so for this problem I have drawn three different aromatic compounds and
they're all gonna undergo bromination and so the fact that we have some
different functional groups on that benzene ring, we're gonna get some
different products, we need to be able to number one: draw the resulting products
and then number two: rank these reactions from slowest to fastest, so if
this looks unfamiliar we're gonna go ahead and take a look at electrophilic
aromatic substitution as well as the tutorial that I have on directing group
abilities to be able to draw these products and rank them accordingly
I've gone ahead and drawn the correct products for these brominations, now the
key to drawing these products is understanding the directing capabilities
of these functional groups, so a chloro group or any halogen is an ortho/para
director so we're gonna get a combination of ortho and para products
so this is the ortho product, this is the para product, and so same over
here with this phenol, the hydroxyl group is also going to be an ortho/para
director so here is our ortho substituted product, here is our para substituted
product, now these are both ortho/para directors even though the
halogens are electron withdrawing, typically electron-withdrawing groups
are meta directors but remember that halogens is that one exception, halogens
are still ortho/para directors and hydroxyl groups are ortho/para
directors because they are electron donating, and then lastly the nitro group
the nitro group because it is strongly electron-withdrawing, that is going to be
a meta director so we do get the meta substituted product. so number one we do
want to talk about the regioselectivity of these in order to be able to
draw the the dominant products but then we also want to talk about kinetics
we wanna talk about the speed of the reaction to see which ones are going to go the
fastest. well we know that the intermediate in an electrophilic aromatic
substitution reaction
is an arenium ion intermediate, it is a positively charged intermediate, so when
we have electron withdrawing groups that are already on the ring they're
going to further destabilize the arenium ion intermediate and so they're gonna
make the reaction proceeded more slowly. now here that nitro group is the
strongest deactivator, it's the strongest electron-withdrawing group so
it is gonna destabilize the intermediate the most and therefore it will be the
slowest reaction. now the halogens over here
halogens are also withdrawing, not by resonance the way that a nitro groups do
but actually by induction, just by the the electronegativity of that carbon
halogen bond, so those are also destabilizing, they're withdrawing and
they're gonna make the reaction proceed more slowly than if it was just
benzene, than if there was not a halogen there so that's going to be the second
slowest. however we have an electron-donating group like a
hydroxyl group or an amino group that are putting lone pairs into the ring by
resonance, that is stabilizing the positively charged arenium ion intermediate
they're putting excess electron density into the ring to stabilize that positive
charge so electron-donating groups actually speed up the reaction
compared to if it was just benzene. so this one is actually going to go faster
than it was benzene by itself. these two go a little bit slower than if it was
benzene by itself, and so the correct order is gonna be 1, 2, 3 and then again these are
the correct product mixtures
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