, those versatile carbon-carbon triple bonds, can be transformed into valuable carbonyl compounds through . This process, catalyzed by mercury(II) salts or achieved through , opens up a world of synthetic possibilities for organic chemists.
Understanding the mechanisms and products of alkyne hydration is crucial for predicting and controlling reactions. Whether you're aiming for aldehydes from or ketones from internal ones, mastering these concepts will make you a hydration sensation!
Hydration of Alkynes
Mechanism of mercury(II)-catalyzed alkyne hydration
Mercury(II) salts (\ceHgSO4) catalyze hydration of alkynes by activating the alkyne for by water
Alkyne coordinates to mercury(II) ion making it more electrophilic
Water acts as a nucleophile and attacks the activated alkyne forming an unstable intermediate
Vinyl cation intermediate rapidly rearranges to form an which is a tautomer of the corresponding or
Enol has an \ceOH group attached to a \ceC=C double bond
Keto form has a \ceC=O double bond and no \ceOH group
converts the less stable enol to the more stable ketone or aldehyde product
Involves the shift of a proton and a double bond
Products of terminal vs internal alkyne hydration
hydration
Terminal alkynes (1-butyne) yield aldehydes as the major product
(2-butyne) yield ketones as the major product
Hydroboration-oxidation
Terminal alkynes (1-hexyne) yield aldehydes as the major product
Proceeds through an intermediate which is oxidized to an alcohol then subsequently to an aldehyde
Internal alkynes not compatible due to formation of stable dialkylboranes that resist oxidation
Synthesis applications of alkyne hydration
Synthesize a ketone from an internal alkyne
Choose internal alkyne with desired substituents on either side of triple bond (3-hexyne)
Treat alkyne with mercury(II) salt and water under acidic conditions
Ketone product (3-hexanone) will have same substituents as starting alkyne
Synthesize an aldehyde from a terminal alkyne
Choose terminal alkyne with desired substituent on triple bond (1-octyne)
Method 1: Treat alkyne with mercury(II) salt and water under acidic conditions to directly form aldehyde (octanal)
Method 2: Perform hydroboration-oxidation on the terminal alkyne
Treat alkyne with (\ceBH3) to form alkylborane intermediate
Oxidize alkylborane with (\ceH2O2) under basic conditions to form alcohol
Oxidize alcohol using a mild oxidant () to form the aldehyde (octanal)
Regioselectivity and Addition Patterns
Hydration of alkynes follows mechanism
: Addition of water to unsymmetrical alkynes results in the hydroxyl group attaching to the more substituted carbon
: Can be achieved through hydroboration-oxidation, resulting in the opposite
: A variation of the mercury(II)-catalyzed hydration that proceeds through a mercurinium ion intermediate, also following Markovnikov's rule
Key Terms to Review (26)
Aldehyde: An aldehyde is a class of organic compounds containing a carbonyl group (C=O) where the carbon atom is bonded to one hydrogen atom and one alkyl or aryl group. Aldehydes are important functional groups in organic chemistry and are involved in various reactions and synthesis pathways.
Alkylborane: An alkylborane is an organic compound consisting of a boron atom bonded to one or more alkyl groups. These compounds are important intermediates in organic synthesis, particularly in the hydration of alkynes and the reactions of carboxylic acids.
Alkynes: Alkynes are a class of organic compounds characterized by the presence of a carbon-carbon triple bond. They are an important family of hydrocarbons with unique chemical properties and applications in various fields, including organic synthesis, materials science, and fuel production.
Anti-Markovnikov Addition: Anti-Markovnikov addition is a type of electrophilic addition reaction that occurs when a hydrogen-containing molecule, such as water or hydrogen halide, adds to an alkene or alkyne in a way that places the hydrogen on the less substituted carbon. This is in contrast to the Markovnikov addition, which places the hydrogen on the more substituted carbon.
Borane: Borane is a chemical compound with the formula BH3, consisting of a boron atom bonded to three hydrogen atoms. It is a highly reactive and flammable gas that serves as a key intermediate in organic chemistry, particularly in reactions involving alkenes, alkynes, and carboxylic acids.
Electrophilic Addition: Electrophilic addition is a type of organic reaction where an electrophile, a species that is attracted to electrons, adds to the carbon-carbon double bond of an alkene. This results in the formation of a new carbon-carbon single bond and the incorporation of the electrophile into the molecule.
Electrophilic addition reaction: An electrophilic addition reaction is a chemical process in which an electrophile reacts with a nucleophile, typically an alkene or alkyne, forming a new sigma bond by adding across the double or triple bond. This reaction is key in organic synthesis, resulting in the addition of atoms or groups to the carbon atoms involved in the multiple bond.
Enol: An enol is an organic compound that contains a carbon-carbon double bond where one of the carbon atoms is also bonded to a hydroxyl (OH) group. Enols are important intermediates in various organic reactions, including the hydration of alkynes, alpha-substitution reactions of carbonyl compounds, and carbonyl condensation reactions.
Hydration: Hydration is the process of adding water to a chemical compound, typically involving the addition of water across a double bond or the incorporation of water into the structure of a molecule. This term is particularly relevant in the context of organic chemistry, where it plays a crucial role in various reactions and transformations.
