hey it's professor Dave let's talk about Clemmensen reduction.
So Clemmensen reduction is a pretty interesting reaction let's just go over the general
overview we're gonna be starting with a ketone we've got a ketone substrate and
what we're reacting this with is amalgamated zinc so this is zinc
complexed with mercury in hydrochloric acid so these are the conditions that we
ideally want to commit to memory we want to look at this and we want to know okay
we're doing Clemmensen reduction and all we need to know is that Clemmensen
reduction will allow us to go from the ketone all the way to the alkane. in
terms of the mechanism there is actually some dispute there are two hypotheses
one in which the protonated carbonyl actually interacts directly with the
zinc, another which happens more in heterogeneous fashion at the surface of
the zinc metal so we don't really need to concern ourselves with the mechanism
too much all we need to know is that we're going from the ketone to the
alkane so essentially the carbonyl we just we lose it we get rid of that
carbonyl and we're just left with alkane. so that is what the Clemmensen reduction
does. now just to go a little bit further I want to show you one very important
application of Clemmensen reduction so let's say we want to do some
friedel-crafts chemistry, let's say we've got benzene and we want to do a
friedel-crafts alkylation right let's say that we want to attach this three
carbon alkyl group right there we're gonna use our we're gonna use our our
alkyl chloride and and AlCl3 catalyst and we're thinking this is gonna be
great I'm gonna put on my three carbon linear fragment right there and this is
gonna work very well. but the thing is we have to recall in terms of the mechanism
of friedel-crafts alkylation we know that there's a carbocation intermediate
and so the problem is this once this chlorine atom interacts with this
aluminum, and you can go back and check out that tutorial if you want to review
that mechanism, once this chlorine interacts with that aluminum and then
that carbon chlorine bond breaks we're gonna be left with a carbocation and if
that carbocation can rearrange, it will do so and so we're
gonna go from there we're gonna get a hydride shift right
and so we're actually gonna have this and so this is gonna be what would
interact with benzene and therefore we would not get this straight chain alkyl
group because this would be reacting with this isopropyl cation instead and
so we would not get this we would get an isopropyl group on there and therefore
this strategy is actually not going to work. we can't do this, we can't go from
here to here with friedel-crafts alkylation. but that's where this other
technique comes in. we know that we can get a similar situation with
friedel-crafts acylation so remember we talked about friedel-crafts acylation
and so we can have this 3 carbon acyl chloride, so 1 2 3 carbons right there
and it's again it's a very similar mechanism we've got aluminum trichloride
catalyst and we know that this is going to allow for us to attach this group
here on to the molecule so now we've got that and we do have 3 carbons there
where we want them and now all we have to do is perform Clemmensen reduction
like we just learned. and we know that Clemmensen reduction is going to allow
us to get rid of that carbonyl right it is a reduction and we're just going to
lose that carbonyl so we're going straight to alkane and so via a two-step
process we are able to get what we originally intended ok so just to
summarize Clemmensen reduction we're using amalgamated zinc so zinc complex
with mercury in hydrochloric acid that is allowing us to go from the ketone to
the alkane and one very important application of this as we said if we
want to get a straight chain alkyl group onto a benzene ring whereas we might
think we could do friedel-crafts alkylation in fact it will not work but
if we want to get that straight chain on there all we have to do is a two-step
sequence we can do friedel-crafts acylation and then simply Clemmensen
reduction to get rid of that carbonyl and get to what we originally intended
so that's a little bit about Clemmensen reduction.