By James Ashenhurst
Synthesis (4) – Reactions of Alkenes
Last updated: March 26th, 2019
In the last post on alkenes we covered the reactions of alkyl halides and it made out tiny little reaction map explode into a cascade.
Here we’re really going to blow up our reaction map, because we’re going to talk about a second very important “hub” for synthesis – alkenes. If you haven’t already noticed…. there are a LOT of alkene reactions. Alkenes are a very versatile building block in organic chemistry, as I hope this post will make clear.
This post is going to assume you’re familiar with these reactions and their products. We’re not going to go into mechanisms or other details here. The point is learning how to apply these reactions so that eventually we can plan syntheses that will take us from one functional group to another. If you need more background on these reactions by all means read this series of posts on alkenes.
As we’ve said many times before, the vast majority of alkene reactions fall into the category of “addition reactions”. That is, we’re breaking a C-C π bond and forming two new bonds to carbon. The new bonds that form, of course, determine the functional group we will be creating. Beneath that, there is a second level of detail – the “regioselectivity” and “stereoselectivity” of the reaction, which you will also need to be familiar with – that will, alas, largely be ignored in our big-picture analysis in this post.
The second category of alkene reactions is “oxidative cleavage”, which involves the cleavage of both C-C bonds and the formation of two new carbonyl [C=O] groups. Depending on conditions, C-H bonds directly attached to the sp2 hybridized carbons of the alkene can also be oxidized to C-OH .
For the purposes of synthesis, we’ll largely be focusing on the new functional groups that are created in each case. If you look at all the reactions of alkenes, draw the products, and categorize according to functional group, you obtain a diagram which looks something like this:
Before we go and update the big “reaction map” from previous posts with these alkene reactions, it’s worthwhile to stop for a second and ask yourself a few questions:
- what are three ways of making alcohols? How do their regioselectivity and stereoselectivity compare?
- what advantage does the oxymercuration reaction have over the other reaction for formation of alcohols that has the same regioselectivity?
- what are two methods of forming alkyl halides form alkenes that have opposite regioselectivity?
- using reactions we’ve learned before, how can the sequence alkane –> alkyl halide –> alkene —> epoxide be accomplished?
- using reactions we’ve learned before, how can the sequence alkane –> alkyl halide –> alkene –> alkane be accomplished?
Feel free to post your answers in the comments below!
Here’s our combined reaction map so far (it incorporates 38 reactions by my count!). Have fun just tracing sequences between different functional groups! In the next post we’ll go through the reactions of alkynes.