Alkene & Alkyne Addition Reactions: Key Terms & Concepts (Ch 8)
Chapter 8: Alkenes & Alkynes II: Addition Reactions
Key Terms and Concepts
Electrophile
An “electron-loving” species; a cation or electron deficient molecule (with a δ+ charge, or an empty p orbital) that can accept a pair of electrons to form a new covalent bond. Examples include:
- Brønsted-Lowry acids (proton donors) such as HBr, H2SO4, etc.
- Lewis acids (electron-pair acceptors) such as BH3, BF3, AlCl3, FeCl3, ZnCl2, etc.
- Metal ions with vacant orbitals such as Ag+, Hg2+, Pt2+, etc.
- Neutral molecules with large polarizable atoms such as Br2, I2, etc.
Electrophilic Addition
An addition reaction in which a polarizable reagent, E-N, is attacked by the pi electrons of an alkene (or alkyne) on the electrophilic portion (E), forming a carbocation intermediate. This intermediate is then attacked by the nucleophilic portion (N). The product has E attached to one carbon and N attached to the adjacent carbon. Also called an ionic addition reaction.
Hydrohalogenation Reaction
An electrophilic addition reaction of a hydrogen halide (HCl, HBr, etc.) to an alkene (or alkyne), producing the corresponding haloalkane (alkyl halide).
Markovnikov’s Rule
When a hydrogen halide, HX, reacts with an unsymmetrical alkene, the hydrogen atom adds to the carbon with the most hydrogens (less substituted), and the halide adds to the carbon with fewer hydrogens (more substituted). This predicts the regiochemistry for ionic addition: the positive portion adds to yield the more stable carbocation intermediate.
Regioselective Reaction
A reaction that favors the formation of one constitutional isomer over others when multiple are possible.
Markovnikov Addition
An electrophilic addition of HX to an unsymmetrical alkene, where hydrogen adds to the less substituted carbon, and the halogen adds to the more substituted carbon, yielding the more substituted alkyl halide.
Anti-Markovnikov Addition
A radical addition of HBr to an unsymmetrical alkene in the presence of peroxides (ROOR). Hydrogen adds to the more substituted carbon, and bromine adds to the less substituted carbon, yielding the less substituted alkyl bromide.
Alkyl Hydrogen Sulfate
The Markovnikov addition product, R-OSO3H, from the reaction of cold concentrated sulfuric acid (H2SO4) with an alkene.
Acid-Catalyzed Hydration of Alkenes
The Markovnikov addition of water (H-OH) to an alkene, forming an alcohol. The acid catalyst (H+) forms the more stable carbocation (which may rearrange), then water adds. After deprotonation, the more substituted alcohol is produced as a mixture of syn and anti addition products.
Oxymercuration-Demercuration of Alkenes
A two-step process converting an alkene to an alcohol with Markovnikov regiochemistry and usually no rearrangements. Oxymercuration adds -OH and -HgOOCCH3 to the double bond. Demercuration replaces the -HgOOCCH3 with hydrogen, producing the more substituted alcohol (syn and anti mixture).
Le Chatelier’s Principle
If a system at chemical equilibrium is disturbed, the equilibrium shifts to counteract the disturbance. For example, adding reactant or removing product shifts the equilibrium towards product formation.
Hydroboration-Oxidation of Alkenes
A two-step process converting an alkene to an alcohol with anti-Markovnikov regiochemistry, syn stereochemistry, and usually no rearrangements. Hydroboration adds BH3 to the alkene. Oxidation and hydrolysis with basic hydrogen peroxide (H2O2) yields the less substituted alcohol.
Halogen Addition to Alkenes
The electrophilic anti addition of halogens to an alkene or alkyne via bridged halonium ions, producing vic-dihalides.
Halonium Ion
A positively charged halogen atom bound to two carbon atoms. The positive charge is partially delocalized. These ions are intermediates in halogen addition to alkenes and alkynes.
Stereospecific Reaction
A reaction where a specific stereoisomer of the substrate yields a specific stereoisomeric product.
Halohydrin
A vicinal halo alcohol with a halogen (-X) and hydroxyl group (-OH) on adjacent carbons. Formed by reacting an alkene with a halogen in water.
Carbene
An uncharged divalent carbon with a lone pair of electrons (e.g., methylene, :CH2). Reacts with alkenes to form cyclopropanes.
Carbenoid
A carbene-like species (e.g., Simmons-Smith reagent from CH2I2/Zn(Cu)). Reacts with alkenes to form cyclopropanes.
Syn 1,2-Dihydroxylation
Syn addition of two hydroxyl groups (-OH) to the same side of an alkene using an oxidizing agent like OsO4, producing 1,2-diols (glycols). Cycloalkenes yield cis-1,2-cycloalkanediols.
Oxidative Cleavage of Alkenes
Cleavage of an alkene at the double bond using a strong oxidizing agent (e.g., KMnO4), yielding ketones, carboxylic acids, and/or CO2.
Ozonolysis
Cleavage of an alkene with ozone (O3), forming a cyclic ozonide. Reduction with (CH3)2S or Zn/acetic acid yields ketones and/or aldehydes.
Synthon
Fragments resulting from the hypothetical disconnection of a bond. The actual reagent used in synthesis is the synthetic equivalent.
Stereoselective Reaction
A reaction that favors the formation of one stereoisomer when a stereogenic center is altered or created. An enantioselective reaction favors one enantiomer; a diastereoselective reaction favors one diastereomer.