Hydration of Alkenes With Aqueous Acid

Hydration of Alkenes to Give Alcohols

• When alkenes are treated with aqueous acid (H₃O+) they can be converted to alcohols.
• Formation of the new C-OH bond tends to occur on the most substituted carbon of the alkene (“Markovnikov” regioselectivity) and is not stereoselective – a mixture of syn and anti addition occurs.
• The reaction proceeds through a carbocation intermediate. Protonation of the alkene occurs such that the most stable carbocation is formed.
• Hydration with acid can be accompanied by rearrangements if a more stable carbocation intermediate can be formed through a hydride or alkyl shift. To avoid this, oxymercuration of alkenes is an alternative that does not give rearrangement. (See article – Oxymercuration of Alkenes)
• Alcohols will also add to alkenes in the presence of acid. If an alkene and acid are present on the same molecule, intramolecular addition can occur.
• Anti-Markovnikov alcohols can be obtained through hydroboration (See article – Hydroboration-Oxidation of Alcohols)

Table of Contents

1. 1. Hydration of Alkenes With Acid
   2. Acid-Catalyzed Hydration of Alkenes Has Markovnikov Regioselectivity
   3. Hydration Is Not Stereoselective
   4. Mechanism for the Acid-Catalyzed Hydration of Alkenes
   5. Reaction Energy Diagram
   6. Carbocation Rearrangements in Hydration
   7. Addition of Alcohols To Alkenes With Acid
   8. Intramolecular Reactions of Alkenes and Alcohols
   9. Summary
   10. Notes
   11. Quiz Yourself!
   12. (Advanced) References and Further Reading

1. Hydration of Alkenes With Acid

When alkenes (olefins) are treated with strong aqueous acid (H₃O⁺) they undergo net addition of water across the double bond (“hydration”). A new C-H bond and and a new C-OH bond are formed, and the C-C pi bond is broken.

If we add up the bond dissociation energies (BDE’s) of the bonds that form (C-H, about 98 kcal/mol, and C-O about 85 kcal/mol) and subtract their sum from those of the bonds that break (C-C pi, about 63 kcal/mol, and H-O, about 111 kcal/mol) the reaction is exothermic by about 10 kcal/mol for the addition of H₂O. [Link to useful table]

H₂O will not add to alkenes by itself, however. Strong acid is required. The reagent for this reaction is commonly written as “H₃O+” which is often just a combination of water and sulfuric acid. (Acids like H₂SO₄ are used since the conjugate base, HSO₄(-), is a poor nucleophile and will not compete with H₂O in attack on the carbocation). [Note 1]

This reaction bears many similarities to the addition of H–X (HCl, HBr, HI) across alkenes in that it is regioselective for formation of the more substituted alcohol (“Markovnikov” selectivity) and also that carbocation rearrangements can occur (See: Rearrangements in Alkene Addition Reactions)

2. Hydration of Alkenes Has “Markovnikov” Regioselectivity

Any time an addition of two different atoms occurs across a double bond there is the possibility of forming constitutional isomers. Hydration of propene, for example, could result in the formation of either 1-propanol or 2-propanol.

A regioselective reaction is one that shows a strong preference for the formation of one constitutional isomer (“regioisomer”) over another.

In the addition of H₂O across an alkene catalyzed by acid, the major product results from formation of the C-OH bond on the most substituted carbon of the alkene – that is, the sp² -hybridized carbon of the alkene directly bonded to the most carbon atoms.

This is known as “Markovnikov” regioselectivity. [See article – Addition of HX to Alkenes and Markovnikov’s Rule]

[Alternatively, you can look at it as the H forms on the carbon containing the most bonds to hydrogen. A useful mnemonic here is “the rich (in C-H bonds) get richer (get another C-H bond)” ].

Here are some more examples of the reactions of H₃O+ with alkenes. The major product has the C-OH bond form on the more substituted carbon in each case.

Depending on the structure of the starting alkene, the resulting product(s) may be a mixture of structural isomers, a mixture of enantiomers, diastereromers, or a single achiral product. [For more on this topic, see What’s A Racemic Mixture, or Types of Isomers]

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See if you can determine the major product in the reaction below:

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3. Acid-Catalyzed Hydration is Not Stereoselective

The pi bond in alkenes is flat and has two faces.

