Markovnikov Addition Of HCl To Alkenes
Last updated: January 17th, 2020 |
Introduction To “Markovnikov Addition” (“Markovnikov’s Rule”)
Onward with alkene addition reactions!
Having discussed the concepts of “regioselectivity” and “stereoselectivity” of alkene addition reactions, let’s go back to “regioselectivity” for a moment.
We said earlier that the reaction of HCl and HBr (among others) with alkenes is “regioselective”. In this post we give several examples of these regioselective reactions and trace them back to the observations of a Russian chemist in the 1880’s, Vladimir Markovnikov. (In the next post, we will show how these observations give us important clues about the mechanism of this reaction. )
Table of Contents
- What Is The Common Pattern In These Three Addition Reactions Of HCl To Alkenes?
- The Major Product Is The One Where Hydrogen Adds To The Carbon Of The Pi Bond With The Most Hydrogens
- Addition Of HCl, HBr, And Other Acids To Alkenes Follows The “Markovnikov Rule”
- Why Does “Markovnikov’s Rule” Work?
Quiz time: let’s see if you can recognize the patterns in the following 3 reactions. Look carefully. What do each of the major products have in common?
Hopefully you can see that in each case, we’re breaking C-C (π) and forming a new C-H and C-Cl bond. But there’s more.
2. The Major Product Is The One Where Hydrogen Adds To The Carbon Of The Pi Bond With The Most Hydrogens
The major product in each case is always the one where the hydrogen adds to the pi-bonded carbon with the most hydrogens, and the chlorine adds to the carbon with the fewest hydrogens.
In other words, this reaction is regioselective.
To describe this, the term “most substituted” is often thrown around a lot, so here is a graphical explanation:
For our purposes,
- the “most substituted” carbon is the carbon of the alkene that is attached to the most carbons (or “fewer number of hydrogens”, if you prefer).
- the “less substituted” carbon is the carbon of the alkene that is attached to the fewest carbons (or “greater number of hydrogens”)
This pattern is not unique to the reaction of HCl with alkenes. It also applies to the reaction of HBr, HI, and other strong acids with alkenes. This empirical observation was first pointed out in 1870 by one Vladimir Markovnikov and this pattern of regioselectivity has become known as “Markovnikov’s rule”:
“when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen to the carbon having the fewer number of hydrogen substituents”
As if to prove the point, look at this counter-example:
Notice how in this case we have an alkene where each side is attached to the same number of hydrogens —> both “equally substituted”. In this case, there is not a clear “major” product. Both products (in this case, 3-chloropentane and 2-chloropentane, if you’re following along with IUPAC) are formed in roughly equal amounts.
Of course the key question is “why might this be”? A chemical rule that merely says that H-Cl will simply add its hydrogen to the carbon containing the most hydrogens doesn’t really help us understand what is happening on a fundamental level.
It also doesn’t help us understand reactions like the following, where something unexpected has occurred. How did the chlorine end up attached to the far carbon?
[It’s a rearrangement reaction]
In the next post, we’ll take all the experimental information and try to come up with a hypothesis for a mechanism that explains all of these observations.
NEXT POST: Markovnikov’s Rule – Why It Works