Lena Savoie
The addition of a double bond to an alcohol results in the formation of an alkene. The hydroxyl group (-OH) acts as the functional group in alcohols, while alkenes are identified by the presence of double bonds between carbon atoms. The process of converting alkenes to alcohols through the addition of water across a double bond is known as hydration. This reaction involves two steps: the initial addition of a proton or acid to the double bond, followed by the addition of water to form an oxonium ion, which eventually yields the alcohol. The IUPAC nomenclature system is used to name these compounds, with specific rules for reflecting the presence of both the double bond and the alcohol functional group.
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What You'll Learn
Hydration
There are two different reactions that can accomplish hydration. The first reaction adds the alcohol (OH group) to the most substituted carbon on the double bond to make the Markovnikov product. The complementary reaction puts the alcohol on the least substituted carbon in the double bond to make the anti-Markovnikov product.
The first reaction involves reacting the alkene with mercuric acetate, Hg(OAc)2, and water, followed by the addition of sodium borohydride, NaBH4. This reaction is called oxymercuration-demercuration. The oxymercuration step involves the attack of the double bond on the mercuric acetate to form a three-membered ring intermediate called a mercurinium ion. Water then attacks the most highly substituted carbon to make the mercurial alcohol, which loses a proton. In the demercuration step, sodium borohydride replaces the mercuric portion with hydrogen.
The second reaction, which forms the anti-Markovnikov product, is called hydroboration. This reaction involves the addition of borane (BH3) in a tetrahydrofuran solvent (THF) to the alkene, followed by the addition of hydrogen peroxide (H2O2) and sodium hydroxide (NaOH). The borane adds to the least substituted side of the double bond to make the alkyl borane. The addition is concerted, meaning that the borane and hydrogen must add to the same face of the carbon-carbon bond. In the second step, hydrogen peroxide in the presence of sodium hydroxide substitutes a hydroxyl group (OH) for the boryl unit (BH2) to make the anti-Markovnikov alcohol.
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Electrophilic addition
Alkenes are a group of unsaturated hydrocarbons with at least one double bond between carbon atoms. Due to the presence of pi electrons, they exhibit addition reactions where an electrophile attacks the carbon-carbon double bond to form addition products. These reactions are known as electrophilic addition reactions.
Alkenes can be converted to alcohols by the net addition of water across the double bond. This process is known as hydration and involves the breaking of the pi bond in the alkene and the formation of a C-H bond and a C-OH bond. The reaction is typically exothermic, but the net free energy change is close to 0. The direct addition of water to an alkene is usually too slow to be significant, but it can be catalyzed by Lewis or Bronsted acids.
The mechanism of hydration involves the electrophilic addition of a proton (or acid) to the double bond, forming a carbocation intermediate. This is followed by the addition of water, resulting in the formation of an oxonium ion. Deprotonation of the oxonium ion yields the alcohol. The proton in the oxonium intermediate can be deprotonated by any base present, including the conjugate base of the acid used as a catalyst or another alkene molecule.
Oxymercuration, a type of electrophilic addition reaction, involves the addition of mercury (II) salts, such as mercuric chloride or mercuric acetate, to an alkene. This reaction results in Markovnikov addition of water to the double bond and the formation of an alcohol. The mercury is eventually replaced by a hydrogen atom through a subsequent reaction with sodium borohydride.
In summary, the conversion of alkenes to alcohols involves electrophilic addition reactions, particularly hydration, which adds a double bond to an alkene through the addition of water and the formation of a carbocation intermediate. Oxymercuration is another electrophilic addition reaction used to introduce a double bond to an alkene, forming an alcohol through a series of steps.
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Oxymercuration
The mercury ion in this complex is coordinated by several water molecules, but they are usually ignored for simplicity. Unlike the carbocations formed by protonation, the complex formed by the addition of mercury is a bridged or cyclic structure. This structure is similar to the cyclic bromonium ion formed during the bromination of alkenes.
The oxymercuration reaction can be divided into three steps:
- Initiation: The nucleophilic double bond of the alkene attacks the mercury ion, resulting in the ejection of an acetoxy group. Simultaneously, the electron pair on the mercury ion attacks one of the carbon atoms in the double bond. This leads to the formation of a mercurinium ion, where the mercury atom carries a positive charge.
