Theo Nash
The addition of alcohol to a double bond is a chemical process that involves converting alkenes to alcohols through hydration. This reaction is similar to the addition of a hydrohalic acid across a double bond. There are two ways to achieve hydration: the first method involves adding an alcohol (OH group) to the most substituted carbon on the double bond, resulting in the Markovnikov product; the second method involves adding the alcohol to the least substituted carbon, forming the anti-Markovnikov product. The Markovnikov rule dictates that the addition of a proton occurs at the less substituted carbon, while the -OH group is added to the more substituted carbon. This process can be facilitated by oxymercuration-demercuration, hydroboration-oxidation, or direct acid-catalyzed hydration.
| Characteristics | Values |
|---|---|
| Mechanism | Hydration, or adding water across a double bond to make an alcohol |
| Reagents | H2O with any catalytic acid such as H2SO4; Grignard reagent; Organolithium reagent |
| Product | Turn double bond into single and add OH to the most substituted carbon from the double bond (Markovnikov rule); Turn double bond into single and add two OHs, one on each carbon from the double bond |
| Stereochemistry | syn (same side, both wedge or both dash) |
| Reaction | Oxymercuration-demercuration; Hydroboration-oxidation |
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What You'll Learn
Using oxymercuration-demercuration
Oxymercuration-demercuration is a two-step organic reaction used to convert alkenes into alcohols. It is a regioselective reaction that follows Markovnikov's rule, where the OH group attaches to the more substituted carbon of the double bond. This is in contrast to hydroboration-oxidation, which gives anti-Markovnikov products. The reaction proceeds as follows:
Oxymercuration
The alkene reacts with Hg(OAc)2 to form a three-membered mercurinium ion intermediate. This is a cyclic mercurinium ion that does not rearrange because much of the positive charge is on the mercury atom. The nucleophilic double bond attacks the mercury ion, ejecting an acetoxy group. The electron pair on the mercury ion then attacks a carbon on the double bond, forming a mercurinium ion with a positive charge on the mercury atom. The electrons in the highest occupied molecular orbital of the double bond are donated to mercury's empty 6s orbital, and the electrons in mercury's dxz (or dyz) orbital are donated to the lowest unoccupied molecular orbital of the double bond.
Demercuration
The nucleophilic water molecule attacks the more substituted carbon, liberating the electrons participating in its bond with mercury. The electrons collapse to the mercury ion and neutralize it. The intermediate organomercury compound is almost never isolated. Instead, demercuration is performed with sodium borohydride (NaBH4), which rapidly breaks the C-Hg bond and forms a new C-H bond. This replaces the organomercury group with hydrogen, yielding the final alcohol product.
Overall, the oxymercuration-demercuration mechanism follows Markovnikov's regioselectivity, with the OH group attached to the most substituted carbon. The reaction is useful because it does not require strong acids, and carbocation rearrangements are avoided as no discreet carbocation intermediate forms. However, it is important to note that mercury is highly toxic.
Oxymercuration-demercuration can also be performed in an alcohol solvent, where the product is an ether. In this case, the alcohol acts as the nucleophile and attacks the mercurinium ion, resulting in an ether product.
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Using hydroboration-oxidation
Hydroboration-oxidation is a valuable laboratory method for the stereoselective and regioselective conversion of alkenes into alcohols. It is a two-step reaction that does not require rearrangement or activation by a catalyst.
The first step involves the hydroboration of the alkene, which is the addition of borane (BH3) to the double bond. This breaks the C-C pi bond and forms a C-H and a C-B bond. The boron adds to the less substituted carbon of the alkene, while the hydrogen adds to the more substituted carbon. This results in an organoborane compound, which is then treated with an oxidant like H2O2 to obtain an alcohol.
In the second step, an oxidant such as hydrogen peroxide is added, usually in the presence of a base such as NaOH or KOH. This breaks the C-B bond and forms a new C-O bond, replacing the boron-carbon bonds with carbon-OH group bonds. The oxidation step converts the boron into boric acid and yields the desired alcohol product.
It is important to note that borane (B2H6) is a toxic gas that ignites spontaneously in air. Therefore, it is commercially available in ether and tetrahydrofuran (THF), which is the commonly used solvent in the hydroboration step.
The hydroboration-oxidation reaction follows an anti-Markovnikov addition pattern, with the hydroxyl group attaching to the less substituted carbon. This provides a stereospecific and complementary regiochemical alternative to other hydration reactions.
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Using Grignard reagent
Grignard reagents are versatile organometallic compounds that react with a wide variety of electrophiles. They are powerful bases that are used synthetically to form new carbon-carbon bonds. They are also used in the reaction with aldehydes and ketones to form alcohols.
The key to the Grignard reagent is understanding the relative electronegativities of carbon (2.5) and magnesium (1.1). The bond between carbon and magnesium is polarized toward carbon, making carbon more electron-rich than magnesium and giving it nucleophilic properties.
