Mason Delacroix
There are several ways to convert an alkene into an alcohol. Alkenes are unsaturated organic compounds that contain carbon-carbon double bonds, while alcohols are hydroxy derivatives of aliphatic hydrocarbons. One method of conversion is acid-catalyzed hydration, which involves reacting the alkene with water to form an alcohol. Oxymercuration-demercuration is another method, where the alkene reacts with mercuric acetate and water to form an intermediate product, followed by the addition of sodium borohydride to replace the mercuric portion with hydrogen. Hydroboration-oxidation is a third process, where the alkene reacts with borane, followed by the substitution of a hydroxyl group for the boryl unit in the presence of hydrogen peroxide and sodium hydroxide.
| Characteristics | Values |
|---|---|
| Number of Common Ways to Convert an Alkene to an Alcohol | 3 |
| First Method | Acid-catalyzed hydration |
| Second Method | Hydroboration-oxidation |
| Third Method | Oxymercuration-demercuration |
| First Step in Acid-catalyzed Hydration | Alkene reacts with water |
| Second Step in Acid-catalyzed Hydration | Formation of alcohol and saturated compound |
| General Reaction in Hydroboration-oxidation | \(R-C=C-R+BH3\to R-C-C-OH\) |
| First Step in Oxymercuration-demercuration | Alkene reacts with mercuric acetate and water |
| Second Step in Oxymercuration-demercuration | Formation of an intermediate |
| Third Step in Oxymercuration-demercuration | Replacement of mercuric portion with hydrogen |
| Alternative Method | Epoxidation followed by a chlorine nucleophile |
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What You'll Learn
Acid-catalyzed hydration
The acid-catalyzed hydration of alkenes involves the addition of water to alkenes to form alcohols. This reaction is also known as electrophilic hydration, which involves the addition of an electrophilic hydrogen from a non-nucleophilic strong acid (a reusable catalyst), such as sulfuric or phosphoric acid. The acid is necessary as the water by itself is a weak acid and cannot protonate the double bond.
The reaction follows a stepwise mechanism: the alkene is protonated by the acid, giving a carbocation. This carbocation intermediate has the trigonal planar geometry of sp2 hybridization, allowing the subsequent reaction with water from either orientation. The protonation could occur on either carbon of the alkene, but it will favour the formation of the most stable carbocation. The carbocation formed is prone to rearrangement, and the possibility of rearrangements during the addition of water to alkenes should be kept in mind.
Once the carbocation is formed, it is attacked by the water, forming an oxonium ion. After the deprotonation of the oxonium ion, the corresponding alcohol is formed as the final product. The proton in the oxonium ion can be deprotonated by any base present, including the conjugate base of the acid used as a catalyst, or even by another alkene molecule.
The acid-catalyzed hydration of alkenes is reversible, and the extent of the reaction is determined by the equilibrium constant. The reaction can be carried out by adding excess water to increase the yield of products. The reverse reaction, the acid-catalyzed dehydration of the alcohol to form the alkene, can be promoted by removing water from the reaction.
The Markovnikov's rule is observed in the acid-catalyzed hydration of alkenes, with the alcohol (OH group) added to the most substituted carbon on the double bond. However, if an unsymmetrical alkene is used, there are two possibilities for where the OH group can be placed.
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Oxymercuration-demercuration
The first step of this mechanism involves the formation of a bond between the pi electrons and mercury, while the lone pair on the mercury simultaneously bonds with the other vinyl carbon, creating a mercurium ion bridge. This bridge forms in conjunction with the loss of an acetate ion. The mercurium ion is positively charged and very reactive, and it is immediately attacked by water in an SN2 fashion, opening up the ring. This results in the formation of a mercurial alcohol.
The second step involves the addition of sodium borohydride (NaBH4) under basic conditions. This reduces the acetoxymercury group and replaces it with hydrogen, forming a new C-H bond. This step is believed to follow a radical mechanism that is not typically covered in undergraduate courses. Overall, the oxymercuration-demercuration mechanism follows Markovnikov's Regioselectivity, with the OH group attached to the most substituted carbon and the H attached to the least substituted carbon.
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Hydroboration-oxidation
In the first step, the hydroboration step, a C-C pi bond is broken, and a C-H bond and a C-B bond are formed. The borane adds to the least substituted side of the double bond to make the alkyl borane. This addition is concerted, meaning that hydrogen and BH2 are added simultaneously, and the borane and hydrogen must add to the same face of the carbon-carbon bond. This is known as syn addition.
