Week 26 of 52: Hydrogenation of alkenes

This is an addition reaction because something is added across a double bond. It's also a reduction reaction because the alkene carbons are losing bonds to each other and gaining bonds to hydrogen. Oxidation/reduction is a very important concept in chemistry, but the terms are really quite terrible and deserve their own post or series of posts. However, when I took organic chemistry, oxidation and reduction were simplified for the sake of our reactions: carbon gaining bonds to more electronegative elements is oxidation and carbon gaining bonds to less electronegative elements is reduction. That simplification doesn't work for inorganic chemistry at all, but for our purposes, just note that this is a reduction.

Like other addition reactions, this one follows the pattern of RC=CR' → RZC=CZR', where "Z" is the thing being added across the double bond. Or to represent it pictorially, since you still don't get it for some reason, here's the reaction with hydrogen...
As you can see, it's not our most confusing reaction. This is a simple way to turn an alkene into an alkane, or to remove C=C bonds in general. The catalyst here is a metal, often palladium. As far as I know, the metal catalyst is mixed into charcoal to maximize the surface area for the reaction, but I would imagine that there are other methods used in some cases. If this is done with palladium, a typical abbreviation for the catalyst is "Pd-C" (standing for palladium on carbon).


Also, π-bonds and rings are known as degrees of unsaturation. Each π-bond or ring in a molecule counts as one degree of unsaturation. So replacing the double bond with bonds to hydrogen is a way of "saturating" the molecule. You've probably encountered this concept with saturated and unsaturated fats. But I explain no further. Good day to you.

Week 25 of 52: Acetylide as a nucleophile

In Week 18, I covered the use of terminal alkynes as weak acids. At the end of that post, I casually remarked that the conjugate base of such an acid, an acetylide ion, can itself be used in reactions. Since that post went up, you've been waiting in agony for a post about a reaction using an acetylide ion. Your wait is finally over.

Perhaps I erred in the earlier post, actually. I mean, it is accurate that terminal alkynes are very weak as acids, but just leaving it at that seems sort of pointless. Oh look, we took some very strong base and we protonated it. Lots of things can do that. For Week 18, what I should have done was emphasize that the point of that reaction would be to prepare acetylides. We can then use the acetylides we just prepared in nucleophilic substitution reactions.

Overall, this reaction is a fairly straightforward example of the S_N2 reaction, which was the first post in this whole series. I did write about that reaction first because it was important. After mentioning nucleophilic substitution so much in these subsequent posts, I think it should start to become clear just why it's important.

The textbook I've been using for most of this takes the opportunity to use this reaction as a starting point for discussion of multistep synthesis, which we've only seen a little so far on this blog, and retrosynthetic analysis. The whole point of learning these reactions in the first place is to understand how to convert something into something else. It's transmutation. I hope to have time in the near future to create a reaction map for use here, charting all the functional groups I've covered and all the reactions with them that have been shown so far. If you've been paying attention, and I know you haven't, this should give you an idea of the big picture.