ok guys for today's practice problem we're gonna cause benzene to undergo a
series of three reactions. these are all electrophilic aromatic substitution
reactions so if you're unfamiliar with those go and check out my lecture on
electrophilic aromatic substitution as well as the directing groups lecture
because that is going to be relevant here and we're gonna generate product A
undergo another transformation, product B, undergo another transformation
products C. so what I want you to do is go ahead and draw possible structures for A, B, and C.
ok so I've gone ahead and drawn the structures for A, B, and C, or at least one
possible route for you here, so let's take a look at one reaction at a time. first of
all we're starting out with benzene and these these conditions, we're talking
about a Friedel Crafts alkylation, in a Friedel Crafts alkylation we take this
alkyl fragment and we just pop it onto the benzene ring. now in this
first reaction we have an unsubstituted benzene ring so it is completely arbitrary
which carbon you decide to put this isopropyl group on
arbitrarily decided to put on the top carbon but if we rotate this is it can
anywhere you put that it's going to be the same molecule, so but after that
there begins to become a discrepancy in the legitimacy of it because here
this is going to be a nitration, these are the conditions for nitration of the
benzene ring but this group right here
alkyl groups are ortho/para directors so that means that this isopropyl group
must direct the incoming nitro group to the ortho or para position. once again to
reiterate in terms of ortho/meta/para directing groups, it is the group that is
already on the ring that directs the incoming electrophile to its position
not the other way around, in other words it does not matter what that the
incoming group is a nitro group and that nitro groups are meta directors because
it is the alkyl group on the ring already that is doing the directing
and is informing where the incoming group has to go so this is
gonna be an ortho/para director and so I've arbitrarily chosen to put the nitro
group at the para position although not entirely arbitrarily because there is
some issue in terms of sterics so I think that the para position is slightly more
energetically favorable although there are two ortho positions but at any
rate both of these are correct so I've chosen to substitute para right there
and then once I have this disubstituted benzene the third step here
we've got a bromination, these are the conditions for bromination of the
benzene ring and now what we have here, this is ortho/para and this is a
meta director, and so what that means is that the ortho/para positions here
well this is ortho, this is ortho, and then this is para which is already
occupied and then in terms of being a meta director, the nitro group is also
directing the incoming electrophile to those positions so fortunately in
this case the two groups agree about where the incoming electrophile has to
go and in addition both of these these are chemically equivalent positions
because if you put it here
flip it over it's over there so they both result in the same product and so
this is going to be one possible product for this sequence of transformations
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