Drawing 3-D Structures of Organic Molecules – Part 2

Drawing 3-D Structures of Organic Molecules – Part 2


We’ll follow the same approach. The first
thing we want to do is show the skeleton, the basic connectivity. So we have the CH3
here, and then we have a CH. It doesn’t matter the angles that you show in the basic skeleton
because we’re not trying to show geometry yet so you can just draw up, down, straight
across, it doesn’t matter. We have another CH and then another CH and an oxygen. So definitely
missing a lot of octets. We’re going to have to add in some lone pairs, some pi bonds to
make every atom have its octet and have a stable configuration. Remember, carbon likes
to have four bonds so I think one thing I can do – both of these carbons are missing
a bond so if I put a pi bond between these two to make a double bond then that would
satisfy both of those carbons. So this has four bonds, this now has four bonds, this
has four bonds. Then what would we do on this end carbon? it’s the same thing with the oxygen.
We can form a double bond between the carbon and oxygen so that carbon has four bonds and
what’s the oxygen missing? Oxygen likes to have two bonds an two lone pairs. So we’ll
add on those lone pairs and now it’s got an octet as well. That would have the right formal
charge. There’s 1,2,3,4,5,6. Oxygen has six and it wants six so that would be a neutral
oxygen. So really, the Lewis structures we should attempt to do by inspection, rather
than adding up all the valence electrons and so on. With experience, we should be able
to put these together fairly rapidly. how about the 3-D sketch? Now again, atom by atom
we want to determine the hybridization because that’s going to give us our geometry. Our
first carbon has 1,2,3,4 regions of electron density so that is sp3 hybridized. Here we
have 1,2,3 regions of electron density. That give us sp2 hybridization. Notice that if
I kind of add up the exponents here: if I have one s and two p’s that’s what gave me
my three hybrid orbitals to accommodate the three regions of electron density. Hopefully
this is something that end up being somewhat intuitive. This carbon has three regions to
it’s sp2. This carbon, sp2. And the oxygen has two lone pairs and the double bond. We’ll
count all those as separate regions so it’s also sp2. A lot of sp2 atoms here. We can
start anywhere we want. Let’s go left to right again so our tetrahedral carbon for sp3 is
tetrahedral. So we draw two bonds in the plane, a little wider than 90, and one bond as a
wedge and one bond as a dash. That’s what every tetrahedral atom is going to look like.
What we have to put here are three hydrogens and a carbon. Now what you should do is you
should put the carbon in one of these positions in the plane because what we want to do is
put as many atoms in the plane as possible. If I put my carbon out here, out in front
of the board, now the whole rest of the molecule is going to be floating in front of the page
and that’s going to be impossible to draw. So we want to locate as many atoms as possible
in the plane, so let’s keep the carbon chain in the plane. So then the other three position
are the hydrogens. now this carbon is sp2 hybridized. If you have three regions of electron
density you want to make a trigonal planar to get them as far apart from each other as
possible. So we’ve already drawn one bond here and now 120 degrees, we go down and up
about 120. Again, we know what 90 looks like so we want a little wider than that. Really,
120 look very much like 110. It’s not a big difference there. So we have a hydrogen and
a carbon that we want to put in. We have a little more room to work down here so let’s
put the hydrogen up here and the carbon down here. Okay. This next carbon is also trigonal
planar so we’ll draw a bond straight down and up that way – that’s about 120 degrees,
the corners of a triangle. We put the hydrogen down here and the carbon up here. Then finally
we have and oxygen and a hydrogen, another trigonal planar. It doesn’t matter which way
we put them. Who goes where – it’s all good. Now we want to show our pi bonds. We have
a double bond between these two carbons and between the carbon and oxygen. Again, if we
draw the trigonal planar in the plane as shown then the p orbitals are orthogonal to that,
they’re perpendicular. They’re sticking straight out and straight back. So we can draw them
as a wedge and dash on each carbon. There’s one pi bond. And then on the carbon and oxygen,
wedge and dash. It’s like we’re tilting it just slightly so we can see both halves of
the p orbital. How about these lone pairs? They’re also trigonal planar so they’re in
this region. We can kind of draw a line and show them. In the last example we showed the
orbital. Another option, sometimes, we can just draw a line, not as a bond but just to
show the geometry. If we take a model of this. If we have it flipped over here…trigonal
planar, trigonal planar, this part of the molecule is flat. You can see that zig-zag
shape there and this last carbon is tetrahedral. You can rotate it any way you want. We had
the one where the hydrogen is up here. You could have also chosen to put this CH3 group
up here. That would actually be a different molecule so it’s kind of bent this way. This
would be called a cis conformation, this would be a trans conformation about that double
bond. Those would both be okay because this condensed formula doesn’t tell us what shape
the molecule is. We have a couple different options there. Those would both be acceptable.

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