When addition occurs such that both new bonds to carbon form on the same face of the alkene, we call this syn addition. When the two bonds are formed on opposite faces of the alkene, it is called anti addition.

(“syn”-  and “anti” are subtly different from “cis” and “trans” in that they refer to dihedral angle, as opposed to orientation about a double bond or small ring. See Staggered vs. Eclipsed Conformations of Ethane)

Some reactions, like hydroboration, are stereoselective for syn addition products. Others, like bromination of alkenes, are stereoselective for anti addition products. [See posts – Hydroboration of Alkenes, Halogenation of Alkenes]

In contrast, hydration of alkenes is not stereoselective. In the example below, the product where C-OH and C-H have added to the same face of the cyclohexane ring (“syn addition”) is formed in about equal proportion to the product where C-OH and C-H are formed on opposite faces of the cyclohexane ring (“anti addition”). [Reference]

(Note also that in the products above, the syn and anti products are each formed as a racemic mixture of enantiomers.)

See if you can draw the products of the following reaction involving a deuterated alkene.

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4. Mechanism of Acid-Catalyzed Hydration

So how does this reaction work, anyway?

Well, any mechanism for hydration will have to explain some important observations.

• The high selectivity for Markovnikov products
• The lack of stereoselectivity
• The formation of products that arise from carbocation rearrangements.  More on this below  [For a full discussion, see Carbocation Rearrangements in Alkene Addition Reactions]

The best mechanistic explanation for this reaction is that it proceeds through a carbocation intermediate, similar to the addition of H-X to alkenes.

In the first step, the alkene is protonated by strong acid, giving a carbocation:

Protonation could potentially occur on either carbon of the alkene, but protonation that results in the most stable carbocation will be favored.

Since carbocations tend to increase in stability as the number of substituents are added, this helps to explain the observed Markovnikov selectivity. (See article: Carbocation Stability) 

(More specifically, the reaction will proceed through the lowest-energy transition state, which is the one where the carbon best able to stabilize positive charge will be favored. ]

Carbocations are excellent Lewis acids, having only six valence electrons and an empty p-orbital. They will readily combine with the best nucleophile present in solution.

In this case the best nucleophile is H₂O, which adds to the carbocation to give R-OH₂(+). (For hydration, sulfuric acid is often used as opposed to hydrohalic acids like H-Cl or H-Br, since the conjugate base HSO₄(-) is poor nucleophile and less likely to compete with H₂O than Cl(-) or Br(-) )

Deprotonation of the oxonium ion by a weak base then gives the neutral alcohol. Note that in this case, H₃O+ is regenerated, so acid acts as a catalyst here.

5. The Reaction Energy Diagram For Acid-Catalyzed Hydration

Reaction energy diagrams are helpful for visualizing changes in energy as reactants are transformed into products. Changes in energy (Δ E) are graphed on the y-axis, while reaction progress from reactants to products is graphed on the x-axis.

In a reaction energy diagram, the local maxima (“peaks”) represent transition states which include partial bonds and partial charges and exceptionally brief lifetimes.

Local minima (“valleys”) represent intermediates with full charges and bonds, and can (at least potentially) be observed in solution. 

The rate-determining step is the step with the greatest difference in energy (Δ E) between reactants and the transition state.

In hydration, the rate-determining step corresponds to the first step – the protonation of the alkene with strong acid, which then results in the carbocation intermediate. In the second step (transition state #2) water adds to the carbocation, giving the protonated alcohol (oxonium ion) intermediate. This is followed by a third step (transition state #3) –  deprotonation to give the neutral alcohol.

Although not shown in the diagram above,  protonation of the alkene to give the less substituted carbocation would proceed through a significantly higher-energy transition state and also result in a significantly higher-energy carbocation intermediate.

6. Carbocation Rearrangements

Hydration of alkenes can be accompanied by rearrangements if a hydride or alkyl shift results in a more stable carbocation. (See, for example – Rearrangement Reactions – Hydride Shifts)

For example, hydration of the alkene below results in a considerable amount of a tertiary alcohol instead of the expected secondary alcohol product that would result from a simple Markovnikov addition.

