By James Ashenhurst
Addition Pattern #1: The “Carbocation Pathway”
Last updated: March 26th, 2019
MOC: What were some of your biggest roadblocks in learning organic chemistry?
OCI: Not learning the patterns. I think I wasn’t told that there were patterns.
-from this post
A Key Pattern For Alkene Addition Reactions: The Carbocation Pathway
The last several posts have primarily dealt with one reaction: the addition of HCl to alkenes. As we’ve seen, the reaction proceeds through attack of the alkene [the nucleophile] upon a proton [the electrophile], leading to formation of a carbocation. The carbocation, being an electrophile, is then attacked by chloride ion to give the alkyl halide.
The major product will be that which proceeds through the most stable carbocation, giving rise to the regioselective formation of “Markovnikov” products where the chloride adds to the most substituted carbon of the alkene.
Since the reaction proceeds through a carbocation, and nucleophiles may attack carbocations from either face of their empty p orbital, this reaction pathway has no inherent stereoselectivity. A mixture of syn and anti products will be formed [where possible, of course].*
Here’s the good news. If you understand how this reaction works, congratulations – you now understand how hydrobromination and hydroiodination of alkenes work as well!
These proceed through the exact same mechanism as we just described. So instead of having to learn three separate reactions, these are essentially three variations of the same reaction.
The Exact Same Pattern, Repeated For Two More Reactions
By learning this mechanism, you’ve also learned the key steps in the mechanism for the acid catalyzed addition of water to alkenes (“hydration”) and the acid-catalyzed addition of alcohols to alkenes. There’s just one extra step we have to add at the end to make it complete.
Since our nucleophile is neutral, it will bear a positive charge after attacking the carbocation. This positive charge can be removed through deprotonation by a weak base. One little assumption here: we are using H2O (or ROH in the second case) as solvent, so there is a whopping excess around to act in this capacity. **
Why This Is A Big Deal
All five of these reactions have the following features in common:
- They proceed through a carbocation intermediate.
- The most stable carbocation will be formed preferentially (giving rise to “Markovnikov” regioselectivity)
- There is no inherent preference for syn or anti products (not stereoselective)
Learn One Mechanism, Learn Them All
Do you see the power of understanding mechanisms in organic chemistry? Reactions that go through a similar mechanism are providing similar outcomes. All that’s changing is the identity of the atoms. This is the power of understanding mechanisms in organic chemistry. It can help us identify patterns.
It’s a little like learning a song on piano or guitar and then adapting it to a different key. Learning the song the first time is hard, but changing the key is easy since the relationships are preserved. The important thing is to notice this pattern.
In summary: we can group these five reactions into a family, that all proceed through the same key steps. They all share the same pattern of regioselectivity and stereoselectivity.
There’s one last wrinkle with this family of reactions: rearrangements are possible. We’ll talk about that next.
NEXT POST: Rearrangements in Alkene Addition Reactions
*Although the reaction mechanism has no inherent bias for syn or anti stereochemistry, there are many cases of reactions where this reaction will be stereoselective on account of the three-dimensional structure of the starting alkene.
**Even though the acidity of the protonated product and the protonated solvent are roughly equal, because solvent is present in high excess relative to our product, equilibrium will favor formation of the deprotonated product. In practice, the reaction will be subjected to a mildly basic workup that neutralizes the excess strong acid, giving us the neutral product in the end.