easy organic problem please help !

[shiu1]

Provide a structure for the dibromo-product expected from the reaction of 1,2,3-

trimethylcyclohexene with Br2, and use this product to describe the difference between

the terms "absolute stereochemistry" and "relative stereochemistry".

[spin-1/2-nuclei]

Hi... I'll do my best to take a stab at this question.

the Br2 will add trans across the double bond. So you will get a total of 4 products if your stereocenter at carbon 3 is racemic. You will get 2 products if it is not.

 

The 2^n rule applies here, where n is the number of stereogenic centers... in this case because your reaction is facially selective you only get 4 possible products instead of 8.

 

You will get 2 enantiomers and 2 diasteriomers here. The "absolute stereochemistry" will be different for each of the 4 molecules, however the relative stereochemistry for the bromines of carbons 1 and 2 will be the same for each of the 4 molecules. This is because the reaction is facially selective therefore you get only trans addition across the double bond. If the addition of Br2 was not facially selective then you would not have any relative stereochemistry.

 

hopefully this was useful.

 

edit: I should clarify that the relative stereochemistry is the same for all of the 4 possible products in the sense that we can know for sure that the bromines in all 4 products on carbons 1 and 2 will be trans to each other. What we can't know is if they added from the top face or the bottom face - therefore you get 2 products from addition to the top and 2 products from addition to the bottom - thus always trans to each other but we cannot know if the center formed is R or S... absolute stereochemistry is knowing that carbon 1 is S carbon 2 is R carbon 3 is S and so forth. Typically you must have a chiral reagent to introduce absolute stereochemistry into the product or you must have a way to separate each enantiomer and disasteriomer from each other.

 

I should also add that because the center at carbon 3 is undefined - this is why you will get 2 products from addition to the top face and 2 products from addition to the bottom face. This is probably called something like Re face and Si face in your notes.. but if carbon 3 were defined you would just have 2 products 1 from the Re side and one from the Si side.

 

So a helpful hint is to always remember to divide the possible number of products by 2 for each stereocenter that is already fixed and by 2 if you have a controlled reaction (i.e.) facially selective etc...

Edited May 3, 2010 by spin-1/2-nuclei