Hydroboration-Oxidation: Hydroboration-oxidation is a two-step reaction sequence that allows for the anti-Markovnikov addition of water to alkenes and alkynes, resulting in the formation of alcohols. This process involves the initial hydroboration of the carbon-carbon double or triple bond, followed by an oxidation step to yield the final alcohol product.
Hydrogen Peroxide: Hydrogen peroxide (H2O2) is a colorless, slightly viscous liquid that is a common oxidizing agent used in a variety of chemical reactions. It is an important compound that plays a role in several organic chemistry topics, including the hydration of alkenes, oxidation of alkenes, hydration of alkynes, preparation of alcohols, and reactions of carboxylic acids.
Internal Alkynes: Internal alkynes, also known as disubstituted alkynes, are a class of organic compounds where the triple bond is positioned between two carbon atoms that are not at the end of the carbon chain. Unlike terminal alkynes, where the triple bond is at the end of the chain, internal alkynes have the triple bond located within the molecule.
Keto-Enol Tautomerism: Keto-enol tautomerism is the reversible chemical equilibrium between a keto (carbonyl) form and an enol form of a compound. This process is particularly relevant in the context of carbonyl chemistry, as it affects the reactivity and properties of these compounds.
Ketone: A ketone is a functional group in organic chemistry that consists of a carbonyl group (a carbon-oxygen double bond) bonded to two alkyl or aryl groups. Ketones are widely encountered in various organic chemistry topics, including the hydration of alkynes, oxidative cleavage of alkynes, organic synthesis, oxidation and reduction reactions, and the chemistry of aldehydes and ketones.
Markovnikov Addition: Markovnikov addition is a fundamental organic chemistry concept that describes the regiochemical outcome of the addition of a polar molecule, such as hydrogen halides or water, to an unsymmetrical alkene or alkyne. It predicts the formation of the more stable carbocation intermediate, leading to the addition of the electrophilic component to the carbon atom that can best stabilize the resulting positive charge.
Mercury(II) Sulfate: Mercury(II) sulfate, also known as mercuric sulfate, is an inorganic compound with the chemical formula HgSO4. It is a white crystalline solid that is soluble in water and has various applications, including in the hydration of alkynes.
Mercury(II)-catalyzed: Mercury(II) catalysis is a technique used in organic chemistry, particularly in the context of the hydration of alkynes. It involves the use of mercury(II) salts, such as mercury(II) acetate or mercury(II) sulfate, to facilitate the addition of water across a carbon-carbon triple bond, resulting in the formation of a carbonyl compound.
Nucleophilic Attack: Nucleophilic attack is a fundamental chemical reaction in which a nucleophile, an electron-rich species, attacks an electrophilic (electron-deficient) center, forming a new covalent bond. This process is central to understanding many organic reactions, including polar reactions, addition reactions, and substitution reactions.
Oxymercuration: Oxymercuration is a chemical reaction that involves the addition of water to an alkene or alkyne in the presence of a mercury(II) salt, typically mercury(II) acetate. This process is used to introduce a hydroxyl group (-OH) to the molecule, effectively hydrating the carbon-carbon double or triple bond.
Oxymercuration–demercuration: Oxymercuration–demercuration is a two-step organic chemical reaction process that adds water (H2O) across the double bond of alkenes, in a regioselective manner, to form alcohols without the rearrangement typically seen in acid-catalyzed hydration. First, oxymercuration involves the addition of mercury acetate and water across the alkene's double bond, followed by demercuration, where sodium borohydride reduces the intermediate to produce an alcohol.
PCC: PCC, or Pyridinium Chlorochromate, is a versatile oxidizing agent used in organic chemistry for the selective oxidation of alcohols to aldehydes and ketones. This powerful reagent is widely employed in various reactions across multiple topics, including the hydration of alkynes, the oxidation of alcohols, and the preparation and oxidation of aldehydes and ketones.
Regioselectivity: Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule, leading to the formation of one regioisomeric product over another. This concept is particularly important in the context of electrophilic addition reactions of alkenes, electrophilic aromatic substitution, and other organic transformations.
Sulfuric Acid: Sulfuric acid (H2SO4) is a highly corrosive, dense, and oily liquid that is one of the most important and widely used industrial chemicals. It is a strong mineral acid that plays a crucial role in various chemical reactions and processes.
Terminal Alkynes: Terminal alkynes are a class of organic compounds that have a carbon-carbon triple bond at the end of the carbon chain. These unique functional groups are characterized by the presence of a terminal alkyne, which has significant implications in various organic chemistry reactions and transformations.
Vinyl Cation: A vinyl cation is a carbocation with a positively charged carbon atom that is directly bonded to a carbon-carbon double bond. These reactive intermediates are important in the context of the hydration of alkynes, where they can form as part of the reaction mechanism.
β Diketone: A β-diketone is an organic compound containing two ketone groups separated by a carbon atom, which is the beta (β) position relative to each ketone group. These molecules are characterized by the presence of hydrogen atoms on the carbon between the two carbonyl (C=O) groups, making them acidic and prone to enolate ion formation.