Alkene Reactions
Markovnikov Addition Of HCl To Alkenes
Last updated: November 17th, 2022 |
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
- 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?
- Notes
- (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:
- 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. - 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). - Experimental Procedure
SPIROANNELATION OF ENOL SILANES: 2-OXO-5-METHOXYSPlRO[5.4]DECANELee, 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.
Thank you James. But i have a question. Some sources say that the minor product we considered here does not form. So it does form or not?
A small amount of the “anti-Markovnikov” product will always form. A ballpark estimate is 80 / 20 ratio of markovnikov to anti-markovnikov but it can vary considerably.
You are correct. Fixed, finally!
TYPO: In the diagram in section 2, the red and blue labels are the wrong way round.
Does markovnikov cause steric hindrance?
No, it is irrelevant to steric hindrance.
These notes helps me to clear my doubts and also helps me alot in doing my tutorials
Fantastic explanation
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.