Friedel - Crafts Reaction - Acylation Video Lecture | Chemistry for JEE Main & Advanced

laya hair from leprosy calm and in this
video i'll take you through the
friedel-crafts isolation reaction and
mechanism in the friedel-crafts
isolation reaction we have benzene
reacting with an acyl group or an acid
chloride in the presence of a catalyst
such as the alcl3 lewis acid to form an
acyl group on the benzene ring this is
very similar to the friedel-crafts
alkylation reaction except for the
carbonyl on the product but since the
two are related I definitely recommend
going back to the alkylation reaction
first to make sure you understand the
similarities and differences between the
reactions you can find my complete
series on electrophilic aromatic
substitution by visiting my website
layer for sicom slash EAS in the EAS
intro video we talked about the super
electrophile so let's see how we form it
for this reaction the molecule that adds
to the benzene is an acyl group and in
this case we have an acid chloride this
is a carboxylic acid derivative where we
replace the O H from carboxyl with a
chlorine the activation of the super
electrophile is very similar to the
activation you see in aromatic
halogenation and friedel-crafts
alkylation we have the acyl chlorine as
the chlorine that doesn't know when to
give up
and aluminum which is an exception to
the octet rule in that it can have just
six valence electrons and three bonds
and still be considered as a complete
octet chlorine doesn't respect that
exception and attacks the aluminum
forming a bond between itself and
aluminum this results in an aluminum
atom with a total of four bonds and
eight electrons and a formal charge of
negative one and a chlorine atom with
two bonds and four electrons for a
formal charge of positive one chlorine
as a halogen is highly electronegative
and dislikes that positive charge and so
to compensate it will grab the electrons
from the carbonyl carbon break the bond
and take the electrons for itself the
resulting ALCL 4 still has a negative
charge on the aluminum but chlorine is
now neutral we'll leave this on the side
but now let's see what happened to the
acyl group the acyl group
and we'll show the carbon atom here
looks like it should be stable because
we have a carbon double bound to an
oxygen
the problem is when chlorine broke away
we didn't replace anything on the carbon
atom and so carbon has an incomplete
octet and a positive charge now while
you have seen stable carbo cations in
the past keep in mind that the carbo
cation that starts out as an sp3 carbon
with four bonds and then loses a pair of
electrons for an incomplete octet while
not stable is still stable enough in
this case we started out with an sp2
hybridized carbon and then added the
positive charge making it very very
unstable Carbon really does not like
having a PI bond and the carbo cation on
the same atom and so it will pull on the
electrons from oxygen giving us a
resonance structure where carbon is now
triple bound to oxygen the resulting
resonance structure has a stable carbon
with four bonds but an oxygen atom with
three bonds one lone pair and a positive
charge this structure is called the a
cilium ion and this is the more stable
of the two resonance because even though
oxygen does not like that positive
charge in a choice between a complete
octet positive oxygen or incomplete
octet double bound positive carbon the
oxygen is more stable now that we have
our super electrophile let's see how we
add it to the benzene ring while the
eighth ilium ion has a positive oxygen
the charge is not completely on the
oxygen remember that with resonance
there is a constant back and forth and
so the electrons between carbon and
oxygen are drawn towards the oxygen atom
making carbon very partially positive so
even though oxygen is the positive Adam
when benzene reaches out to attack its
the carbon atom and not the oxygen that
gets attacked the electrons that were
being drawn towards oxygen are now able
to completely break away from carbon and
rest as a lone pair on the oxygen atom
the resulting intermediate has a neutral
carbon double bound to oxygen and single
bound to an R group we also have a
hydrogen on that same benzylic carbon
atom
making it sp3 hybridised and incapable
of resonance and finally we have a
positive charge on the other carbon on
benzene where the PI bond broke away
from to reform the aromaticity we bring
back the ALCL form - specifically the
chlorine that didn't know when to give
up the chlorine will break away from
aluminum and use those electrons to form
a bond between itself and hydrogen but
it only reaches for the hydrogen nucleus
leaving the electrons to collapse away
from hydrogen and towards that positive
carbon on benzene these electrons are
what reformed the benzene aromaticity
giving us our final product and acyl
group on benzene we also form a molecule
of HCl and the alcl3 catalyst is
regenerated and ready to proceed with
another reaction friedel-crafts
alkylation and isolation have important
differences and similarities and so be
sure to join me in the next video where
I show you what you have to understand
as similarities and differences between
the two reactions you can catch my
entire series on electrophilic aromatic
substitution by visiting my website
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