Preparation of Aryl Halides by Electrophilic Substitution Video Lecture | Chemistry for JEE Main & Advanced

so normally carbon-carbon double bonds
react to via addition so that's where
you have a carbon-carbon double bond
plus something makes one product you're
adding stuff these are the five that you
need to know for SL but here's the
strange thing benzene has carbon-carbon
double bonds but it does not react fire
addition benzene normally reacts via
substitution which is another way of
saying swapping for example benzene and
hydrogen well you'd expect one of the
double bonds between the carbons to
replace by a single bond with the
hydrogen's on either side like what
happens with an alkene but that isn't
what happens
that's addition and benzene doesn't
undergo addition normally it undergoes
substitution so the chlorination of
benzene a chlorine is going to swap out
for a hydrogen on the benzene ring
now the benzene ring does have a ring of
electrons above and below it
and these delocalized electrons give the
ring very big stability so it's going to
be hard to break that ring and keep it
broken
but it is susceptible to electrophiles
things that love electrons so an example
let's look at benzene plus chlorine
which gives me chlorobenzene and HCL and
you can see that one of the H's from the
benzene ring has substituted with a
chlorine now how it's chlorine an
electrophile had as chlorine love
electrons chlorine itself is neutral ah
you need something called a halogen
carrier and the details of that a higher
level but the halogen carrier makes the
chlorine molecule produce a CL plus and
anything positive is going to be an
electrophile it's going to want to be
new electrons via electrostatic
attraction so what if I bang on a bit
more chlorine well I would make a dye
chlorobenzene I could make one two
dichlorobenzene or the isomer one three
dichlorobenzene or even more excitingly
maybe I would have made one for
dichlorobenzene of course the one five
is the one three and the one sixth is
the one two and is ignored now let's
have a little think what about if I had
the chlorines across a single bond and
the chlorines across a double bond are
those different isomers well you know
they're not because in actual fact
they're the same because there are no
single and double bonds in benzene its
delocalized it's two rings of electrons
wow I feel other guy from the matrix
explain the truth to you and the second
one that you need to know about
electrophiles is nitric acid reacting
with benzene so it looks like the proton
in the nitric acid is the electrophile
it is after all positive but in
fact you mix it all up in sulfuric acid
and that makes no2 plus and that's the
electrophile now since its substitution
is going to swap out with one of those
hydrogens now what am I going to be left
with I've got a couple of hydrogen's and
an oxygen left over so that's going to
be water and again in higher-level you
need to know this in much more detail