today we're going to look at a few Grignard reactions so you probably went
over this in lecture in your class or you watched my tutorial on Grignard
reactions and so very simply probably on a test you have to be able to draw
the products of some Grignard reaction so I've drawn three right here
here's a substrate and a Grignard reagent another substrate and another Grignard
reagent and then here we have an ester substrate and an excess of a
Grignard reagent, that should give you a hint of what to do there so go ahead and
give these Grignard reactions a shot
ok so we're doing Grignard reactions the key thing to remember is that it is
always the carbon that is attached to the magnesium and the carbonyl carbon on
the substrate that are going to generate a new carbon-carbon bond. this is because
the insertion of the magnesium atom into the carbon-bromine bond
reverses that polarity, we're going to have a partially negatively charged
carbon atom so that is a source of nucleophilic carbon and then once again
we know that a carbonyl looks like that, we've got a partially positive
carbon, carbon is partially positive so any reaction in chemistry is
always electron excess, electron deficiency, so that's how Grignard
reactions work. so in this case we know that this carbon is gonna go ahead and
attack there, pop that up, a little bit of acidic work-up and we'll get the
alcohol. so this is the new bond that we have formed right there
ok these two carbons came from the Grignard reagent, and these four
were already there. so let's number them 1, 2, 3, 4 and then this is
gonna be 5 and 6. 1, 2, 3, 4, 5 and 6. so 5 from the Grignard reagent, the one that was
nucleophilic, and 4, the carbonyl carbon now have that new bond between them so
remember that is always going to be the case for Grignard reactions. now once
again right here
let's number these 1, 2, 3, 4 and then we've got 5, 6, 7, 8, 9 it is going to be once again
this is the carbon that is connected to magnesium, this is the carbonyl carbon
you're going to have that attack here
pop that up, it'll get protonated upon work-up and we've got our alcohol so
here we need to make that new bond, we need to show that new bond but then a
very common error is to create the correct connectivity but then have
something weird happened with the other carbons. remember that the only thing
that has happened is a new bond between this carbon and this carbon
everything else that is there simply remains where it is, so that's why it's
sometimes good to number our carbons like that, we know that we've made a bond
between 2 and 5 so there's five or so there's 2 and here's 5 and then
let's number around the ring like we had 6, 7, 8, 9 and then connected to carbon 2
on this substrate was carbon 1, the methyl so that's still there nothing happened at
that, and then 3 and 4 right there, those are still there nothing happened
to those, so if you're having a difficult time visualizing the
transformation itself usually you can number the carbons and then if you just
say to yourself, 5 and 2, there's a new bond there and then everything else that
those were connected to they're still connected to, so 2, there's 1, 3 and
4, etc. so there's our product for that one, now this one's a little bit different
because we have an ester substrate. if you recall from my Grignard lecture we know
that an ester substrate in the presence of excess Grignard reagent can undergo
two consecutive Grignard reactions because what happens is, so this is, all
we have is this methyl here so that is going to attack that can go up there and
then we get the oxygen and but then instead of protonating this can
actually come back down and kick off the alkoxy group so we have added our our
Grigarnd, we have added our
are methyl Grignard reagent right there that's the new methyl and we've
kicked off an alkoxy but in kicking off the alkoxy group we regenerated the
carbonyl and we know that any ketone or aldehyde is susceptible to attack from a
Grignard reagent
excess Grignard reagent at least two equivalents that means that we can do
this again, another Grignard reagent that's swimming around can find that ketone
and now all of a sudden we have added another methyl Grignard reagent, and so
what we can see here is by kicking off the alkoxy group we regenerated the
carbonyl, there's one equivalent of our Grigarnd reagent and then
we can go ahead and do it a second time because that was regenerated and we've
got another methyl group attached onto there
these are the products of some of these Grignard reactions
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