– Many times in chemistry
we’ll see different molecules
that have the same constituent atoms.
For example, these two molecules here,
they both have four carbons.
One, two, three, four.
One, two, three, four.
So if I were to write
their chemical formula,
it would be C4 and then they both have,
one, two, three, four, five,
six, seven, eight, nine, ten.
One, two, three, four, five,
six, seven, eight, nine, ten
hydrogens.
So both of them, both of them
have the chemical formula
C4H10.
C4H10, but they’re still
fundamentally different molecules
and you can see that because
they have different bonding.
For example, over here we
have a carbon that is bonded
to three other carbons and a hydrogen.
Over here I can’t find
any carbon that’s bonded
to three other carbons.
I can find ones that are
bonded to two other carbons,
but not one that’s bonded
to three other carbons.
So, how we’ve put the atoms
together, is actually different.
They’re bonded to different things.
And so when we have the
situation where you have
the same constituent atoms,
where you have the same
chemical formula, but
you’re still dealing with
different molecules
because either how their
bonds are made or what their shape is,
we call those isomers.
So an isomer, isomer,
you have the same chemical formula,
same chemical formula.
But you could have different
bonding but different,
different bonding,
bonding or shape,
bonding, shape or orientation.
Orientation.
So over here you have
just different bonding
and this type of isomer is
called a structural isomer.
So these characters
are structural isomers,
same constituent atoms,
but different bonding.
Structural isomers.
So that’s structural
isomers right over there.
Now when you look at this pair
or this pair,
you’ll say those don’t look
like structural isomers.
Not only do they have
the same constituents,
both of these for example
have four carbons,
four carbons and they both have
one, two, three, four, five, six,
one, two, three, four,
five, six, seven, eight,
and they both have eight hydrogens.
So these are both C4H8,
it’s looks like they’re bonded similarly.
For example, I mean the
left hand side here,
these look identical
and one the right hand side,
you have a carbon bonded to another carbon
that’s bonded to three hydrogens,
carbon bonded to another carbon
that’s bonded to three hydrogens.
Carbon bonded to a hydrogen,
carbon bonded to a hydrogen,
so it looks like the
structure of the bonding,
everything’s bonded to the same things,
but you might notice a difference.
Over here, on the right hand side,
this CH3 is on the bottom right,
while over here it’s on the top right
and you might say okay well we know,
what’s the big deal there,
these, you know, all these molecules,
they’re all moving around,
maybe they’re rotating
with respect to each other
and these things could,
this thing could have rotated down
to become what we have up here.
If this was a single bond.
A single bond would allow
for that type of rotation,
it would allow for these things
to rotate around each other.
For the molecule to
rotate around that bond,
but a double bond does
not allow that rotation.
So this fixes these two things,
this fixes these two things in place.
And because of that, these are actually
two different molecules.
Over here on the top,
you have the CH3 groups,
they’re both, they’re both,
I guess you could say,
facing down or their both on the same side
of the double bond,
while over here they’re on different sides
of the double bond
and so this type of isomerism,
where you have the same constituents
and you even have the same bonding,
this is called stereoisomerism.
So over here we’re caring
much more about how things sit
in three dimensions.
We don’t just care about
what’s bonded to what
or the constituents
and actually this one is, as we’ll see,
is also a stereoisomer because this carbon
is bonded to the same
things in either case.
So these are both, these
are both situations,
there are both stereoisomers,
stereoisomers,
and this particular
variation of stereoisomer
is called a cis trans isomer.
Cis is when you have the
two groups on the same side,
cis,
and trans is when you have the two groups
on the opposite sides of the double bond.
Cis trans isomers.
Cis trans isomers.
Isomers,
and these are often
called geometric isomers.
Geometric,
geometric isomers.
So that’s a subset, so when
I’m talking about cis trans
or geometric, I’m talking about
these two characters over here.
They are a subset of the stereoisomers.
Now what’s going on over here?
I have no double bond, I’m not
talking about cis and trans.
The carbon, as I’ve just said,
is bonded to fluorine, chlorine,
bromine, and a hydrogen,
fluorine, chlorine,
bromine, and a hydrogen.
How are these two things different?
And the way that they’re different
is if you were to actually
try to superimpose them
on each other.
You will see that it is impossible.
There are mirror images of each other
and because there’s four
different constituents here,
you can actually not
superimpose this molecule
onto this molecule over here
and actually because of that,
they actually have different
chemical properties,
and so this over here,
these two characters,
which is a subset of stereoisomers.
Stereoisomers are
concerned with how things
are positioned in three dimensions,
not just how their bonding is different,
but this subset where you
have these mirror images
that cannot be superimposed,
we call these enantiomers.
So these two characters,
these are enantiomers.
Enantiomers,
and enantio comes from Greek,
the Greek word or the Greek root opposite.
So these are opposites of each other,
they cannot be superimposed,
they’re mirror, they’re mirror images.
So all of these are different
variations of isomers
and once again, you might say,
okay theses are clearly
two different molecules
that have different bonding,
but even cis trans isomer will have
different chemical properties.
These two in particular,
they aren’t that different
but they do have different
chemical properties,
but sometimes they’re so different
that one might be able to
exist in a biological system
while the other is not.
One might be okay for your health,
and the other might not
be okay for your health.
Same thing for enantiomers.
One might be biologically
active in a certain way
and the other one might
not be biologically active
in that same way.

