Theo Nash
There are several ways to convert an alkene to an alcohol, including acid-catalyzed hydration, hydroboration-oxidation, and oxymercuration-demercuration. Acid-catalyzed hydration involves adding water across the carbon-carbon double bond in the presence of an acid catalyst, such as sulfuric or phosphoric acid. Hydroboration-oxidation involves reacting the alkene with borane, followed by the addition of hydrogen peroxide and sodium hydroxide. Oxymercuration-demercuration involves reacting the alkene with mercuric acetate and water, followed by the addition of sodium borohydride. These reactions can result in the formation of Markovnikov or anti-Markovnikov products, depending on whether the alcohol is added to the most or least substituted carbon on the double bond.
| Characteristics | Values |
|---|---|
| Common methods | Acid-catalyzed hydration, hydroboration-oxidation, oxymercuration-demercuration |
| Acid-catalyzed hydration reaction | $CH_{2}=CH_{2}+H-OH\to H-CH_{2}-CH_{2}-OH |
| Hydroboration-oxidation reaction | $R-C=C-R+BH_{3}\to R-C-C-OH |
| Oxymercuration-demercuration | Involves the formation of a mercurinium ion and the addition of water to make mercurial alcohol |
| Indirect method | Conversion of alkyl halides synthesized from alkenes |
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What You'll Learn
Acid-catalysed hydration
One of the three common ways to convert an alkene to an alcohol is through acid-catalysed hydration. This process involves replacing the pi bond on an alkene with a water molecule, resulting in the formation of an alcohol and the conversion of an unsaturated compound to a saturated compound. The reaction can be represented as follows:
$$CH_2=CH_2 + H-OH \rightarrow H-CH_2-CH_2-OH$$
In this reaction, water (OH and H) is added across the pi bond to form an alcohol. The acid acts as a catalyst, with the strong acid dissociating in solution to form hydronium (H3O+) automatically. The acid catalyst dissociates to give H+ in solution, which is then picked up by a solvent molecule, typically water, to form a protonated solvent molecule, hydronium. The rate-determining step in this reaction is the first step, which involves the protonation of the alkene with strong acid, resulting in the formation of a carbocation intermediate.
The acid-catalysed hydration of alkenes follows Markovnikov's rule, where the alcohol is added to the most substituted carbon on the double bond. This can be visualised using reaction energy diagrams, where changes in energy (Δ E) are graphed on the y-axis, and reaction progress is graphed on the x-axis. The local maxima ("peaks") represent transition states, while local minima ("valleys") represent intermediates.
It is important to note that the reaction may undergo a carbocation rearrangement, and if the product is chiral, it will be racemic due to the carbocation intermediate. The nucleophilic pi bond attacks a proton in solution, breaking the pi bond in the process. The carbocation forms on the more substituted carbon, and a water molecule uses its oxygen electrons to attack the carbocation, forming oxonium. Finally, another water molecule in the solution removes the extra proton, resulting in the formation of an alcohol with a hydroxyl group on the more substituted carbon.
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Hydroboration-oxidation
The first step of hydroboration-oxidation involves treating the alkene with borane (BH3) or similar reagents, breaking the C-C pi bond and forming a C-H and C-B bond. This results in an organoborane compound, which is then immediately treated with an oxidant to give the desired alcohol product. The borane reagent can come in various forms, including borane itself (BH3), BH3•THF, B2H6, BH3•OEt2, and substituted boranes like disiamyl borane and 9-BBN.
In the second step, an oxidant such as hydrogen peroxide (H2O2) is added, usually in the presence of a base like NaOH or KOH. This substitutes the boryl unit (BH2) with a hydroxyl group (OH), forming the anti-Markovnikov alcohol. The hydrogen and boron must add to the same face of the carbon-carbon bond, known as syn addition. This stereoselectivity is another key feature of the hydroboration-oxidation reaction.
The hydroboration-oxidation reaction was first reported by Herbert C. Brown in the late 1950s, and he received the Nobel Prize in Chemistry in 1979 for this work.
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Oxymercuration-demercuration
In the first step of oxymercuration-demercuration, the pi electrons of the alkene form a bond with mercury, while the lone pair on the mercury simultaneously bonds to the other vinyl carbon, creating a mercurium ion bridge. This step is stereospecific and regioselective because the mercurium ion stabilizes the carbocation intermediate, preventing it from rearranging. The mercurium ion forms in conjunction with the loss of an acetate ion.
In the second step, a water molecule reacts with the most substituted carbon, opening the mercurium ion bridge. This is followed by a proton transfer to a solvent water molecule, neutralizing the addition product.
The third step is the demercuration step, where the organomercury intermediate is reduced under basic conditions with sodium borohydride (NaBH4). This step breaks the C-Hg bond and forms a new C-H bond. The mechanism of demercuration often involves a free-radical intermediate and is not covered in introductory textbooks.
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Using alkyl halides
Converting alkenes to alcohols can be done in two main ways: acid-catalyzed hydration and hydroboration-oxidation. The first method follows Markovnikov's rule, while the latter places the OH group in the anti-Markovnikov position. An indirect way of converting alkenes to alcohols is by first converting them to alkyl halides and then performing a Zaitsev or Hofmann elimination.
Alkyl halides can be converted to alcohols by reacting them with hydroxides or water. Hydroxides are good nucleophiles and strong bases, making them ideal for converting primary alkyl halides. Tertiary alkyl halides, on the other hand, should not be used with hydroxides as this will result in the formation of the corresponding alkene as the major product. Instead, water, a weak nucleophile, should be used with tertiary alkyl halides to promote the SN1 conversion to alcohols. Secondary alkyl halides can undergo both SN1 and SN2 reactions, but a strong base like hydroxide is preferred to encourage E2 elimination.
To summarise, the choice of reagent (hydroxide or water) depends on the type of alkyl halide:
- Primary alkyl halides: Use hydroxides such as NaOH, KOH, or LIOH.
- Secondary alkyl halides: Can use either hydroxides or water, depending on whether SN1 or SN2 is desired.
- Tertiary alkyl halides: Use water to prevent the formation of alkenes as the major product.
It is important to consider the possibility of rearrangement during these reactions. For instance, secondary alkyl halides can undergo a rearrangement to form a tertiary alkyl halide. To avoid this, one can convert the OH group into a mesylate or tosylate and then perform a Zaitsev or Hoffman elimination. Another strategy to prevent rearrangements in the conversion of alkenes to alcohols is to use mercury acetate (Hg(OAc)2) in a process called oxymercuration-demercuration.
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Markovnikov's rule
In organic chemistry, Markovnikov's rule, also known as Markownikoff's rule, describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870.
The rule can be illustrated by the reaction of propene with hydrobromic acid. In this reaction, the hydrogen ion (H) attaches to the carbon with more hydrogen substituents, while the bromine ion (Br) attaches to the carbon with more alkyl substituents. This results in the formation of 1-bromopropane as the major product.
It's important to note that some reactions do not follow Markovnikov's rule, and anti-Markovnikov products are observed. This occurs when the halogen adds to the less substituted carbon, opposite to a Markovnikov reaction. Anti-Markovnikov behaviour is observed in certain chemical reactions, such as the hydration of phenylacetylene by auric catalysis, which results in acetophenone.
In the context of converting alkenes to alcohols, Markovnikov's rule is relevant in the oxymercuration-demercuration reaction. This reaction involves the addition of mercuric acetate (Hg(OAc)2) and water to the alkene, followed by the addition of sodium borohydride (NaBH4). The Markovnikov product is formed when the alcohol (OH group) adds to the most substituted carbon on the double bond.
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