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Alkene Reactions

By James Ashenhurst

Markovnikov Addition Of HCl To Alkenes

Last updated: February 27th, 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

  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?

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

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?

products from addition of hcl to alkenes are markovnikov selective give major product

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:

most substituted carbon has fewer hydrogens least substituted carbon has more hydrogens

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:

carbons that are equally substituted attached to equal numbers of hydrogens

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?

addition of hcl to alkene with rearrangement reaction alkyl shift

[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 


(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.

Comments

Comment section

6 thoughts on “Markovnikov Addition Of HCl To Alkenes

  1. I happened to stubble on this website which she is great amazing.! However the last problem of. CH3
    CH3-C-CH=CH2
    CH3
    hydrochlorination rxn contradicts what you write on the next article linked to this one. At the portion of the alky shift you show the H taken from HCl to follow Markovnikov’s; however, not here.. I may be wrong but this example shown above has caused me great confusion with its contradiction on thethe Article of: Markovnikov’s Rule – Why It Works.

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