Preparation of Aryl Halides by Sandmeyers Reaction Video Lecture | Chemistry for JEE Main & Advanced

in this online lecture we're going to
discuss the diazonium salt formation
mechanism and what we're going to see
here is that key point number one the
diazonium salt formation mechanism
involves addition of n o followed by a
series of acid-base reactions that turn
the oxygen to water and creates a triple
bond between the two nitrogens the water
is then booted off as a leaving group
we're also going to see number two is
that diazonium salt formation is only
possible with primary errol and alkyl
amines so let's dive into this here
remember what we've learned so far is
that to create a diazonium salt you need
an NH 2 on a benzene ring you're adding
nano2 and HCL and that gives you this
right here the diazonium salt as a
product now to understand the mechanism
behind this reaction let's first talk
about how the nano2
and the HCL react with each other that's
where this mechanism begins so let's
look at that here and what I'm going to
do is write out the nano2 group in its
Lewis dot form light here so here's the
first step of the mechanism one of the
oxygens on the enno group gets
protonated by the HCL acid so this is
just an acid-base reaction what we end
up with is this as a product then
another HCl in solution protonates that
same oxygen again that turns in now into
water which remember is a much better
leaving group and that's exactly what
happens here water is booted off as a
leaving group so you're ending up with
water as a product and what's left over
here is the N O group giving the n a
positive formal charge this thing is
called a nitro so Neum ion so going back
to the big picture here all we're saying
is that the nano2 reacts with the HCL to
create this n o+ group here and it is
this thing right here that is now ready
to react with the benzene ring with the
mene on it so let's look at the
mechanism of how this is going to happen
the first step here involves the nh-2
group on the benzene ring being
attracted to this positive and creating
a bond between the two nitrogens
giving us this result right here take a
few seconds here to make sure you see
the actual matching up here of these two
molecules now let's do this
the nh-2 group in our product here let's
expand out these two hydrogens to get a
better look this sets us up for the next
step of the mechanism what we then do is
add some sort of base here doesn't
matter there's a base in the reaction
that comes along and rips off this
hydrogen right here and leaves the
electrons behind on that nitrogen we end
up with this as a result right here
which happens to be called a nitro so
amine and the mechanism keeps going here
at this point we add an acid now careful
here I'm going to be generic I'm just
going to call it HB remember our
diazonium reaction is run in HCl so
there's plenty of acid available and
watch what happens here the electrons on
one of the nitrogen here fall down
between these two nitrogen's and the pi
electrons in the N o bond go out and
grab this hydrogen right here causing
the electrons between H and B to jump up
on the B what we end up with is
something that looks like this and since
our H be donated a proton to the
molecule here's how he exists right now
as B - and the reason why that's
important to know is watch what happens
here now that he's B - what he does here
is grab off this next hydrogen right
here and again same kind of step here
the electrons jump up on this nitrogen
right here and you're ending up with
this as a result and as a side product
since we added the hydrogen back to the
B he's now back to being HB the molecule
we've created so far in the MEC
happens to be called an N hydroxy as o
compound but we're still not yet at the
diazonium let's keep going here right
now remember we still have our HB what
happens now is we protonate again this
oxygen right here and the electrons
between H and B of course jump up on the
B and now our Oh H has been turned into
water which remember we've seen this
many times in organic chemistry
mechanisms anytime we're turning o H
into water it usually means that he's
being prepared to leave and that exactly
is what happens here and the next step
of the mechanism the electrons on the
nitrogen right here fall down between
these two nitrogen's and boot off water
right here and at this point look what
we have here the diazonium ion and as a
side product we would have the leaving
group water right here so what you saw
here is the overall mechanism for
diazonium salt formation remember we
showed in the mechanism how the
diazonium ion is formed but remember
since this reaction involves HCl the CL
minus in the reaction would of course be
attracted to the diazonium ion therefore
making the diazonium salt now you might
be thinking to yourself that's a lot of
steps and that's a pretty hard core
