Addition Pattern #1: The “Carbocation Pathway”

The Mechanism For Hydrohalogenation of Alkenes, Acid-Catalyzed Hydration of Alkenes, and Acid-Catalyzed Addition of Alcohols To Alkenes: The Carbocation Pathway In Alkene Addition Reaction Mechanisms

There are lots of patterns in organic chemistry. The sooner you see them, the further along you’ll get.

MOC: What were some of your biggest roadblocks in learning organic chemistry?

OCI: Not learning the patterns. I think I wasn’t told that there were patterns.

-from this post

Today we’ll talk about a key pattern in alkene addition reactions, followed by hydrohalogenation of alkenes (3 reactions), acid catalyzed hydration of alkenes, and acid catalyzed addition of alcohols to alkenes. It’s one of three key mechanistic pathways we’ll go through. This is the Carbocation Pathway.

Table of Contents

1. The Mechanism For Hydrohalogenation of Alkenes Goes Through A Carbocation Intermediate
2. The Mechanism For Acid-Catalyzed Hydration of Alkenes Goes Through The Exact Same Pattern
3. What Do The Mechanisms Of These Five Reactions Have In Common?
4. A Key Pattern In Alkene Addition Reactions: The Carbocation Pathway 
5. Notes
6. (Advanced) References and Further Reading

1. The Mechanism For Hydrohalogenation of Alkenes Goes Through A Carbocation Intermediate

The last several posts have primarily dealt with one reaction: the addition of HCl to alkenes. As we’ve seen, the reaction proceeds through attack of the alkene [the nucleophile] upon a proton [the electrophile], leading to formation of a carbocation. The carbocation, being an electrophile, is then attacked by chloride ion to give the alkyl halide.

The major product will be that which proceeds through the most stable carbocation, giving rise to the regioselective formation of  “Markovnikov” products where the chloride adds to the most substituted carbon of the alkene.

Since the reaction proceeds through a carbocation, and nucleophiles may attack carbocations from either face of their empty p orbital, this reaction pathway has no inherent stereoselectivity. A mixture of syn and anti products will be formed [where possible, of course] [Note 1]

Here’s the good news. If you understand how this reaction works, congratulations – you now understand how hydrobromination and hydroiodination of alkenes work as well!

These proceed through the exact same mechanism as we just described. So instead of having to learn three separate reactions, these are essentially three variations of the same reaction.

2. The Mechanism For Acid-Catalyzed Hydration of Alkenes Goes Through The Exact Same Pattern

By learning this mechanism, you’ve also learned the key steps in the mechanism for the acid catalyzed addition of water to alkenes (“hydration”) and the acid-catalyzed addition of alcohols to alkenes. There’s just one extra step we have to add at the end to make it complete.

Since our nucleophile is neutral, it will bear a positive charge after attacking the carbocation. This positive charge can be removed through deprotonation by a weak base. One little assumption here: we are using H₂O (or ROH in the second case) as solvent, so there is a whopping excess around to act in this capacity. [Note 2]

3. What Do The Mechanisms Of These Five Reactions Have In Common?

All five of these reactions have the following features in common:

• They proceed through a carbocation intermediate.
• The most stable carbocation will be formed preferentially (giving rise to “Markovnikov” regioselectivity)
• There is no inherent preference for syn or anti products (not stereoselective)

4. A Key Pattern In Alkene Addition Reactions: The Carbocation Pathway 

Do you see the power of understanding mechanisms in organic chemistry? Reactions that go through a similar mechanism are providing similar outcomes. All that’s changing is the identity of the atoms. This is the power of understanding mechanisms in organic chemistry. It can help us identify patterns.

It’s a little like learning a song on piano or guitar and then adapting it to a different key. Learning the song the first time is hard, but changing the key is easy since the relationships are preserved. The important thing is to notice this pattern. 

In summary: we can group these five reactions into a family, that all proceed through the same key steps. They all share the same pattern of regioselectivity and stereoselectivity.

Learn one mechanism, learn them all!

There’s one last wrinkle with this family of reactions: rearrangements are possible. We’ll talk about that next.

NEXT POST: Rearrangements in Alkene Addition Reactions

Notes

Note 1.  Although the reaction mechanism has no inherent bias for syn or anti stereochemistry, there are many cases of reactions where this reaction will be stereoselective on account of the three-dimensional structure of the starting alkene.

Note 2. Even though the acidity of the protonated product and the protonated solvent are roughly equal, because solvent is present in high excess relative to our product, equilibrium will favor formation of the deprotonated product. In practice, the reaction will be subjected to a mildly basic workup that neutralizes the excess strong acid, giving us the neutral product in the end.

(Advanced) References and Further Reading

Addition of HX to alkenes:

1. 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: 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. 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: 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. 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.
4. The Addition of Hydrogen Bromide to Simple Alkenes
   Hilton M. Weiss
   Journal of Chemical Education 1995, 72 (9), 848
   DOI: 1021/ed072p848
   A simple experiment suitable for undergraduate organic chemistry laboratory courses that demonstrates that it is possible for the intermediate carbocation to rearrange and give different products.Hydration of alkenes in H₃O⁺:
5. The Electrolyte Effects in the Hydration of Isobutene
   Frank G. Ciapetta and Martin Kilpatrick
   Journal of the American Chemical Society 1948, 70 (2), 639-646
   DOI: 1021/ja01182a062
   An early paper on the acid-catalyzed hydration of alkenes.
6. The Dependence of the Rate of Hydration of Isobutene on the Acidity Function, H₀, and the Mechanism for Olefin Hydration in Aqueous Acids
   Robert W. Taft Jr.
   Journal of the American Chemical Society 1952, 74 (21), 5372-5376
   DOI: 1021/ja01141a046
   This early paper demonstrates that the rate of olefin hydration increases with the acidity of the medium, providing evidence that a carbocation is the intermediate in the reaction – these become increasingly stable as the acidity of the medium increases.
7. Reversible hydration of 1,3-cyclohexadiene in aqueous perchloric acid
   J. L. Jensen and D. J. Carre
   The Journal of Organic Chemistry 1971, 36 (21), 3180-3183
   DOI: 10.1021/jo00820a022
   The authors mention in the introduction that the objective of this paper was to investigate the hydration of simple dienes, as a separate class from isolated alkenes, styrenes, and styrenes conjugated to another alkene.
8. General acid catalysis in the hydration of simple olefins. Mechanism of olefin hydration
   A. J. Kresge, Y. Chiang, P. H. Fitzgerald, R. S. McDonald, and G. H. Schmid
   Journal of the American Chemical Society 1971, 93 (19), 4907-4908
   DOI: 10.1021/ja00748a043
   This paper addresses the simple mechanism of hydration of simple olefins and gives evidence that it can be generalized.
9. Structural effects on the acid-catalyzed hydration of alkenes
   Vincent J. Nowlan and Thomas T. Tidwell
   Accounts of Chemical Research 1977, 10 (7), 252-258
   DOI: 1021/ar50115a004
   This is a useful account that reviews all the work done on investigating the acid-catalyzed hydration of alkenes up to that point. It also ties this topic to carbocation chemistry – the 2-norbornyl cation, which was a hot topic at the time, is mentioned towards the end.
10. Enthalpies of hydration of alkenes. 4. Formation of acyclic tert-alcohols
    Kenneth B. Wiberg and Shide Hao
    The Journal of Organic Chemistry 1991, 56 (17), 5108-5110
    DOI: 1021/jo00017a022
    A nice calorimetric study on the hydration of alkenes, determining the enthalpy of this reaction.