Markovnikov Addition Of HCl To Alkenes

Introduction To “Markovnikov Addition” (“Markovnikov’s Rule”)

• When we add H-Cl (or other acids) to an alkene, the major product is the one where Br adds to the most substituted carbon of the alkene (i.e. the carbon with the most carbons directly attached)  and the H adds to the least substituted carbon (i.e. the carbon with the least carbons directly attached).
•  This is  known as “Markovnikoff’s rule”.
• It works because these reactions proceed through an intermediate carbocation and carbocation stability increases with the number of attached carbons.

Table of Contents

1. What Is The Common Pattern In These Three Addition Reactions Of HCl To Alkenes?
2. The Major Product Is The One Where Hydrogen Adds To The Carbon Of The Pi Bond With The Most Hydrogens
3. Addition Of HCl, HBr, And Other Acids To Alkenes Follows The “Markovnikov Rule”
4. Why Does “Markovnikov’s Rule” Work?
5. Notes
6. (Advanced) References and Further Reading

1. What Is The Common Pattern In These Three Addition Reactions Of HCl To Alkenes?

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. )

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. (See Alkene Addition Patterns: Regioselectivity)

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”)

3. Addition Of HCl, HBr, And Other Acids To Alkenes Follows “Markovnikov’s Rule”

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.

4. Why Does “Markovnikov’s Rule” Work?

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 – See Alkene Addition Reactions With Rearrangements]

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 

Notes

(Advanced) References And Further Reading:

1. Early example
   The Stereochemistry of the Addition of Hydrogen Bromide to 1,2-Dimethylcyclohexene
   George S. Hammond and Thomas D. Nevitt
   Journal of the American Chemical Society 1954 76 (16), 4121-4123
   DOI: 10.1021/ja01645a020
   Early paper from the 50’s by Prof. George Hammond (of Hammond’s Postulate) on the mechanism of HBr addition to 1,2-dimethylcyclohexane. He prefers a concerted pathway, although that might due to the conditions he employs – in pentane, a very nonpolar solvent, polar intermediates are disfavored.
2. Mechanistic studies
   Hydrochlorination of cyclohexene in acetic acid. Kinetic and product studies
   Robert C. Fahey, Michael W. Monahan, and C. Allen McPherson
   Journal of the American Chemical Society 1970 92 (9), 2810-2815
   DOI: 10.1021/ja00712a034
   Detailed kinetic studies of the addition of HCl to cyclohexene in acetic acid, discussing a possible third-order mechanism (rate = k[cyclohexene][HX]²).
3. Experimental Procedure
   
   SPIROANNELATION OF ENOL SILANES: 2-OXO-5-METHOXYSPlRO[5.4]DECANE

   Lee, T. V.; Porter, J. R. Org. Synth. 1995, 72, 189
   DOI: 10.15227/orgsyn.072.0189
   The first reaction in the above procedure involves two steps – addition of HBr across the double bond and converting the aldehyde to a dimethyl acetal.