mechanism which it is but let me show
you how to memorize the mechanism if you
have two on your or go exam the first
thing you have to remember is that the
actual species that reacts with the
benzene ring with the nh-2 group is the
n o+ group here on the right and that
this is how they first connect to each
other to make this product right here of
this step and remember let's expand
these hydrogen's right here and here's
the key to remembering the mechanism
here all that is really happening here
the big picture is remember we have this
base B minus here and what he does is
simply remove these two hydrogen's right
here overall he takes them off like this
and then simply puts them
on the molecule but at a different
location and that is on this oxygen
right here so that he could become water
then eventually what happens is the two
nitrogens right here remember we got to
get a triple bond between them to make
the diazonium so eventually steps in the
mechanism create a triple bond between
these nitrogen's and leave behind just a
single bond between this nitrogen in the
water and again since the water is a
good leaving group
we eventually boot him off like this
this creates our diazonium ion if you
remember that this is what's happening
overall it can help you remember the
individual mechanistic steps that are
necessary to get to this result remember
we learned in a previous online lecture
all the mechanistic moves that are
possible in organic chemistry and we
also talked about how seeing what's in
front of you and knowing where you're
going can help you get through a
mechanism so that's our mechanism again
for the diazonium salt formation now
some other things I'd like you to know
here just in case is that you don't have
to have a benzene ring to make a dyes
own ium for instance this is just a
simple alkyl amine notice if I use the
same reagents right here nano2 and HCl
you can actually turn the nh-2 group
into the diazonium salts so again the
benzene ring is not necessary for this
reaction to take place but however there
are some restrictions here that I'd like
you to know in order for a diazonium
salt to form the amine has to be at
least primary and remember what that
means is that the nitrogen has to have
two hydrogen's and only one R group
connected to it and we can see that that
our group could be a benzene ring or an
alkyl group so that means primary amines
only so if for instance look at this
molecule right here notice that nitrogen
has only one hydrogen and two R groups
two are groups
make him a secondary amine if you try to
form a diazonium salt with this molecule
again adding the n o+ group here the
nitrogen will still attack the other
nitrogen here and create this thing
right here as a result and if you add a
base to this right here
he'll still rip off remember this
hydrogen right here and the electrons
will fall on this nitrogen like this
this gives us this product right here a
nitro so amine but here's what we're
learning the reaction stops at this
point think about it you don't have
another hydrogen to rip off and
therefore put on the oxygen so that you
could turn the oxygen into water and
boot him off so just know that secondary
amines will not form a diazonium salt
and I'd like you to know that this is
also true for tertiary amines in this
particular case when you add the n o+
group here the nitrogen tries to attack
this nitrogen here and make a connection
to make this product right here but
notice in this case there's no
hydrogen's to rip off that nitrogen with
the plus charge so what happens here is
that this bond breaks and let's go which
means we go right back to where we
started so this is why the diazonium
salt can't form for tertiary amines but
just in case I'd like you to know what
can actually happen here instead
remember the n o+ group that we're
adding is technically because of his
positive formal charge an electrophile
which means the n o+ and the benzene
ring on the Left can participate in an
electrophilic aromatic substitution
reaction EAS with again the n O acting
as the electrophile and remember
nitrogen's on a benzene ring or
ortho/para directing activators so the n
o electra
we'll add either ortho or para to this
ring if given the choice here in this
case we prefer the para product because
having those methyls on the nitrogen is
slightly bulky so the enno group would
prefer to be far away from that as
possible placing him para to the N ch3
group so what have we learned here key
points number one the diazonium salt
formation mechanism involves addition of
n o followed by a series of acid-base
reactions that turn the oxygen into
water and creates a triple bond between
two nitrogens the water is then booted
off as a leaving group we also saw that
number two dies on 'i'm salt formation
is only possible with primary errol and
that is primary amines that are on
benzene rings and primary alkyl amines
you