Isomers | Properties of carbon | Biology | Khan Academy
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42 thoughts on “Isomers | Properties of carbon | Biology | Khan Academy

  • July 7, 2015 at 3:15 pm
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    🙂

    Reply
  • July 9, 2015 at 1:29 pm
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    Cis-trans? Triggered!

    Reply
  • September 17, 2015 at 9:50 pm
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    WOW! Thanks!!

    Reply
  • September 20, 2015 at 1:24 am
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    Made easy 🙂 thank u

    Reply
  • October 27, 2015 at 5:08 pm
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    you draw so neatly c:

    Reply
  • October 30, 2015 at 4:37 am
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    funny how i paid $400 dollars for a textbook and $800 for a class that doesn't teach me jack, but you put it in a way where i can learn the topic in under 10 minutes for free… I LOVE THE INTERNET!

    Reply
  • November 21, 2015 at 12:19 pm
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    my chemistry teacher pronounces it as iiizzzz…
    .ooommmeerrss

    Reply
  • February 28, 2016 at 12:14 am
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    thanks I've been trying to figure this out all day 🙂

    Reply
  • April 24, 2016 at 9:02 am
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    Why do we need to know about the isomers?

    Reply
  • September 17, 2016 at 4:51 pm
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    salman khan my man!

    Reply
  • September 19, 2016 at 3:52 am
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    Khan academy backgrounds are garbage. Bozeman science >

    Reply
  • September 30, 2016 at 12:55 pm
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    hello what about functional isomers for any type they are related

    Reply
  • October 25, 2016 at 3:06 pm
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    thanks !

    Reply
  • November 29, 2016 at 11:57 am
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    thanks a lot

    Reply
  • December 3, 2016 at 1:43 pm
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    indian academy? nice video. makes science easier.

    Reply
  • February 10, 2017 at 7:35 pm
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    That last BIOLOGICALLY made me laugh hard!!!😂😂
    at 6:43

    Reply
  • February 27, 2017 at 6:42 pm
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    But you don't have to say a word over and over as you write it… I can't complain too much because this is free though…

    Reply
  • March 26, 2017 at 1:17 am
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    Awesome!! That's simple! Thanks a lot!!

    Reply
  • June 7, 2017 at 11:33 pm
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    This is cool and all but how do scientists figure out the shape of all these molecules? Are they visible under special microscopes?

    Reply
  • June 15, 2017 at 1:59 pm
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    thank u so much dude…..

    Reply
  • July 23, 2017 at 1:31 pm
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    plz improve your way of presentation

    Reply
  • September 2, 2017 at 6:50 pm
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    good stuff!

    Reply
  • September 5, 2017 at 4:43 pm
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    Best explanation ever and quick, easiest!!!

    Reply
  • September 6, 2017 at 5:54 pm
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    Great video sir!!!!

    Reply
  • October 22, 2017 at 1:34 am
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    It really helped me to understand the topic.

    Reply
  • November 17, 2017 at 5:52 pm
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    His voice reminds me of mark Jefferson from life is strange 😂 am I alone on this?

    Reply
  • November 25, 2017 at 7:00 am
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    does they share the same chemical properties?

    Reply
  • January 25, 2018 at 7:48 am
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    I love his voice, it's like he loves teaching

    Reply
  • February 21, 2018 at 5:16 am
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    Excellent

    Reply
  • February 23, 2018 at 2:05 pm
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    What is coal an allotrope or compound??

    Reply
  • March 8, 2018 at 12:18 pm
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    perfectly explained, but please also write the formula for the enantiomere example

    Reply
  • March 28, 2018 at 12:00 pm
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    I just keep liking! Thank you so much 🙂

    Reply
  • April 20, 2018 at 6:47 pm
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    Can you specifically define what the word superimposeble means.

    Reply
  • June 13, 2018 at 4:22 pm
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    Thank you very much you're always saving my life!!

    Reply
  • June 23, 2018 at 6:38 pm
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    Your voice is calming. I was tense before I started watching your video, but I'm completely relaxed now.

    Reply
  • August 2, 2018 at 1:44 pm
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    Thank you so much. You are a great teacher

    Reply
  • September 4, 2018 at 4:37 am
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    I could not understand AP Bio textbook explanation but this video was so easy to understand.

    Reply
  • September 7, 2018 at 8:13 pm
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    Does this guy know everything?

    Reply
  • September 20, 2018 at 12:50 pm
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    Could u even make a video on types of structural isomerism? Like functional isomerism, chain isomerism, tautomerism, metamerism and positional isomerism?

    Reply
  • May 8, 2019 at 12:45 am
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    How about constitutional isomers?

    Reply
  • August 2, 2019 at 5:02 pm
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    Sir why the rotation of OH and H group in the 3rd carbon of glucose is different.. Please answer me

    Reply

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