hey guys, professor Dave here, i wanna
talk to you a little bit about addition reactions so addition reactions are a little bit
like the opposite of elimination reactions whereas in elimination reactions two
groups are removed from adjacent carbons and replaced by a pi bond here we can see that an addition reaction involves two groups being added to adjacent carbons with the pi bond, and that pi bond
is now gone and now those sp2 carbons have become sp3 carbons so it's a little bit like the
opposite of elimination so let's take a look at one to start with we look at hydrogenation so hydrogenation is telling us that its
hydrogen atoms that are being added to these carbons and typically this is done
with some kind of a transition metal catalyst, a lot of
times platinum so what's happening here is two hydrogen
atoms are being added to the two carbons participating in the
double bond and in this case because this is
heterogeneous catalysis this means that the catalyst is in a
different phase than the substrate so it's like a surface that the substrate will have to approach and then hydrogen atoms which are sort of
sticking out the top of the material are going to be added therefore to the substrate on the same side so the first thing we
have to understand about addition reactions is that they can be stereospecific so in this case we have to learn the terms syn and anti so those are terms that discuss the
stereospecificity of a particular addition reaction in
this case hydrogenation over platinum metal is a syn addition because the two groups that are being added to the substrate
are being added from the same side so if we take two hydrogen atoms and
those are added to the two carbons like that now the implied hydrogens that were added are on implied wedge bonds, the methyl groups end up dashes and so this was a syn addition. now it's very important to
make the distinction between syn and anti versus cis and trans it is true that when two groups are
added to a substrate in syn fashion that they will typically end up cis to one another in the product however
cis and trans are discussing spatial relationships between groups on a molecule independent of the context
of any kind of reaction whereas syn and anti are terms that
are specifically describing the mechanism of operation of a reaction so it's within the
context of a reaction this is a syn addition reaction therefore the hydrogens that were added
will end up cis to one another in the product so addition reaction, two hydrogen atoms added syn there they are on the product, they're implied, and that's what we have. thanks for watching guys, subscribe to my channel for more tutorials and as always feel free to email me with questions