In the reaction of Grignard reagents with aldehydes, the carbon attacks the carbonyl carbon and performs a 1,2-addition to give an alkoxide. Acid is then added to yield the alcohol. The nucleophilic addition of a Grignard reagent to a carbonyl is a powerful tool in organic synthesis because it forms a C-C bond.
The Grignard reagent reaction with ethylene oxide produces a primary alcohol containing two more carbon atoms than the original Grignard reagent. The preparation of Grignard reagents requires an ether solvent, usually diethyl ether. The reaction of a Grignard reagent with water provides a way to convert a haloalkane to an alkane in two steps.
An important reaction of Grignard reagents is their addition to epoxides to form carbon-carbon bonds. They tend to add to the less substituted end of the epoxide, which is the less sterically hindered end. After the addition of acid, an alcohol is obtained.
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Using Organolithium reagent
Organolithium reagents are nucleophilic chemical compounds that contain carbon–lithium (C–Li) bonds. They are important in organic synthesis and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. They are also used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers.
The first step of the reaction is the addition of the organolithium to the aldehyde or ketone. This is the end of the reaction until workup with mild acid, whereupon the negatively charged oxygen ("alkoxide") is protonated with mild acid to give the final alcohol product. Organolithium reagents can add to electrophilic carbonyl double bonds to form carbon–carbon bonds. They can react with aldehydes and ketones to produce alcohols. The addition proceeds mainly via polar addition, in which the nucleophilic organolithium species attacks from the equatorial direction and produces the axial alcohol.
The addition of lithium salts such as LiClO4 can improve the stereoselectivity of the reaction. When the ketone is sterically hindered, using Grignard reagents often leads to the reduction of the carbonyl group instead of addition. However, alkyllithium reagents are less likely to reduce the ketone and may be used to synthesize substituted alcohols. For example, ethyllithium addition to adamantone produces a tertiary alcohol.
Organolithium reagents are also better than Grignard reagents in their ability to react with carboxylic acids to form ketones. They are powerful bases and nucleophiles and can react with electrophiles, such as epoxides, to form new carbon-carbon bonds. However, the outcome of the reaction can depend on the sterics and electronics of the substrate, as well as the reaction conditions, which can lead to different products or even the starting material being regenerated. Acid quenching is a technique used to terminate reactions by adding an acid, which can protonate reactive intermediates and stabilize the final product.
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Using acid-catalyzed hydration
The process of adding alcohol to a double bond is known as the acid-catalyzed hydration of alkenes. This process involves the addition of a hydroxyl group and a hydrogen across the double bond. The first step in this process is the protonation of the less substituted end of the double bond to form a more substituted carbocation. This is followed by the addition of water, which acts as a nucleophile and attacks the carbocation to create an oxonium ion. Finally, water, acting as a base, deprotonates the oxonium ion to yield an alcohol. This reaction is favoured at lower temperatures, where the formation of the alcohol is favoured over the alkene.
The acid used in this process is a non-nucleophilic strong acid, such as sulfuric or phosphoric acid, which acts as a catalyst. The acid participating in the reaction is the hydronium ion, which is consumed in the first step and regenerated in the final step. The rate of the reaction increases with the acidity of the medium, and the reaction is dependent on temperature and acid concentration.
The mechanism of acid-catalyzed hydration involves the electrophilic addition of a proton (or acid) to the double bond, forming a carbocation intermediate. The carbocation formed during the reaction is prone to rearrangement if a more stable intermediate can be formed. Transition metals can also be used as acids for hydration reactions, with mercury (II) salts being a classic example.
The direct addition of water to an alkene is too slow to be significant, but the addition can be catalyzed by acids to form an alcohol. This reaction is exothermic, with a negative enthalpy term, and has an entropy change of approximately -35 to -40 cal/mol K. The net free energy change is close to 0, and the equilibrium constant is near unity.
The acid-catalyzed hydration of alkenes can be carried out by adding excess water to increase the yield of products. The reverse reaction, the dehydration of alcohol to form an alkene, can be promoted by removing water from the reaction.
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Frequently asked questions
The general process is known as hydration, which involves adding water across a double bond to make an alcohol. This is done through acid-catalyzed addition, which creates a carbocation intermediate.
There are a few different reactions that can be used, including oxymercuration-demercuration, hydroboration-oxidation, and direct acid-catalyzed hydration.
Oxymercuration-demercuration involves the addition of mercuric acetate to the double bond, forming a mercurinium ion. Water then attacks the most highly substituted carbon to form a mercurial alcohol. Finally, sodium borohydride replaces the mercuric portion with hydrogen.
Hydroboration-oxidation involves the addition of borane to the least substituted side of the double bond to form an alkyl borane. In the second step, hydrogen peroxide in the presence of sodium hydroxide substitutes a hydroxyl group for the boryl unit to form an anti-Markovnikov alcohol.
