Class 12 Exam > Class 12 Videos > Meta Directors I
Meta Directors I Video Lecture - Class 12
FAQs on Meta Directors I Video Lecture - Class 12
| 1. What is Meta Directors I Class 12? |  |
Ans. Meta Directors I Class 12 refers to a specific subject or course offered in the curriculum for the 12th grade. It is a program or study material that focuses on the concept of meta directors in organic chemistry.
| 2. What are meta directors in organic chemistry? | |
Ans. Meta directors are functional groups or substituents that direct the incoming groups to the meta position in a benzene ring during electrophilic aromatic substitution reactions. These groups include nitro (-NO2), carbonyl (-CO), and sulfonyl (-SO3H) groups.
| 3. How do meta directors affect the reactivity of benzene compounds? | |
Ans. Meta directors decrease the reactivity of benzene compounds towards electrophilic aromatic substitution reactions. This is because they withdraw electron density from the benzene ring, making it less nucleophilic and less susceptible to attack by electrophiles.
| 4. What are the common examples of meta directors? | |
Ans. Some common examples of meta directors in organic chemistry include nitro groups (-NO2), carbonyl groups (-CO), and sulfonyl groups (-SO3H). These groups are electron-withdrawing and direct the incoming groups to the meta position in a benzene ring.
| 5. How can meta directors be identified in a molecule? | |
Ans. Meta directors can be identified by analyzing the electron-withdrawing nature of the functional groups or substituents attached to a benzene ring. These groups are often electron-withdrawing due to the presence of electronegative atoms or electron-withdrawing groups within their structure.
Text Transcript from Video
Now that we understand
ortho/para directors,
let's take a look at an
example of a meta director.
And so we start
with nitrobenzene.
And if our goal is to
nitrate nitrobenzene
we need to add
something like fuming
nitric acid, sulfuric
acid as our catalyst,
and we also need to heat it up.
So we need to
force this reaction
to occur because
nitrobenzene turns out
to be not as reactive
as benzene itself.
And so the end result
is to add a nitro group
into the meta position.
So we added a nitro group
onto the meta position
on our benzene ring.
And so the meta product
dominates here, not
the ortho/para.
Let's see if we
can figure out why
by looking at some resonance
structures for this mechanism.
And so we'll start
with and ortho attack.
So in our first
example here we'll
do an ortho attack where we add
the nitro group onto the ortho
position.
So remember, the function of the
nitric acid and sulfuric acid
is to generate your
electrophile, which
is the nitronium ion
right here with a plus 1
formal charge on the nitrogen.
And so that's our electrophile.
The benzene ring is going to
function as the nucleophile
and so these pi
electrons here are
going to attack this nitrogen,
pushing these electrons off
onto the oxygen.
So let's go ahead and
look at the results
of our nucleophilic attack.
We have a nitro
group right here.
I want to show an
ortho attack, so I'm
going to show the NO2 adding
onto the ortho position.
So here's the ortho
position with an NO2.
That carbon also has a
hydrogen bonded to it.
Let's go ahead and
highlight some electrons.
So these pi electrons
right here are
forming a bond
between the nitrogen
and the carbon on our ring.
So that takes a bond
away from this carbon.
And so that's where our plus 1
formal charge is going to go.
And then of course we
still have these pi
electrons in our ring like that.
Let's go ahead and draw
a resonance structure
for this ion here.
So we have a pi bond next
to a positive charge.
So we could take
these pi electrons
and move them over to here.
And let's go ahead and draw
that resonance structure.
So we have our ring
top nitro group.
Over here we have
the other nitro group
in the ortho position.
We have these pi
electrons at the top.
We have these pi
electrons over here
now, so let's highlight those.
So the pi electrons in blue
have moved over to here,
taking a bond away
from this carbon.
So that carbon gets a plus
1 formal charge like that.
So there's a
resonance structure.
Let's go ahead
and draw one more.
So I could show, once again,
these pi electrons moving down
to here.
And we have our ring.
We have this top nitro
group, which I'm actually
going to go ahead and draw
in the formal charges.
And you'll see why in a second.
So this oxygen has a
negative 1 formal charge
and this nitrogen has a plus
1 formal charge like that.
So over here we have
our other nitro group
in the ortho position.
We have pi electrons
here, and we just
moved some pi electrons
over to this position.
So let's go ahead and
highlight those too.
So over here in red,
these pi electrons
have moved over to
here, taking a bond
away from this top carbon here.
So that's where our
plus 1 charge goes now.
So we have a plus 1 formal
charge on this top carbon here.
And this is a destabilizing
resonance structure.
And we know that because we
have a positive 1 formal charge
on this nitrogen
and a positive 1
formal charge on this
carbon on our ring.
