# 8.5: Hydration of alkenes - addition of H2O by oxymercuration (2023)

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##### Goals

After completing this section, you should be able to.

1. Write an equation for the hydration of an alkene with sulfuric acid.
2. Write an equation for the formation of an alcohol from an alkene by the oxymercuring-demercuring process.
3. Identify the alkene, the reactants, or both that must be used to produce a specific alcohol by the oxymercuring-demercuring process.
4. Write the reaction mechanism of an alkene with mercury(II) acetate in aqueous tetrahydrofuran (THF).
##### key terms

Make sure you can define the key terms below and use them in context.

• hydration
• oxymercuration
##### study notes

oxymercurationis the reaction of an alkene with mercury(II) acetate in aqueous THF followed by reduction with sodium borohydride. The final product is an alcohol.

It is important that you recognize the similarity between the mechanisms of bromination and oxymercuration. Recognizing these similarities will help you reduce the amount of factual information you need to remember.

Mercury acetate, or mercury(II) acetate to give it the preferred IUPAC name, is written as Hg(OAc).2; comparing this formula with the formula Hg(O2CCH3)2, you can equate Ac with -COCH3. In fact, Ac is an abbreviation for the acetyl group with the structure shown below, as well as other similar abbreviations you will come across.

 Ac (acetyl) I (methyl) Y (ethyl) PRnorte(norte-propyl) PRUE(isopropyl) OT(Third-Butyl) Ph (phenyl)

## What is electrophilic hydration?

Electrophilic hydration is the process of adding electrophilic hydrogen to a strong non-nucleophilic acid (a reusable catalyst, examples include sulfuric and phosphoric acids) and applying appropriate temperatures to break the alkene double bond. Aftercarbocationis formed, water combines with the carbocation to form a first, second, or third alcohol in the alkane. Electrophilic hydration is the reverse dehydration of alcohols and finds practical application in the production of alcohols for fuels and reagents for other reactions. The basic reaction at certain temperatures (given below) is as follows:

Electrophilic hydrogen is essentially a proton: a hydrogen atom stripped of its electrons. Electrophilic hydrogen is commonly used to break double bonds or to regenerate catalysts (seeSN2for more details).

## How does electrophilic hydration work?

### Mechanism for the 3rd alcohol (1st and 2nd mechanisms are similar):

Hydration is the process of adding water to an alkene to give an alcohol. Acid catalyzed hydration is when a strong acid is used as a catalyst to start the reaction, but let's look at the mechanism below and discuss the steps.

Step 1: A hydrogen atom from the acid is attacked by the nucleophilic pi electrons in the double bond. This is similar to the other reactions of alkenes that we have seen so far. In this process, a new C-H bond is formed to create the more stable carbocation.

Step 2: A nucleophilic water attacks or donates a lone pair to the positively charged carbon in the carbocation intermediate generated in the first step. There is a new C-O bond, with O having a formal charge of +1. The product is a protonated alcohol.

Step 3: To obtain the neutral alcohol product, the final step is to deprotonate the oxygen atom with the +1 formal charge using the acid. This final step regenerates the acid catalyst and produces the neutral alcohol product.

(Video) Oxymercuration Demercuration Reaction Mechanism

### Temperatures for types of alcohol synthesis

Heat is used to catalyze electrophilic hydration; Since the reaction is in equilibrium with the dehydration of an alcohol, which requires higher temperatures to form an alkene, lower temperatures are required to form an alcohol.The exact temperatures used are highly variable and depend on the product being formed.

### What is regiochemistry and how is it used?

Regiochemistry is concerned with where the substituent bonds attach to the product.Zaitsev'arenaMarkovniksThe rules relate to regiochemistry, but Zaitsev's rule applies when synthesizing an alkene, while Markovnikov's rule describes where the substituent attaches to the product. In the case of electrophilic hydration, Markovnikov's rule is the only ruleImmediatelyis applicable. The following is a detailed explanation of the regiochemical-Markovnikov explanation: Radical additions - Formation of anti-Markovnikov products

In the mechanism shown above for a third alcohol, the H adds to the least substituted carbon attached to the nucleophilic double bonds (has the fewest carbons attached). This means that the carbocation forms on the third carbon, which makes it very stable.hyperconjugationElectrons in close sigma (single) bonds help fill the empty p orbital of the carbocation, thereby reducing the positive charge. More substitution on one carbon means more sigma bonds are available to "help" with the positive charge (using overlap), creating a greatercarbocation stability. In other words,Carbocations form on the most substituted carbon.connected to the double bond. Carbocations are also stabilized by resonance, but resonance is not an important factor in this case, since any carbon-carbon double bond is used to start the reaction, and other molecules with double bonds can produce an entirely different reaction.

