Anti and Syn Stereoactivity

[PatelDentist]

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I am having a hard time understanding when to use the Anti addition and Syn addition? For example, Br2/CCl4 reagent uses Anti addition but what is the reason behind the adding the 2 Br in an Anti way....Why is it not syn addition? I keep getting things wrong on my practice problems because I don't know how to determine them.

THANK U!

Could it be because its a SN2 reaction and usually SN2 reactions add inversely?

 

[BRZ24]

I am having a hard time understanding when to use the Anti addition and Syn addition? For example, Br2/CCl4 reagent uses Anti addition but what is the reason behind the adding the 2 Br in an Anti way....Why is it not syn addition? I keep getting things wrong on my practice problems because I don't know how to determine them.

THANK U!

when you are adding Br2/CCl4 there is a reason behind adding it anti way because there might be a substitutent on the carbon. A lot of questions like to trick you by putting a substituent and NOT the things you are actually adding (they might be in a dashed postion or wedge)

You need to understand the mechanism in order to see why it is NOT syn addition. I believe chad talked about it in the video. But for the purpose of this test, you don't need to know a lot of mechanisms.

There is a worksheet in chads videos and he summarized all the reactions nicely and showed which rxns is syn, anti, etc...

 

[412smb412]

I am having a hard time understanding when to use the Anti addition and Syn addition? For example, Br2/CCl4 reagent uses Anti addition but what is the reason behind the adding the 2 Br in an Anti way....Why is it not syn addition? I keep getting things wrong on my practice problems because I don't know how to determine them.

THANK U!

Could it be because its a SN2 reaction and usually SN2 reactions add inversely?

there is a bridge formation of the alkene reacting with the Br+, then there is an nucleophilic attack by the Br-... but u will get a racemic mixture if there are chiral centres regardless of what substituents are also bound to the carbon. if i were you... CHAD!!!!!!!!!!!

 

[aqz]

It's something like this:

....Br+
../.....\
R.......R

So then if you imagine a nucleophilic attack with the second Br, it'll invert the stereochem and be anti.

But honestly...don't worry too much about the reasons...

Only things that are anti, where X = halogen:

X2 addition

X2/H2O addition or X2/R-OH addition.

While I'm at it, the only anti-markovnikov ones are:

BH3/NaOH, H2O2

and HBr/ROOR......but NOT HCl/ROOR.

Those are the ONLY exceptions, so I would just memorize them instead. Everything else is Syn and markovnikov.

Correct me if I'm wrong.

 

[412smb412]

It's something like this:

....Br+
../.....\
R.......R

So then if you imagine a nucleophilic attack with the second Br, it'll invert the stereochem and be anti.

But honestly...don't worry too much about the reasons...

Only things that are anti, where X = halogen:

X2 addition

X2/H2O addition or X2/R-OH addition.

While I'm at it, the only anti-markovnikov ones are:

BH3/NaOH, H2O2

and HBr/ROOR......but NOT HCl/ROOR.

Those are the ONLY exceptions, so I would just memorize them instead. Everything else is Syn and markovnikov.

Correct me if I'm wrong.

and oxymercuration! -> markovnikov but not rearrangement

lol aqz i swear u come up to every thread i post on and think to urself: how can i make this guy look like an idiot?.... lol joking, appreciate u correcting my errors, made a couple bad ones b4!!!!!

 

[aqz]

and oxymercuration! -> markovnikov but not rearrangement

lol aqz i swear u come up to every thread i post on and think to urself: how can i make this guy look like an idiot?.... lol joking, appreciate u correcting my errors, made a couple bad ones b4!!!!!

But....that's exactly what I try to do 🙁

just helping out as much as I can. I won't be on so frequently much longer, going back on campus tomorrow and getting my life back

 

[PatelDentist]

Thank you that was very helpful!