Electrophilic Hydration to Make Alcohols
Table of contents
Electrophilic Hydration is the act of adding electrophilic hydrogens from a non-nucleophilic strong acid (which is a reuseable catalyst- examples include sulfuric and phosphoric acid) and applying certain temperatures to break the alkene's double bond. After a carbocation is formed, the water bonds with the carbocation to later form a 1º, 2º, or 3º alcohol on the alkane.
What Is Electrophilic Hydration?
Electrophilic Hydration s the exact reverse of a dehydration of an alcohol Alkenes by Dehydration of Alcohols It is used in "real life" to make alcohols for fuels and reagents for other reactions. The basic Reaction under certain temperatures (provided later) is:
Electrophilic hydrogens literally mean electron loving hydrogens (while nucleophile means nucleus loving). An electrophilic hydrogen means that it is basically a proton- a hydrogen stripped of its electrons. Electrophilic hydrogens are commonly used to help break double bonds or restore catalysts. For more about electrophiles and nucleophiles: SN2
How Does Electrophilic Hydration Work?
Mechanism for 3º Alcohol (1º and 2º function the same way):
Temperatures for Types of Alcohol Synthesis
Heat is used to catalyze the elctrophilic hydration, and since it is in equilbrium 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.
But...Why Does Electrophilic Hydration Work?
What is Regiochemistry and How Does It Apply?
Regiochemistry deals with where the substituent will bond on the product. Zaitsev's and Markovnikov's rules address regiochemistry, but Zaitsev's Rule applies to making an alkene while Markovnikov's Rule describes where the substituent will bond onto the product. Since we are synthesizing alcohol by electrophilic hydration, Markovnikov's Rule is the only rule that directly applies. See here for another in-depth explanation of regiochemistry Markovnikov explanation: Radical Additions--Anti-Markovnikov Product Formation
In the mechanism for a 3º alcohol drawn above, the red H was added to the least substituted carbon that was connected to the nucleophilic double bonds (it has less carbons attached to it). This means that the carbocation forms on the 3º carbon, causing it to be highly stabilized by hyperconjugation- electrons in nearby sigma (single) bonds will help fill the empty p orbital of the carbocation which lessens the positive charge. More substitution on a carbon means more sigma bonds are available to "help out" (by using overlap) with the positive charge, which means more carbocation stability. This translates into: carbocations will form on the most substituted carbon that was connected to the double bond. Carbocations are also stabilized by resonance, but resonance is not a large factor here since any carbon-carbon double bonds are used to initiate the reaction and other double bonded molecules can cause a completely different reaction.
If the carbocation does originally form on the less substituted part of the alkene, carbocation rearrangements will occur to form more substituted products by:
The nucleophile will attack the positive charge that has been formed on the most substituted carbon connected to the double bond, since the nucleophile is seeking that positive charge. In the mechanism for a 3º alcohol drawn above, water is the nucleophile. When the green H is removed from the water molecule, it is clear the alcohol has added to the most substituted carbon that was connected to the double bond. Hence, electrophilic hydration follows Markovnikov's Rule.
What is Stereochemistry and How Does It Apply?
Stereochemistry is how the substituent will bond on the product directionally. Dashes and wedges denote stereochemistry- whether the molecule or atom is going into or out of the plane of the board. Whenever the bond is a simple single straight line, the molecule that is bonded is equally likely to be found going into the plane of the board as it is out of the plane of the board. This means the product is a racemic mix.
Electrophilic hydration follows a stereochemistry where the substituent will be equally likely to bond going into the plane of the board as it is going out of the plane of the board. The 3º alcohol product could look like either of these:
Note: Whenever a straight line is used along with dashes and wedges on the same molecule, it could be denoting that the straight line bond is in the same plane as the board. Practice with a molecular model kit and attempting the practice problems at the end would help solve any unclarities about what a straight line is denoting.
Is this a Reversible Synthesis?
Yes, it is because an alkene in water is in equilibrium with the alcohol product. To sway the equilibrium one way or another, the concentration of the non-nucleophilic, strong acid and the temperature can be changed. For example:
Is There a Better Way to Add Water to Synthesize an Alcohol From an Alkene?
A better way does exist! How about Oxymercuration - Demercuration: A Special Electrophilic Addition? Oxymercuration does not allow for rearrangements, but it does require the use of mercury, which is highly toxic. Detractions for using electrophilic hydration to make alcohols include:
Write out the product of the reaction. Try to answer all 5 before checking your work under "Answers to Practice Problems"
1) Here is a test for your basic understanding:
2) What does the cyclopropane group do?
3) (Hint: What's different about this problem?)
4) (Hint: Careful of your stereochemistry!)
5) Write out any shifts as well as the major product:
Answers to Practice Problems
1) This is a basic electrophilic hydration.
2) What does the cyclopropane group do? The answer is additional side products, but the major product formed is still the same (the product shown). Depending on the temperatures used, the cyclopropane may open up into a straight chain which makes it unlikely that the major product will form on it (you may have seen the tip of the triangle as a 3º carbon, but after the reaction, it is unlikely that it will be a 3º carbon).
3) A hydride shift actually occurs from the top of the 1-methylcyclopentane to where the carbocation had formed.
4) This reaction will not have very good yields due to a very unstable intermediate. For a brief moment, carbocations can form on the two center carbons, which are more stable than the outer two carbons. The carbocations have an sp2 hybridization, and when the water is added on, the carbons change their hybridization to sp3. This makes the methyl and alcohol groups equally likely to be found going into or out of the plane of the paper- the product is racemic.
5) In the first picture shown below, an alkyl shift occurs but a hydride shift (which occurs faster) is possible. Why doesn't a hydride shift occur? The answer is because the alkyl shift leads to a more stable product. There is a noticeable amount of side product that forms where the two methyl groups are, but the major product shown below is still the most significant due to the hyperconjugation that occurs by being in between the two cyclohexanes.
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