If the carbocation originally formed in the less substituted portion of the alkene, rearrangements of the carbocation occur to form more substituted products:

• Hydride Changes:a hydrogen atom attached to a carbon atom adjacent to the carbocation lets that carbon bind to the carbocation (after the hydrogen removes the two electrons from the single bond, it is called a hydride). As a result, the formerly adjacent carbon becomes a carbocation and the former carbocation becomes an adjacent carbon atom.

• rental displacements:if no hydrogen atoms are available for a hydride change, a complete methyl group will undergo the same change.

The nucleophile attacks the positive charge formed on the most substituted carbon attached to the double bond because the nucleophile searches for that positive charge. In the mechanism shown above for a third alcohol, water is the nucleophile. After one H atom is removed from the water molecule, the alcohol is attached to the more substituted carbon. With that,Electrophilic hydration follows the Markovnikov rule.

### What is stereochemistry and how is it applied?

Stereochemistry deals with how substituents form directional bonds with the product.. Dashes and wedges denote stereochemistry, showing whether the molecule or atom is moving in or out of the panel plane. As long as the bond is a simple straight line, the bonded molecule has the same probability of entering the plane of the plate as it is of leaving the plane of the plate. That showsthe product is a racemic mixture.

(Video) Hydrocarbons 15 : Properties of Alkenes 4 : Addition of Water : OxyMercuration - Demercuration

Electrophilic hydration adopts a stereochemistry in which the substituent is just as likely to attach pointing in the plane of the plate as pointing away from the plane of the plate. The third alcoholic product of the following reaction can resemble any of the following products:

There is no stereochemical control in acid-catalyzed hydration reactions. This is due to the trigonal plane, sp2Nature of the Intermediate Carbocation. Water can act as a nucleophile to form a bond on both sides of the carbocation, leading to mixed stereochemical results.

Note: Whenever a straight line is used along with dashes and wedges in the same molecule, it can mean that the straight line is in the same plane as the checkerboard. Practice with a molecular model kit and at the end try to solve the practice problems to clear up any confusion.

### Is this a reversible synthesis?

Electrophilic hydration is reversible, since an alkene in water is in equilibrium with the alcoholic product. To affect the equilibrium in one way or another, the temperature or the concentration of the strong non-nucleophilic acid can be changed. For example:

• Less sulfuric or phosphoric acid and an excess of water help to synthesize more alcoholic product.
• Lower temperatures help to synthesize more alcoholic product.

### Is there a better way to add water to synthesize an alcohol from an alkene?

There is a more efficient way: oxymercuration - demercuration, a special type of electrophilic addition. Oxymercuration does not allow rearrangements, but requires the use of mercury, which is highly toxic. The advantages and disadvantages of using electrophilic hydration to produce alcohols include:

• Consideration of carbocation rearrangements
• Low yields because reactants and products are in equilibrium.
• Allow mixes of products (such as a (R)-enantiomer and one (S)-Enantiómero)
• With sulfuric or phosphoric acid

Oxymercuration is a special electrophilic addition. It is antistereospecific and regioselective. Regioselectivity is a process by which the substituent chooses a direction in which it is preferred over all other possible directions. The good thing about this reaction is that there is nocarbocation rearrangementdue to stabilization of the reactive intermediate. A similar stabilization can also be seen inbromationas well as alkenes.

## Introduction to Oxymercuration

One of the main advantages of oxymercuration is that rearrangements of carbocations cannot occur under these conditions (Hg(OAc)2, h2EITHER). Carbocation rearrangement is a process in which the intermediate carbocation can undergo a methyl or alkyl change to form a more stable ion. due to a possiblecarbocation rearrangement, the following reaction would not produce the product shown in high yields. Rather, the oxymercuration reaction would continue to form the desired product.