And so like charges repel,
and therefore destabilize
this resonance structure,
So we have a destabilizing
resonance structure.
And remember, it's actually
really a [INAUDIBLE]
of our resonance structures
for our sigma complex.
But one of those
resonance structures
is destabilizing, which means
that this sigma complex is not
very likely to form.
So let's go ahead and
look at a meta attack.
And you'll see that we will not
have a destabilizing resonance
structure when we
do a meta attack.
So let's go ahead
and once again show
our nitrobenzene and
our nitronium ion.
And this time we will
do a meta attack.
So if I want to show a nitro
group adding onto the meta
position I would once again
use these pi electrons.
So nucleophilic attack
pushes these electrons off.
And so we're going to once again
show the resulting carbocation.
So we have a nitro
group right here.
And this time we're showing
the nitro group adding on meta.
And once again there's a
hydrogen attached to our ring.
And these pi electrons
here are forming the bonds
between this carbon
and our nitrogen,
taking a bond away
from this carbon.
So that carbon gets a
plus 1 formal charge.
And we still have, of course,
pi electrons in our ring.
And so that's our first
resonance structure.
We can draw another one.
I could take these pi electrons
and move them over to here.
So let's go ahead and show
the next resonance structure
with our ring.
Our nitro group here, our nitro
group in the meta position,
and hydrogen also
attached to that carbon.
Pi electrons here,
and pi electrons
have moved over to here.
So let me highlight those.
So these pi electrons
have moved over to here,
taking a bond away
from this carbon.
So we get a plus 1
formal charge here.
We can draw another
resonance structure
taking these pi electrons
and moving them over to here.
So let's go ahead and do that.
We have our ring once again.
We have a nitro group
in the top carbon.
We have a nitro group
in the meta position.
Once again, we have hydrogen,
we have these pi electrons,
and we now have moved
the pi electrons over
to here, so let me
highlight those.
So in red, these pi
electrons have moved over
to this position, taking a
bond away from this carbon.
So we get a plus 1 formal
charge on that carbon.
And so these are the
three resonance structures
that show up for a meta attack.
And notice, we don't
have a destabilizing one.
So in our three resonance
structures, none of them
have the two positive charges
right next to each other
as we saw in the
previous example.
So it's not so much that the
sigma complex for a meta attack
is extra stable.
It's just that the sigma
complex for a meta attack
doesn't have any
destabilizing like charges
repelling each other.
And so because there
is no destabilization
the meta sigma complex
becomes the most stable one
and the one that's most likely
to form in your mechanism.
And so that's why a
nitro group on your ring
is going to function
as a meta director.
Let's go ahead and
do a para attack
so that you can see that a para
attack is going to give you
the same situation
as an ortho attack.
So if I wanted to add this nitro
group into the para position,
I would have to use these pi
electrons over here this time.
So nucleophilic attack,
so nucleophile attacks
the electrophile, once again
pushing those electrons off.
And so we have our
nitro group here.
We have these pi electrons.
We are now going to show
the nitro group in the para
position.
So if I'm saying these
electrons in magenta
are going to form a
bond with this nitrogen
right here, taking a bond
away from that carbon.
So that gets our plus 1
formal charge right here.
So a resonance structure would
be to take these pi electrons
and move them over to here.
So let's go ahead and
show the next one.
So we have our ring.
And this time I'm
going to draw out
the formal charges
on my nitro group.
So I have an oxygen
that's negatively charged.
This nitrogen has a plus
1 formal charge on it.
And my pi electrons, I
have pi electrons here.
I have my nitro group once
again in the para position.
And I have some pi
electrons over here as well.
So let's highlight those.
So the pi electrons in blue
have moved over to here,
taking a bond away
from this top carbon.
And so I can go ahead and
draw a plus 1 formal charge
on this top carbon.
And once again, this is
our destabilizing resonance
structure.
I have a positively
charged carbon
on my ring right next to my
positively charged nitrogen.
So like charges repel.
This is the destabilizing
resonance structure
which of course destabilizes
the sigma complex for a para
attack.
Just to be complete, I can draw
another resonance structure.
So I could take these pi
electrons over to here.
So let's go ahead and
draw our third one.
So I have my nitro group,
I have pi electrons here,
pi electrons here.
And I have a nitro
in the para position.
And let me go ahead and
highlight those electrons one
last time.
So I have these electrons
in red have moved over
to here, taking a bond
away from this carbon.
And so that will get
our plus 1 formal charge
for our last
resonance structure.
So once again, remember
that the sigma complex
is a hybrid of these
resonance structures.
