00:05
Okay, so we have this alkyne and we are reacting it with br2, so it's addition of bromine across a double bond.
00:22
And you do have the correct product shown to an extent, right? but what you need to show is the stereochemistry.
00:33
Right.
00:34
So recall that this reaction proceeds through a brimonium ion.
00:42
Intermediate.
00:43
What do i mean by that? it means this double bond swings open to attack bromine at the same time that this bromine attacks the other carbon of the double bond while this bromine leaves, and that what we end up with is bromine being attached to two carbons, but attached at the same face of those carbons, right? so bromine is being bound to both of those.
01:16
It's got a positive charge.
01:17
Bromene with two bonds is always going to have a positive charge.
01:21
And we still got br minus, right? but the idea is that this is sort of like, it's almost a bit like an epoxide, right? they're on the same.
01:33
The bromine has to be on the same face.
01:35
Now, when this happens, there's two ways that this can happen, right? so it can add both to the top face, right? both bonds to bromine are on the top face, or we can end up with a situation where both bonds to bromine are on the bottom face.
01:53
Right so it's kind of like was the bromine above the double bond or was it beneath the double bond okay now the final step after this is that this br minus will do backside attack on one of these carbons similarly this bromine will do backside attack on one of these carbons which in this case means coming from the top okay um and so we're going to get a mixture of products because note theoretically, this bromine can attack here or it can attack here.
02:43
Now, just like when we consider the opening of epochsides, we may note, hey, this is a tertiary carbon, and this is a secondary carbon.
02:53
Is there a preference in terms of attack for bromine? and in fact, there is.
02:59
Bromene is more likely to attack the more highly substituted carbon...