In this reaction, a mercury acting as a reagent attacks the double bond of the alkene to form aMercurinio-Ionenbrücke. A water molecule then attacks the more substituted carbon to open the mercury ion bridge, followed by proton transfer to the water molecule from the solvent.

(Video) Oxymercuration Demercuration Reduction - Alkene Reaction Mechanism by Leah Fisch

The organomercury intermediate is then reduced with sodium borohydride; the mechanism for this final step is beyond the scope of our discussion here. Note that the general oxymercuration-demercuration mechanism follows Markovnikov regioselectivity, with the OH group attached to the more substituted carbon and the H attached to the less substituted carbon. However, the reaction is useful because strong acids are not required and carbocation rearrangements are avoided because a discrete intermediate carbocation is not formed.

It is important to note that for the mechanism shown above, the enantiomer of the product shown is also formed. This is the result of the formation of the mercury ion below the alkene in the first step.

## references

1. Vollhardt, K. Peter C. Structure and function of organic chemistry. New York: W. H. Freiman, 2007.
2. Smith, Michael B., and Jerry March. Reactions, mechanisms and structure of March Advanced Organic Chemistry (March Advanced Organic Chemistry). New York: Wiley-Interscience, 2007 2007.
3. Roderic P. Quirk, Robert E. Lea, Reductive demercuration of hex-5-enyl-1-mercuric bromide by metal hydrides. rearrangement, isotopic effects and mechanism,Jelly. chemistry soc., 1976, 98 (19), S. 5973–5978.

## Some practical problems

##### practice problems

What are the end products of these reactants?

Respondedor

The end product of these exercises is quite similar. First locate where the double bond is on the reactant side. Then look at which substituents are attached to each side of the double bond and add the OH group to the side with the stronger substituent and the hydrogen to the side with the smaller substituent.

##### but problems

Predict the product of each reaction.

1)

2) How does the cyclopropane group affect the reaction?

(Video) 8.3 Acid Catalyzed Hydration, Oxymercuration Demercuration, and Hydroboration Oxidation

4) (Hint: keep stereochemistry in mind.)

5) Report all shifts as well as the main product:

Respondedor

1) This is a simple electrophilic hydration.

2) The answer is additional by-products, howeverthe principal product formed remains the same(the product shown). Depending on the temperatures used, cyclopropane can open up into a linear chain, making formation of the main product unlikely (after reaction, the third carbon atom is unlikely to remain as such).

3) There is actually a hydride shift from carbon 3 of 3-methylcyclopentene to the place where the carbocation was formed.

4)This reaction has low yields due to a very unstable intermediate.. Carbocations that are more stable than the outer two carbons can briefly form on the middle two carbons. Carbocations have a sp2hybridization, and when water is added, the carbons change their hybridization to sp3. This makes the methyl and alcohol groups equally likely to be inside or outside the plane of the paper: the product is racemic.

5) In the first figure below, an alkyl change is taking place, but a hydride change (which occurs faster) is possible. Why doesn't hydride displacement occur? the answer is becausealkyl change leads to a more stable product. A notable number of by-products form where the two methyl groups are located, but the main product below remains the most important due to the hyperconjugation that occurs when it is located between the two cyclohexanes.

• Lance Peery (UCD), Duyen Dao-Tran (UCD)
• Organic chemistry with a biological approachVontim soderberg(University of Minnesota, Morris)

• jim clark (Chemguide.es)

• Layne Morsch (University of Illinois, Springfield)
• Dra. Krista Cunningham

• Lauren Reutenauer (Amherst College)
(Video) Oxymercuration-Demercuration and Hydroboration-Oxidation Basics, Regiochemistry

## Videos

1. Alkene to Alcohol (Oxymercuration Mechanism)
(chemistNATE)
2. Oxymercuration-Demercuration
(Professor Dave Explains)
3. Oxymercuration-demercuration of an alkene
(J Michelle Leslie)
4. Oxymercuration Demercuration/ Addition of water
(Prakash Raja)
5. Oxymercuration and Demercuration of Alkene | Hydration of Alkene by Oxymercuration
(Chemistry Vidya)
6. 8.3b Hydration Oxymercuration Demercuration
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