Br2 + alkene

[joonkimdds]

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which one do u guys think is the right answer?
I know that it is anti-addition but I can't tell which is anti when they put it in this fischer projections.

 

[iwannabadentist]

the correct answer would be E. With anti addition, you will always get enantiomers. A and D are enantiomers of each other. B is clearly wrong because Br2 attaches to 2 different Cs. Choice C is wrong because both of the chiral carbons are 'S' whereas with Choice A and D, they are R and S. At a first glance I would pick C just because the Br's "appear to be" anti, but if you compare the R and S configurations, they are both S, which would not happen with anti addition.

 

[PooyaH]

E!

 

[iwannabadentist]

It'd be C. One would be on the left and the other would be on the right!

haha you fell for the trick! Its E, am I right?

 

[PooyaH]

haha you fell for the trick! Its E, am I right?

haha yeah, as soon as I clicked on submit, I was like wait a sec! LOL I wasn't even thinking about the Fischer Projection lol

 

[joonkimdds]

the correct answer would be E. With anti addition, you will always get enantiomers. A and D are enantiomers of each other. B is clearly wrong because Br2 attaches to 2 different Cs. Choice C is wrong because both of the chiral carbons are 'S' whereas with Choice A and D, they are R and S. At a first glance I would pick C just because the Br's "appear to be" anti, but if you compare the R and S configurations, they are both S, which would not happen with anti addition.

U r right, but if I change this fischer projection into newman projection, both Br are in eclipsed position so it looks like there was a syn addition. Maybe addition happen only when they are in staggered position?

 

[iwannabadentist]

y would u change it to newman? thats so much extra work and isnt even definite. just determine if its R,S and if its S,R on the 2nd one - you have your answer.

 

[ethanyoo726]

the way i thought about it was....Br2 adds across the alkene so that the product has one of the Br coming out the plane (wedge) and the other Br going into the plane (dashed)....so the carbons must have different absolute configurations. One should be R and one should be S and it doesn't matter which carbon has which configuration just as long as the other carbon has the opposite configuration.

So going through answer choices, the only ones that meet that criteria are A & D, so E is your answer.

 

[joonkimdds]

y would u change it to newman? thats so much extra work and isnt even definite. just determine if its R,S and if its S,R on the 2nd one - you have your answer.

so if one chiral is R and the other chiral in the same structure is S then they are anti to each other?

 

[PooyaH]

so if one chiral is R and the other chiral in the same structure is S then they are anti to each other?

For adjacent carbon atoms yes!

 

[iwannabadentist]

of course. because before the reaction, it was an alkene and both of the carbons were sp2 hybridized and achiral. the only new addition is the Br, so as long as they are different (R and S), you know its anti. If it helps you, do this problem using the wedge and dotted line method, and you will see that if its anti, both of the chiral carbons have to have a different conformation.