Onward with addition reactions!
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?
The major product in each case is always the one where the hydrogen adds to the 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:
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?
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