This is really quite interesting. When we
draw out the skeleton, we have a three-carbon chain, two hydrogens on each end and so right
away we see none of the carbons have a filled octet. They all need four bonds so the way
we can achieve that is to make this a double bond and make this a double bond. So we have
two double bonds on top of each other, sharing the same carbon. This is called a cumulated
double bond. This molecule is actually called allene, so you can look that up and see some
examples of that. The hybridization of these end carbons with 1,2,3 regions of electron
density is sp2. These end carbons are sp2 and this middle carbon is sp hybridized – it
has just two bonds, I mean two groups so it’s sp. What’s interesting about allene is most
of the sp hybridized carbons we’ve seen have triple bonds. This is an example where we
have two double bonds so we’ll see what that does to the geometry. So let’s start with
our first carbon here. sp2 is trigonal planar so we can draw that very nice in the plane
like so. Put our carbon here and hydrogen, hydrogen. Where does our pi bond go? If we’ve
drawn the trigonal planar in the page, then our p orbital is out here, perpendicular.
We can overlap them to form the pi bond. Then this carbon is sp hybridized. sp means linear
so we can draw this straight out this way. But now, when we think about how to form this
second pi bond, it’s going to be another p orbital. Where is the second p orbital for
sp hybridized? sp hybridized atoms always have two unhybridized p orbitals. We’ve already
shown the one that’s in the X orbital, I’m sorry in the Z orbital – straight out and
back, so where’s the second one? It’s actually in the Y orbital, I mean Y axis, perpendicular
in the plane. So if we draw that on each carbon, we see that’s where this second pi bond is.
So it turns out for allene we have a cumulated diene like this, the two pi bonds are orthogonal
to each other – they’re perpendicular to each other. Which means now when we draw these
hydrogens and the p orbital is in the plane, what does that do to those hydrogens? They
can no longer be in the plane. They have to be perpendicular to that so it turns out that
one is a wedge and the other is a dash. So allene has a twisted geometry. It is no longer
planar. These two hydrogens are in the plane and the other two hydrogens are perpendicular.
That’s because here’s the p orbital that connects to this carbon is in this direction and the
p orbital that does the double bond on this side is in this direction. This is called
allene and it has this twisted geometry – a really interesting molecule when we consider
the 3-D sketch.