- Propagation: A nucleophilic water molecule attacks the more substituted carbon atom in the mercurinium ion, resulting in the liberation of electrons that were participating in the bond with mercury.
- Termination: The intermediate organomercury compound formed is almost never isolated. Instead, a demercuration step is performed using sodium borohydride (NaBH4) or sodium borodeuterohydride (NaBD4), which breaks the C-Hg bond and forms a new C-H bond. This step can lead to the formation of deuterated products if NaBD4 is used.
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Markovnikov's Rule
The rule states that the acid hydrogen (H) or electropositive part attaches to the carbon with more hydrogen substituents, while the halide (X) group or electronegative part attaches to the carbon with more alkyl substituents. This is in contrast to Markovnikov's original definition, where the X component is added to the carbon with the fewest hydrogen atoms, and the hydrogen atom is added to the carbon with the most hydrogen atoms.
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. Therefore, the major product of the addition of HX (where X is an atom more electronegative than H) to an alkene has the hydrogen atom in the less substituted position and X in the more substituted position. However, the less substituted and less stable carbocation will still be formed to some extent and will be the minor product with the opposite, conjugate attachment of X.
Some reactions do not follow Markovnikov's Rule, and anti-Markovnikov products are isolated. This is the case for certain radical-induced additions of HX, such as the addition of hydrogen bromide to isobutylene in the presence of benzoyl peroxide or hydrogen peroxide. In these reactions, the halogen adds to the less substituted carbon, opposite to a Markovnikov reaction.
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Transition metals as acids
Transition metals are chemical elements in the d-block of the periodic table (groups 3 to 12). They are lustrous metals with good electrical and thermal conductivity. They form compounds in two or more different oxidation states and bind to various ligands to form coordination complexes. Transition metals can change their oxidation states, making them effective catalysts. They also form stable coordination compounds, where the central metal atom or ion acts as a Lewis acid and accepts one or more pairs of electrons.
Transition metal ions in aqueous solutions invariably form complexes known as aqua ions, wherein six water molecules act as ligands forming an octahedral complex. Ions with an oxidation state of +3 or greater have a high charge density, causing polarisation of the O-H bond in one or more water ligands. This leads to the loss of H+ to a free water molecule, resulting in an acidic solution. For example, the reaction of [Fe(H2O)6]3+(aq) with water produces [Fe(H2O)5OH]2+(aq) and H3O+(aq), the latter being responsible for the acidity of the solution.
Transition metal oxides, composed of oxygen atoms bound to transition metals, are commonly used for their catalytic activity and semiconducting properties. The relative acidity and basicity of these oxides are influenced by the coordination of the metal cation and oxygen anion, affecting their catalytic properties. Structural defects in transition metal oxides, such as oxygen or metal ion vacancies, can significantly impact their acidity or basicity.
The surface of a metal oxide consists of ordered arrays of acid-base centres, with cationic metal centres acting as Lewis acid sites and anionic oxygen centres as Lewis bases. The strength and concentration of these Lewis and Brønsted acid-base sites determine the catalytic activity of the metal oxide. The surface acidity and basicity of metal oxides depend on the crystal structure, surface orientation, charge and radius of the metal ions, as well as the nature of the metal-oxygen bond. Techniques like infrared spectroscopy and calorimetry are used to characterise the acidic and basic sites on the surface of metal oxides.
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Frequently asked questions
The process is called hydration, which involves adding water across a double bond.
The first step is the electrophilic addition of a proton (or acid) to the double bond, forming a carbocation intermediate. The second step involves the addition of water to form an oxonium ion, which, upon deprotonation, gives the alcohol.
Oxymercuration is one of the two reactions involved in the hydration process. It involves the addition of mercuric acetate (Hg(OAc)2) to the alkene, followed by the addition of sodium borohydride (NaBH4).
Markovnikov's Rule states that the addition of a proton occurs at the less substituted carbon, with the -OH group adding to the more substituted carbon.
The anti-Markovnikov product is formed when the alcohol adds to the least substituted carbon in the double bond. This can be achieved through hydroboration, where borane (BH3) is added to the alkene, followed by the addition of hydrogen peroxide (H2O2) and sodium hydroxide (NaOH).