In the second step, the oxidation step, the C-B bond is replaced with a C-OH bond. This is achieved by oxidation of the resulting organoborane with hydrogen peroxide (H2O2) in the presence of sodium hydroxide (NaOH). This substitutes a hydroxyl group (OH) for the boryl unit (BH2) to make the anti-Markovnikov alcohol.
The notable outcome of hydroboration-oxidation is the formation of an alcohol on the least substituted carbon of the alkene, also known as "anti-Markovnikov" regioselectivity. This is in contrast to the “Markovnikov” selectivity for the formation of alcohols from alkenes using acid-catalyzed hydration or oxymercuration.
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Markovnikov's Rule
In organic chemistry, Markovnikov's rule (or Markownikoff's rule) describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870.
The chemical basis for Markovnikov's Rule is the formation of the most stable carbocation during the addition process. Adding the hydrogen ion to one carbon atom in the alkene creates a positive charge on the other carbon, forming a carbocation intermediate. The more substituted the carbocation, the more stable it is, due to induction and hyperconjugation. The major product of the addition reaction will be the one formed from the more stable intermediate.
Not all reactions follow Markovnikov's rule, and some exhibit anti-Markovnikov behaviour. Anti-Markovnikov behaviour is observed when the halogen adds to the less substituted carbon, the opposite of a Markovnikov reaction. To make the anti-Markovnikov product, hydroboration is used, where borane (BH3) is added to the alkene, followed by the addition of hydrogen peroxide (H2O2) and sodium hydroxide (NaOH).
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Anti-Markovnikov alcohol
Turning an alkene into an alcohol involves a process called hydration, which adds water across the alkene's double bond. The addition of the alcohol group (OH) to the most substituted carbon results in a Markovnikov product, whereas adding it to the least substituted carbon results in an anti-Markovnikov product.
The hydroboration-oxidation reaction is a two-step pathway used to produce anti-Markovnikov alcohols. In the first step, borane (BH3) is added to the least substituted side of the double bond, forming an alkyl borane. This addition is concerted, meaning that the borane and hydrogen must add to the same face of the carbon-carbon bond (known as syn addition). In the second step, hydrogen peroxide (H2O2) and sodium hydroxide (NaOH) are introduced, substituting a hydroxyl group (OH) for the boryl unit (BH2) to create the anti-Markovnikov alcohol. This reaction is stereospecific, meaning that hydroboration occurs on the same face of the double bond, resulting in cis stereochemistry.
The oxymercuration-demercuration reaction is another method for producing alcohols from alkenes. It involves attacking the double bond with mercuric acetate (Hg(OAc)2) to form a three-membered ring intermediate called a mercurinium ion. Water then attacks the most highly substituted carbon, creating a mercurial alcohol after losing a proton. Subsequently, sodium borohydride (NaBH4) is added, replacing the mercuric portion with hydrogen to yield the final alcohol.
The choice between hydroboration-oxidation and oxymercuration-demercuration depends on the desired product. Hydroboration-oxidation is typically employed to obtain anti-Markovnikov alcohols, while oxymercuration-demercuration leads to Markovnikov-product alcohols.
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Frequently asked questions
There are three common ways to convert an alkene into an alcohol: acid-catalyzed hydration, hydroboration-oxidation, and oxymercuration-demercuration.
Acid-catalyzed hydration involves the reaction of an alkene with water, resulting in the formation of an alcohol. The reaction can be carried out by adding excess water to increase the yield of products.
Hydroboration-oxidation involves reacting an alkene with borane, resulting in the formation of an alcohol. The addition of borane (BH3) and hydrogen must occur on the same face of the carbon-carbon bond.
Oxymercuration-demercuration involves reacting an alkene with mercuric acetate and water to form an intermediate. This intermediate then reacts with sodium borohydride to form the alcohol.
The mechanism of hydration involves the electrophilic addition of a proton (or acid) to the double bond in the alkene, forming a carbocation intermediate. Addition of water in the second step results in the formation of an oxonium ion, which, upon deprotonation, gives the alcohol.
Other methods include epoxidation followed by reaction with a chlorine nucleophile, such as hypochlorous acid or HCl.