But since we can draw a
destabilizing resonance
structure, this
sigma complex is not
the one that's most
likely to form.
So the meta attack is preferred.
So one more quick thing
about meta directors,
so an easy way to
recognize a meta director.
So we've seen in this
example our substituent has
an atom as a positive charge
right next to our benzene ring.
And so one way to look for a
meta director would be here's
just a generic
substituent y directly
bonded to our benzene ring.
So the atom directly bonded
to your benzene ring,
if there's a plus 1
formal charge on it,
we've just seen that resonance
structures are destabilized
for an ortho/para attack.
Therefore, meta
attack is preferred.
So you could look for
a plus 1 formal charge
or you could look for
a partial positive.
And so that's just my
really fast and easy way
of figuring out a meta director.
So in the next video, we'll
look in much more detail
about some other meta directors.
ortho/para directors,
let's take a look at an
example of a meta director.
And so we start
with nitrobenzene.
And if our goal is to
nitrate nitrobenzene
we need to add
something like fuming
nitric acid, sulfuric
acid as our catalyst,
and we also need to heat it up.
So we need to
force this reaction
to occur because
nitrobenzene turns out
to be not as reactive
as benzene itself.
And so the end result
is to add a nitro group
into the meta position.
So we added a nitro group
onto the meta position
on our benzene ring.
And so the meta product
dominates here, not
the ortho/para.
Let's see if we
can figure out why
by looking at some resonance
structures for this mechanism.
And so we'll start
with and ortho attack.
So in our first
example here we'll
do an ortho attack where we add
the nitro group onto the ortho
position.
So remember, the function of the
nitric acid and sulfuric acid
is to generate your
electrophile, which
is the nitronium ion
right here with a plus 1
formal charge on the nitrogen.
And so that's our electrophile.
The benzene ring is going to
function as the nucleophile
and so these pi
electrons here are
going to attack this nitrogen,
pushing these electrons off
onto the oxygen.
So let's go ahead and
look at the results
of our nucleophilic attack.
We have a nitro
group right here.
I want to show an
ortho attack, so I'm
going to show the NO2 adding
onto the ortho position.
So here's the ortho
position with an NO2.
That carbon also has a
hydrogen bonded to it.
Let's go ahead and
highlight some electrons.
So these pi electrons
right here are
forming a bond
between the nitrogen
and the carbon on our ring.
So that takes a bond
away from this carbon.
And so that's where our plus 1
formal charge is going to go.
And then of course we
still have these pi
electrons in our ring like that.
Let's go ahead and draw
a resonance structure
for this ion here.
So we have a pi bond next
to a positive charge.
So we could take
these pi electrons
and move them over to here.
And let's go ahead and draw
that resonance structure.
So we have our ring
top nitro group.
Over here we have
the other nitro group
in the ortho position.
We have these pi
electrons at the top.
We have these pi
electrons over here
now, so let's highlight those.
So the pi electrons in blue
have moved over to here,
taking a bond away
from this carbon.
So that carbon gets a plus
1 formal charge like that.
So there's a
resonance structure.
Let's go ahead
and draw one more.
So I could show, once again,
these pi electrons moving down
to here.
And we have our ring.
We have this top nitro
group, which I'm actually
going to go ahead and draw
in the formal charges.
And you'll see why in a second.
So this oxygen has a
negative 1 formal charge
and this nitrogen has a plus
1 formal charge like that.
So over here we have
our other nitro group
in the ortho position.
We have pi electrons
here, and we just
moved some pi electrons
over to this position.
So let's go ahead and
highlight those too.
So over here in red,
these pi electrons
have moved over to
here, taking a bond
away from this top carbon here.
So that's where our
plus 1 charge goes now.
So we have a plus 1 formal
charge on this top carbon here.
And this is a destabilizing
resonance structure.
And we know that because we
have a positive 1 formal charge
on this nitrogen
and a positive 1
formal charge on this
carbon on our ring.
And so like charges repel,
and therefore destabilize
this resonance structure,
So we have a destabilizing
resonance structure.
And remember, it's actually
really a [INAUDIBLE]
of our resonance structures
for our sigma complex.
But one of those
resonance structures
is destabilizing, which means
that this sigma complex is not
very likely to form.
So let's go ahead and
look at a meta attack.
And you'll see that we will not
have a destabilizing resonance
structure when we
do a meta attack.
So let's go ahead
and once again show
our nitrobenzene and
our nitronium ion.
And this time we will
do a meta attack.
So if I want to show a nitro
group adding onto the meta
position I would once again
use these pi electrons.
So nucleophilic attack
pushes these electrons off.
And so we're going to once again
show the resulting carbocation.
So we have a nitro
group right here.
