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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: G O D I V A on November 13, 2009, 08:45:35 PM

Title: Conformational Isomers question
Post by: G O D I V A on November 13, 2009, 08:45:35 PM
Which of the following are conformational isomers? (number them 1,2,3,4 starting from the left)

Is it 3 and 4?

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg121.imageshack.us%2Fimg121%2F5564%2F81111276.jpg&hash=3e98cbb2d5c8b2e742a9db6435dbc332257c77ac)
Title: Re: Conformational Isomers question
Post by: Suprachiasmatic Nucleus on November 13, 2009, 08:52:33 PM
Quote from: G O D I V A on November 13, 2009, 08:45:35 PM
Which of the following are conformational isomers? (number them 1,2,3,4 starting from the left)

Is it 3 and 4?

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg121.imageshack.us%2Fimg121%2F5564%2F81111276.jpg&hash=3e98cbb2d5c8b2e742a9db6435dbc332257c77ac)

Hi, I believe it is #2 and #4, by rotating the sigma bonds.

#2 is: R,R.
#4 is: R,R.

#1 is: S,R
#3 is: S,S; therefore it SHOULD be 2,4 if I am correct.
Title: Re: Conformational Isomers question
Post by: G O D I V A on November 13, 2009, 09:34:56 PM
Quote from: Suprachiasmatic Nucleus on November 13, 2009, 08:52:33 PM
Quote from: G O D I V A on November 13, 2009, 08:45:35 PM
Which of the following are conformational isomers? (number them 1,2,3,4 starting from the left)

Is it 3 and 4?

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg121.imageshack.us%2Fimg121%2F5564%2F81111276.jpg&hash=3e98cbb2d5c8b2e742a9db6435dbc332257c77ac)

Hi, I believe it is #2 and #4, by rotating the sigma bonds.

#2 is: R,R.
#4 is: R,R.

#1 is: S,R
#3 is: S,S; therefore it SHOULD be 2,4 if I am correct.

I get something different for 2 and 4

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg687.imageshack.us%2Fimg687%2F163%2F92904549.jpg&hash=c5d1ea547ebde1546c16c010f9da27990e50a057)

For 2 I get S, R not R, R

and for 4, how do you know which way to go from the bromine?
Title: Re: Conformational Isomers question
Post by: Mitsunobo on November 13, 2009, 10:18:46 PM
When assigning R and S, it would be easier if you position the hydrogen at the back, then assign priorities to the remaining substituents. I think you will agree with the previous poster if you do that.
Title: Re: Conformational Isomers question
Post by: G O D I V A on November 14, 2009, 02:50:34 AM
Quote from: Suprachiasmatic Nucleus on November 13, 2009, 08:52:33 PM


Hi, I believe it is #2 and #4, by rotating the sigma bonds.



Quote from: Mitsunobo on November 13, 2009, 10:18:46 PM
When assigning R and S, it would be easier if you position the hydrogen at the back, then assign priorities to the remaining substituents. I think you will agree with the previous poster if you do that.


Alright thanks guys.  I have a related question to this.  I'm horrible at this 3d rotation thing in my head.

If you reacted (E)-2-butene with Br2 which of those (1-4) would you get?  I was thinking 3 only because it would be anti addition right?  So the bromine have to be like 180 apart so that excludes 1,2 and 4