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Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: Big-Daddy on April 30, 2014, 02:24:10 PM

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

A sample of 2-bromooctane gives [α]²⁵_D = +30.0; it is then reacted with NaOH to form 2-octanol. This 2-octanol then gives [α]²⁵_D = -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]²⁵_D = +6.0 for the 2-octanol.
Title: Re: Practice problem 6
Post by: Big-Daddy 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.
Title: Re: Practice problem 6
Post by: momoho on May 05, 2014, 12:41:39 PM: 87.46 percent for a?
Title: Re: Practice problem 6
Post by: Big-Daddy on May 05, 2014, 05:33:02 PM: Quote from: momoho on May 05, 2014, 12:41:39 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.
Title: Re: Practice problem 6
Post by: momoho 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?
Title: Re: Practice problem 6
Post by: momoho on May 06, 2014, 01:06:33 PM: I'm sorry...i made a ridiculous mistake...my answer is 84.953 %
Title: Re: Practice problem 6
Post by: Big-Daddy on May 06, 2014, 03:24:49 PM: Quote from: momoho on May 06, 2014, 01:06:33 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.
Title: Re: Practice problem 6
Post by: momoho on May 13, 2014, 04:15:06 PM: Why didn't anyone ask another question?
I mean practice problem 7!!!
Title: Re: Practice problem 6
Post by: Rutherford on May 14, 2014, 02:08:01 AM: That should be you, because you gave the answer. Also the problem isn't solved completely.
Title: Re: Practice problem 6
Post by: momoho 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!!!
Title: Re: Practice problem 6
Post by: Borek on May 14, 2014, 10:11:46 AM: Quote from: momoho on May 14, 2014, 09:49:13 AM
i don't want to give a problem because i don't have any problems!!!

That's your problem.
Title: Re: Practice problem 6
Post by: Big-Daddy on May 14, 2014, 10:16:09 AM: Quote from: momoho 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!!!

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.