives [α]25D = +30.0; it is then reacted with NaOH to form 2-octanol. This 2-octanol then gives [α]25D = -6.0.

a.   Calculate the fraction of stereocenters which have inverted during the course of the reaction.
b.   Consider the symmetry of the expression obtained, by calculating the fraction of stereocenters which would have been inverted if we had observed instead [α]25D = +6.0 for the 2-octanol.

Offline Big-Daddy

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Re: Practice problem 6
« Reply #1 on: May 03, 2014, 12:02:41 PM »
No correct answers yet so I'll post a hint which hopefully will not make things too obvious:

We must suppose that inversion of centres is symmetrical and random in this process - i.e. whatever fraction of (R)-2-bromooctane was inverted to make (S)-2-octanol is the same as the fraction of (S)-2-bromooctane inverted to make (R)-2-octanol.

Offline momoho

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Re: Practice problem 6
« Reply #2 on: May 05, 2014, 12:41:39 PM »
87.46 percent for a?

Offline Big-Daddy

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Re: Practice problem 6
« Reply #3 on: May 05, 2014, 05:33:02 PM »
87.46 percent for a?

Close (within a few percent) but no dice unfortunately  :(

In a couple of days I will post another hint.

Offline momoho

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Re: Practice problem 6
« Reply #4 on: May 06, 2014, 12:44:38 AM »
I didn't use the hint in my first attempt.
Now i use it and i think it's 69.91 percent for a.
Correct?

Offline momoho

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Re: Practice problem 6
« Reply #5 on: May 06, 2014, 01:06:33 PM »
I'm sorry...i made a ridiculous mistake...my answer is 84.953 %

Offline Big-Daddy

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Re: Practice problem 6
« Reply #6 on: May 06, 2014, 03:24:49 PM »
I'm sorry...i made a ridiculous mistake...my answer is 84.953 %

Correct! The exact value is 175/206 = 84.9515%. Do you want to explain how you got there?

Any ideas for the second part? It's basically a repeat of the calculation, but using the second value, and note down what strikes you about the situation.

Offline momoho

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Re: Practice problem 6
« Reply #7 on: May 13, 2014, 04:15:06 PM »
Why didn't anyone ask another question?
I mean practice problem 7!!!

Offline Rutherford

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Re: Practice problem 6
« Reply #8 on: May 14, 2014, 02:08:01 AM »
That should be you, because you gave the answer. Also the problem isn't solved completely.

Offline momoho

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Re: Practice problem 6
« Reply #9 on: May 14, 2014, 09:49:13 AM »
The second part is almost same with the first and i don't want to give a problem because i don't have any problems!!!

Offline Borek

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Re: Practice problem 6
« Reply #10 on: May 14, 2014, 10:11:46 AM »
i don't want to give a problem because i don't have any problems!!!

That's your problem.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Big-Daddy

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Re: Practice problem 6
« Reply #11 on: May 14, 2014, 10:16:09 AM »
The second part is almost same with the first and i don't want to give a problem because i don't have any problems!!!

why don't you explain your working for the first part, then I'm sure someone else will step up and solve the second and then maybe that person can offer Problem 7.

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