Preparation and Physical Properties of Ethers Video Lecture - NEET

laya hair from labor sicom and in this
video i'll take you through the EAS
halogenation reaction focusing on the
mechanism for chlorination and
bromination you can find my entire
series on electrophilic aromatic
substitution by visiting my website leia
versailles comm /e AAS we'll start with
a quick comparison of alkene
halogenation to aromatic halogenation if
I add a halogen to an alkene in some
inert solvent I wind up with a vicinal
dihalide in this case 2 3 dye bromo
butane however if I try the same
reaction with benzene
even though benzene has 3 PI bonds we
get absolutely no reaction and that's
because the halogen by itself is not a
strong enough electrophile to make
benzene break out of its aromaticity I
talked about this in great detail in the
EAS intro video and I also mentioned
that you need a super electrophile so
let's see how this happens in order to
add a halogen to benzene we have to use
a Lewis acid which is a strong catalyst
that helps us create our super
electrophile you'll commonly see febr3
and alcl3 has your lewis acid but
there's one more difference instead of
adding two halogens to the molecule we
actually replace just one hydrogen
adding a single halogen in this case
bromine and we get a side product of hbr
now let's see how the halogen interacts
with the catalyst to make it a super
electrophile for benzene to attack since
bromination and chlorination follow the
same mechanism let's see what happens
when we add CL 2 to benzene in the
presence of alcl3 the trick that you
want to look out for think of chlorine
as greedy and it simply doesn't know
when to give up let's take a look at the
catalyst we have alcl3 where aluminum as
you should recall is an exception to the
octet rule in that it only needs three
bonds or six electrons for a complete
octet but even though aluminum is happy
it still has that MTP orbital and that
means other atoms may not respect its
exception to the octet rule and we'll
find an
tak that MTP orbital because this
molecule is open to attack by a lone
pair of electrons this is considered a
lewis acid the reaction begins when the
coin that doesn't know when to give up
uses a lone pair of electrons and
attacks that empty p-orbital binding
itself to aluminum the resulting
intermediate is an aluminum with four
bonds if we do a quick formal charge
using the trick of should- has should
being the number of valence electrons in
a neutral atom has meaning directly
attached to it
aluminum should have three but directly
attached to it we have 4 3 minus 4 gives
me a formal charge of negative 1 on
aluminum we'll do the same thing for
chlorine specifically the chlorine that
did the attack chlorine should have 7
but directly attached we have 6 7 minus
6 gives me a formal charge of plus 1 the
overall molecule is neutral because the
plus and minus cancel out but let's face
it halogens are highly electronegative
and they really dislike positive charge
remember I said this chlorine doesn't
know when to give up
in order to compensate for this positive
charge the greedy chlorine grabs the
bond that connects it to the other
chlorine atom completely breaking off
that chlorine the resulting structure
still has a negative charge on the
aluminum but that green chlorine atom is
now neutral this component of the
molecule is fine but don't forget what
else we created in addition to the ALCL
4 negative we also have a free chlorine
and solution with only 6 electrons this
is the super electrophile for two
reasons reason number one chlorine has
an incomplete octet making it unstable
and reason number 2 look at the formal
charge chlorine should have seven
electrons it only has six seven minus
six is positive one
remember halogens really dislike
positive charge and so if you pick a
halogen and give it a positive charge
and an incomplete octet it's so unhappy
so unstable benzene has no choice but to
attack the reaction begins when benzene
uses its PI bond to reach out and attack
the positive chlorine binding it to the
ring
the chlorine is balanced of the rink it
has a complete octet and it looses a
formal charge don't forget we also have
a hydrogen sitting on the same carbon
atom as well as a positive charge on the
next carbon atom if you're not fully
comfortable with this mechanism go back
to EAS video to where I take you through
the entire mechanism in detail
remember that chlorine atom that didn't
know when to give up
it finally figures out that it has to be
useful chlorine will take the electrons
that bind it to the alcl3 catalyst break
away from the catalyst and instead form
a bond to that hydrogen atom chlorine
only wants the hydrogen nucleus not the
bonding electrons and so when hydrogen
is grabbed these electrons will collapse
towards that positive carbon reforming
the aromaticity the final products
include chlorobenzene which has
regenerated its aromaticity and
therefore is very stable
we have HCl and we've also regenerated
the alcl3 catalyst allowing you to
proceed and catalyzed yet another
reaction you'll get the same mechanism
if you react something like br-2 in the
presence of febr3 since chlorine and
bromine follow the same mechanism be
sure to join me in the next video where
I take you through aromatic nitration or
you can catch the entire EAS video
series on my website layer 4 sicom /e
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