And this time we're showing
the nitro group adding on meta.
And once again there's a
hydrogen attached to our ring.
And these pi electrons
here are forming the bonds
between this carbon
and our nitrogen,
taking a bond away
from this carbon.
So that carbon gets a
plus 1 formal charge.
And we still have, of course,
pi electrons in our ring.
And so that's our first
resonance structure.
We can draw another one.
I could take these pi electrons
and move them over to here.
So let's go ahead and show
the next resonance structure
with our ring.
Our nitro group here, our nitro
group in the meta position,
and hydrogen also
attached to that carbon.
Pi electrons here,
and pi electrons
have moved over to here.
So let me highlight those.
So these pi electrons
have moved over to here,
taking a bond away
from this carbon.
So we get a plus 1
formal charge here.
We can draw another
resonance structure
taking these pi electrons
and moving them over to here.
So let's go ahead and do that.
We have our ring once again.
We have a nitro group
in the top carbon.
We have a nitro group
in the meta position.
Once again, we have hydrogen,
we have these pi electrons,
and we now have moved
the pi electrons over
to here, so let me
highlight those.
So in red, these pi
electrons have moved over
to this position, taking a
bond away from this carbon.
So we get a plus 1 formal
charge on that carbon.
And so these are the
three resonance structures
that show up for a meta attack.
And notice, we don't
have a destabilizing one.
So in our three resonance
structures, none of them
have the two positive charges
right next to each other
as we saw in the
previous example.
So it's not so much that the
sigma complex for a meta attack
is extra stable.
It's just that the sigma
complex for a meta attack
doesn't have any
destabilizing like charges
repelling each other.
And so because there
is no destabilization
the meta sigma complex
becomes the most stable one
and the one that's most likely
to form in your mechanism.
And so that's why a
nitro group on your ring
is going to function
as a meta director.
Let's go ahead and
do a para attack
so that you can see that a para
attack is going to give you
the same situation
as an ortho attack.
So if I wanted to add this nitro
group into the para position,
I would have to use these pi
electrons over here this time.
So nucleophilic attack,
so nucleophile attacks
the electrophile, once again
pushing those electrons off.
And so we have our
nitro group here.
We have these pi electrons.
We are now going to show
the nitro group in the para
position.
So if I'm saying these
electrons in magenta
are going to form a
bond with this nitrogen
right here, taking a bond
away from that carbon.
So that gets our plus 1
formal charge right here.
So a resonance structure would
be to take these pi electrons
and move them over to here.
So let's go ahead and
show the next one.
So we have our ring.
And this time I'm
going to draw out
the formal charges
on my nitro group.
So I have an oxygen
that's negatively charged.
This nitrogen has a plus
1 formal charge on it.
And my pi electrons, I
have pi electrons here.
I have my nitro group once
again in the para position.
And I have some pi
electrons over here as well.
So let's highlight those.
So the pi electrons in blue
have moved over to here,
taking a bond away
from this top carbon.
And so I can go ahead and
draw a plus 1 formal charge
on this top carbon.
And once again, this is
our destabilizing resonance
structure.
I have a positively
charged carbon
on my ring right next to my
positively charged nitrogen.
So like charges repel.
This is the destabilizing
resonance structure
which of course destabilizes
the sigma complex for a para
attack.
Just to be complete, I can draw
another resonance structure.
So I could take these pi
electrons over to here.
So let's go ahead and
draw our third one.
So I have my nitro group,
I have pi electrons here,
pi electrons here.
And I have a nitro
in the para position.
And let me go ahead and
highlight those electrons one
last time.
So I have these electrons
in red have moved over
to here, taking a bond
away from this carbon.
And so that will get
our plus 1 formal charge
for our last
resonance structure.
So once again, remember
that the sigma complex
is a hybrid of these
resonance structures.
But since we can draw a
destabilizing resonance
structure, this
sigma complex is not
the one that's most
likely to form.
So the meta attack is preferred.
So one more quick thing
about meta directors,
so an easy way to
recognize a meta director.
So we've seen in this
example our substituent has
an atom as a positive charge
right next to our benzene ring.
And so one way to look for a
meta director would be here's
just a generic
substituent y directly
bonded to our benzene ring.
So the atom directly bonded
to your benzene ring,
if there's a plus 1
formal charge on it,
we've just seen that resonance
structures are destabilized
for an ortho/para attack.
Therefore, meta
attack is preferred.
So you could look for
a plus 1 formal charge
or you could look for
a partial positive.
And so that's just my
really fast and easy way
of figuring out a meta director.
So in the next video, we'll
look in much more detail
about some other meta directors.
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About this Video
Jan 10, 2025
Last updated
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