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