[THC]

When Cl2 is added to an alkene like CH2=CH2, the Cls are "anti" in CH2Cl-CH2Cl... why is this important? There's free rotation around the Cs, so the Cls are only sitting "anti" in the few seconds after the reaction.

[azmanam]

we say 'anti' only in the context of a 'net anti addition.'  as opposed to OsO₄ dihydroxylation, which is a 'net syn addition' - even though there will be free rotation about the C-C single bond after addition here, too.

with respect to the original double bond in the starting material, the two atoms of chlorine add from opposite faces of the pi system, thus the 'net anti addition.'

and what if the double bond were part of a cyclic system, how does that affect our analysis of 'net anti addition'?

additionally, it matters if there are two new stereocenters formed in the reaction.  the 'net anti addition' will have a real impact on the naming of the newly created stereocenters, regardless of the free rotation about the C-C single bond after addition.

good question, though.

[THC]

Thank you for your quick answer.

Suppose I have CBrCl=CBrCl (cis). I then add I2 to the alkene. Would it be possible to determine R/S for the C atoms? Or do I need more information?

[azmanam]

nope, you'd have enough info to name stereocenters.  What's your answer?

[THC]

Both are R, right?

[azmanam]

that's not what I get.  how are you prioritizing the 4 groups?

[THC]

I > Br > Cl > C

?

[azmanam]

ah, ah, ah.  I wasn't thinking about it right.

we happened to do it 2 different (but correct) ways to get both possible enantiomers of the product.  S,S or R,R are both right answers b/c you get a racemic mixture.

at first I was thinking it'd give a meso cpd, but that's not what happens here.

so S,S is an acceptable (but not complete) answer.

[THC]

See attached image.

EDIT: 14 seconds too late I get it now. If it's S,S, Br and Cl switch positions compared to my drawing. Thanks

[azmanam]

Br and Cl switch positions compared to my drawing.

on paper that's true.  do you know what "switch positions" means in real chemical mechanisms, tho?

[THC]

I actually wondered about that. Why is it possible to get both R,R and S,S? I thought that there was no isomers of cis-1,2-dibromo-1,2-dichloroethane, but there is according to the two different products - R,R and S,S. So no, I'm not sure what that means in chemical mechanisms... it's not a rearrangement, right?

[azmanam]

What's the first step of your mechanism?  Explain your arrow pushing in the first mechanistic step in words as detailed as you can.

[THC]

Hmm, I'm not sure, but see attached image.

[azmanam]

eh, watch that middle intermediate.  you're wedges/hashes to the iodine atom are incorrect.  also, make sure the atom with the actual formal positive charge has the plus sign.

Can you describe this mechanism in words?  pay attention to the adjectives you use, and ask your self why the mechanism proceeds in the manner you describe.