Conformations of Alkanes Video Lecture | Chemistry Class 11 - NEET

- [Voiceover] On the left, we have one way
to represent the ethane molecule.
The only problem with this drawing
is it doesn't give us much information
about what ethane looks
like in 3D dimensions.
On the right, is another
drawing of the ethane molecule,
but this drawing gives
us more information.
This is a wedge and dash drawing.
Remember, a wedge has a bond
coming out at you in space
or a bond coming out of
the plane of the page.
A dash is a bond going
away from you in space
or a bond going into
the plane of the page.
And finally, if you just draw
a straight line like this,
that means a bond in
the plane of the page.
This wedge and dash drawing represents
one conformation of ethane.
Conformations are different
arrangements of atoms
that result from bond rotation.
We know that there's free rotation
around this carbon-carbon single bond.
And this wedge and dash drawing right now
represents what's called
the staggered conformation
of ethane.
But if we rotate about
this carbon-carbon bond,
we're gonna get different
arrangements of the atoms
and therefore we would get
different conformations.
This is easiest to see with a model set,
so up next I have a video where
I have a model set of ethane
and I rotate around the carbon-carbon bond
so we can see different conformations.
I'm gonna make these
hydrogens green in the video,
so those green hydrogens
are attached to this carbon,
which will be the front carbon.
And then we have hydrogens attached
to the back carbon that I will make white.
So let's watch the video and look
at the different conformations of ethane.
Here we have the staggered
conformation of ethane,
looking at it from a wedge
and dash perspective.
If we rotate this a little bit,
we'll see the staggered conformation
of ethane from a sawhorse perspective.
If we sight down the carbon-carbon bond,
we'll see the third way of
looking at this molecule.
And this is called a Newman projection.
If I take one of these
hydrogens here and I rotate it,
so I'm keeping the back carbon stationary
and I'm rotating the front carbon,
every time I rotate this, that represents
a different conformation
of the ethane molecule.
So there are many, many possible
conformations in theory.
I rotate back to the
original position here,
so this is the Newman
projection where we just were.
The other conformation that we
really care about for ethane
is what's called the eclipse conformation.
So, I just rotated the green
hydrogens so they're in front
or eclipsing the white
hydrogens in the back.
So, this is the eclipse
conformation as a Newman projection.
I could turn this a
little bit so we can see
what an eclipsed conformation looks like
from a sawhorse view and then finally,
I can turn a little bit so we can see
the eclipse conformation from
a wedge and dash perspective.
Now that we've seen the video,
let's look at three ways
to represent the staggered
conformation of ethane.
And we'll start with the
wedge and dashing drawing.
So you can see that this hydrogen,
this carbon, this carbon,
and this hydrogen are
all in the same plane.
So they're all in the same
plane here which is why
all these bonds are
drawing as straight lines
in our wedge and dash drawing.
If we look at this carbon on the left,
we know that there is a hydrogen in green
coming out at us in
space, a hydrogen in green
going away from us in space,
and then this hydrogen
which is in the plane of the page.
For the hydrogens in white, this hydrogen
is in the plane of the
page, this one's coming out
at us in space, and then we have one
going away from us in space.
And it's pretty easy to see
that in the picture here,
so we're just going to match
the drawings to the pictures.
Our next way to represent
the staggered conformation
is what's called a sawhorse drawing.
So for this sawhorse drawing,
this carbon right here
is this carbon and it has
three hydrogens in green
attached to it.
So, this is a hydrogen in
green, in green, and in green.
So, we also have this carbon-carbon bond
so this bond here on our sawhorse drawing.
And finally, for this carbon in the back,
this is the one that
has hydrogens in white.
So these are the three hydrogens in white.
If we look down that carbon-carbon bond,
if we look down this axis right here,
so if you put your eye, imagine
putting your eye right here,
so here's the eye and
you look down this axis,
you will see the Newman projection
just like we saw in the video.
And so, this dot, this
point here represents
the front carbon.
That's this carbon right here.
So let me go ahead and write that down.
So this point represents the front carbon
and the front carbon is bonded
to our hydrogens in green.
So this would be a hydrogen in green,
so is this one, and so is this one.
The front carbon blocks
the carbon in the back.