Why is that? See if you can draw the mechanism of the reaction that leads to this product:

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In this case, a more stable tertiary carbocation is generated after migration of the CH₃ group on the adjacent carbon.

Note that if one wants to avoid carbocation rearrangements in the formation of alcohols with Markovnikov selectivity, a good strategy is to use oxymercuration-demercuration instead. (See article: Oxymercuration Demercuration of Alkenes) 

For more examples of rearrangement quizzes, see the “Quiz Yourself” section below. 

7. Addition of Alcohols To Alkenes With Acid

If an alcohol is used in place of water, subsequent treatment of alkenes with an acid catalyst can result in ethers.

The reaction is essentially the same as that for the reaction with H₂O. Protonation results in the most stable carbocation, which is then attacked by the alcohol. Deprotonation then gives the ether.

See if you can draw the product in the reaction below:

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This reaction can come in handy for making ethers that are otherwise difficult to make through, say, an S_N2 reaction between an alkoxide and an alkyl halide. [Note 2]

Much later, in the chapter on reactions of aromatic compounds, we’ll see examples of using t-butyl ethers as temporary protecting groups for aromatic alcohols – see this article on Blocking Groups. 

8.  Intramolecular Reactions of Alcohols and Alkenes

If an alcohol and an alkene are present on the same molecule, then treatment with acid can result in the intramolecular formation of an ether, resulting in a new ring.

It works best for the formation of 5- and 6-membered rings, as these rings are formed fastest and also have the least ring strain.

See if you can draw the mechanism of this reaction:

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Watch out for these kinds of examples, as they are commonly found on exams!

9. Summary

Hydration of alkenes and the addition of alcohols to alkenes are two examples of reactions that pass through what I sometimes call the “carbocation pathway” of alkene addition reactions.These reactions, along with addition of H-X to alkenes, all have the following in common.

• They pass through an intermediate carbocation and are selective for Markovnikov products
• They give a mixture of syn and anti– addition products (i.e. are not stereoselective)
• Carbocation rearrangements may occur.

For more, (See article – Alkene Addition Pattern #1 – The Carbocation Pathway)

Notes

Note 1. Even if HSO₄(-) does add to the carbocation to give an alkyl sulfate, these are quite readily hydrolyzed in water to give alcohols.

Note 2. Although it should be noted that adding acid to alcohols also invites the possibility of elimination reactions to give alkenes! For our purposes, something like oxymercuration is probably a better choice for making ethers from alkenes, since it is not accompanied by elimination.

Quiz Yourself!

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(Advanced) References and Further Reading

1. 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: 10.1021/ja01182a062
   An early paper on the acid-catalyzed hydration of alkenes.
2. 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: 10.1021/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.
3. The Steric Course of Hydration of 1,2-Dimethylcyclohexene
   CAROL H. COLLINS and GEORGE S. HAMMOND
   The Journal of Organic Chemistry 1960 25 (6), 911-913
   DOI: 10.1021/jo01076a009
   In this study, 1,2-dimethylcyclohexene was treated with 0.1 M nitric acid for 1 day at 50°C and a 52:48 ratio of trans to cis alcohols was obtained, showing the reaction is not stereoselective.
4. 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
   The key finding of this paper is that 2,3-dimethyl-2-butene and trans-cyclooctene undergo hydration with general acid catalysis and therefore other alkenes do as well (i.e. direct protonation of the alkene with no intermediate pi complex).
5. 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: 10.1021/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.
6. 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: 10.1021/jo00017a022
   A nice calorimetric study on the hydration of alkenes, determining the enthalpy of this reaction.
7. PREPARATION OF tert-BUTYL ARYL ETHERS
   DONALD R. STEVENS
   The Journal of Organic Chemistry 1955 20 (9), 1232-1236
   DOI: 10.1021/jo01126a010
   “tert-alkyl aryl ethers can be prepared in fairly high yields by simply passing isobutylene into the phenol at relatively low temperatures and in the presence of only a trace of sulfuric acid as the catalyst.”