You can't see the carbon in the back
in this drawing but we know it's there
and we have to represent it somehow
so in a Newman projection,
it's this circle back here
that represents the back carbon.
So this is the back carbon, this circle.
Let me write that down.
The circle is the back carbon
even though we can't see it
when you actually have
the model in front of you.
And the back carbon is the one
that has the hydrogens in white.
So this should be a hydrogen in white,
so is this one, and so is this one.
So you can see the
hydrogens in white back here
and you know they're attached to a carbon
but you can't see it
because the front carbon
is blocking it.
So let's talk a little bit
more about Newman projections.
We can talk about the angle
between this hydrogen in white
and this hydrogen in green.
So think about the angle between these.
It's 60 degrees so we have 60 degree angle
between these two hydrogens.
So I can write down
here too. So 60 degrees.
This angle is called the dihedral angle
or the torsional angle.
So I'll write those down,
so you can call this
the dihedral angle or you could
also call it the torsional angle.
Let me write that down here too.
So torsional angle or you could
also call it the torsion angle.
The angle between the
hydrogens will be important
when you're talking about conformations.
Here the angle is 60 degrees which means
that these hydrogens are not
right on top of each other.
There's space between these hydrogens
on the Newman projection.
The green hydrogens are staggered
compared to the white hydrogens.
So if you go around you can see
that there's space between all of these.
And that's why we call this
the staggered conformation
of ethane.
And finally, let's look at the other
important conformation of ethane
which is the eclipsed conformation.
And we'll start with the
wedge and dash drawing.
So we have this hydrogen, this carbon,
this carbon, and this hydrogen.
These are all in the same plane,
so I draw this line here
and we can see these bonds
are all in the same plane up here
on my wedge and dash drawing.
For this carbon on the left,
we can see that we have
a hydrogen in green coming out at us.
So right, here's your hydrogen
in green coming out at you,
a hydrogen in green going
away from you back here,
and then this hydrogen
in the plane of the page.
For the hydrogens in white,
this hydrogen is in the plane
of the page, this hydrogen
is coming out at us in space,
and this hydrogen is going
away from us in space.
For the sawhorse drawings,
let's move on to here.
This carbon is the front carbon,
so that's this carbon right here.
The one that's bonded to the
three hydrogens in green.
So let me highlight those.
So we have one hydrogen, this
one right here is going up.
So that's our hydrogen in green going up.
This hydrogen over here is going down,
a little bit to the
right, and this hydrogen's
going down and to the left.
And then we have the carbon-carbon bond.
So let me draw that in here.
So here's our carbon-carbon bond,
I made it much longer up here just so
the atoms wouldn't
interfere with each other
on our sawhorse drawing.
And then we get to the carbon in the back.
So here's the carbon in the back
which is bonded to three hydrogens.
And I made these the hydrogens in white.
So here's one, two, and three.
So here are the hydrogens
in white on the picture.
Finally, we get to the Newman projection
for the eclipsed conformation.
And remember, the Newman
projection is what we would see
if we stare down the carbon-carbon bond.
So if we sight down
this carbon-carbon bond,
if we put our eye right here
on our sawhorse projection
and we stare down the carbon-carbon bond,
we see this for our Newman projection.
Alright, first we see this
point here repesenting
the front carbon, so that
point is the front carbon
on our Newman projection.
And the front carbon is bonded
to the hydrogens in green.
So here's one, two, and three.
So up here, one, two, and three.
So this would be the bonds,
this represents the bonds
of that front carbon to those hydrogens.
The back carbon is pretty difficult to see
but we know that the circle
represents the back carbon.
The front carbon is in the way.
I'm not sure if you might be able to see
a tiny bit of the back carbon here.
But the back carbon should be
blocked by the front carbon.
The back carbon we know
has our hydrogens in white
so here are the three hydrogens in white.
So back here you can just
barely see them poking out.
This time when you think about the angle
between your hydrogens,
this time your dihedral
or your torsional angle is zero degrees.
Think about this hydrogen
being perfectly upright,
and the hydrogen in the back
being perfectly upright.
So the angle between those is zero.
So your dihedral angle
is zero degrees here
for the eclipsed conformation.
The green hydrogens are
eclipsing the white hydrogens.
So that's why we call this
the eclipsed conformation.