Alkene and Alkyne Reactions: A Comprehensive Guide
Terms for Chapter 7: Alkenes & Alkynes I: Reactions of Alkyl Halides
E-Z System (Sec 7.2)
System for specifying double-bond configuration that is an alternative to cis-trans notation. Rank is determined by the Cahn-Ingold-Prelog system. When higher-ranked substituents are on the same side of the double bond, the configuration is Z. When higher-ranked substituents are on opposite sides, the configuration is E.
Cahn-Ingold-Prelog Notation (Sec 5.7, 7.2)
System for specifying the absolute configuration for R and S, or E and Z stereoisomers, based on the order of attachment of atoms or groups, which are ranked by precedence according to rules based on atomic number.
Hydrogenation (Sec 4.16, 7.3A, 7.13 – 7.15, 12.2)
- Reduction of a compound by adding a pair of radical hydrogen atoms, H•, or by adding both a hydride ion, :H–, and a hydrogen ion, H+.
- An addition reaction in which hydrogen adds to a double or triple bond, often accomplished using a metal catalyst like platinum, palladium, nickel, rhodium, or ruthenium.
Heat of Hydrogenation (Sec 7.3A)
The standard enthalpy change accompanying the hydrogenation of one mole of an unsaturated compound.
Alkene Stability (7.3B)
The greater the number of attached alkyl groups (the more highly substituted the alkene), the greater the alkene’s stability.
Elimination Reaction (Sec 3.1, 6.15 – 6.17, 7.5 – 7.7)
A reaction resulting in losing two groups from a substrate, forming a pi bond. The most common elimination is a β elimination (1,2 elimination), where the two groups are lost from adjacent atoms.
E2 Reaction (Sec 6.15C, 6.16, 6.18A, 7.6)
A bimolecular β elimination where, in a single step, a base removes a proton from a β carbon while a leaving group departs from the α carbon of the substrate, forming a pi bond.
E1 Reaction (Sec 6.15C, 6.17, 6.18B, 7.7)
A unimolecular β elimination where, in a slow, rate-determining step, a leaving group departs from the α carbon of the substrate, forming a carbocation; then, in a fast step, a base removes a proton from a β carbon of the carbocation to form a pi bond.
Dehydrohalogenation (Sec 6.15A, 7.6)
An elimination reaction resulting in losing hydrogen halide, HX, from adjacent carbon atoms of an alkyl halide, forming a pi bond.
Zaitsev’s Rule (Sec 7.6A, 7.8A)
The major product of an elimination reaction using a small, strong base will be the most stable alkene, which has the most highly substituted double bond.
Hofmann Rule (Sec 7.6B, 20.12A)
The major product of an elimination reaction using a bulky, strong base will be the less stable alkene, which has the less substituted double bond.
Coplanar (Sec 7.6C)
A conformation where two adjacent atoms and two vicinal groups bound to them lie in the same plane.
Anti Coplanar (Sec 7.6C)
The five atoms involved in the transition state of an E2 reaction must be coplanar for proper orbital overlap in the developing pi bond of the forming alkene. The anti coplanar transition state is preferred because the base and leaving group are anti (staggered) and separated by the greatest distance.
Syn Coplanar (Sec 7.6C)
The five atoms involved in the transition state of an E2 reaction must be coplanar for proper orbital overlap in the developing pi bond of the forming alkene. The syn coplanar transition state occurs only with rigid molecules unable to assume the more stable anti coplanar arrangement because the base and leaving group are syn (eclipsed) and directly oppose each other.
Dehydration (Sec 7.7, 7.8, 11.11A)
Removal of -OH and -H substituents from nearby atoms; commonly employed in preparing alkenes and ethers by heating alcohols with an acid catalyst.
Rearrangement Reaction (Sec 3.1, 7.8, 11.4)
A reaction where the carbon skeleton of a molecule is rearranged to produce a different isomer of the original molecule.
1,2-Shift (Sec 7.8)
A rearrangement occurring when a less stable secondary carbocation rearranges to a more stable tertiary carbocation through the migration of a hydride or methanide anion from a carbon adjacent to the carbocation.
Hydride Migration (Sec 7.8)
Migration of a hydride ion, :H─ , from a carbon adjacent to a carbocation, usually forming a more stable carbocation. Also called hydride shift or hydride rearrangement.
Methanide Migration (Sec 7.8)
Migration of a methanide ion, H3C:─ , from a carbon adjacent to a carbocation, usually forming a more stable carbocation. Also called methanide shift or methanide rearrangement.
Acetylenic Hydrogen Atom (Sec 4.6, 7.9)
The hydrogen atom bound to a terminal alkyne’s carbon atom, H-C≡C-R.
Geminal (gem-) (Sec 7.10)
Substituents on the same atom (geminus, Latin: twins).
Vicinal (vic-) (Sec 7.10)
Substituents on adjacent atoms (vicinus, Latin: adjacent).
Heterogeneous Catalyst (Sec 7.13, 7.14)
Reactions where the catalyst is insoluble in the reaction solvent.
Heterogeneous Hydrogenation (Sec 7.13, 7.14)
Hydrogenation of a multiple bond, catalyzed by a finely divided metal insoluble in the reaction mixture, such as platinum, palladium, nickel, rhodium, or ruthenium.
Homogeneous Catalyst (Sec 7.13, 7.14)
Reactions where the catalyst is soluble in the reaction solvent.
Homogeneous Hydrogenation (Sec 7.13, 7.14)
Hydrogenation of a multiple bond, catalyzed by an organometallic compound soluble in the reaction mixture, such as complexes of rhodium or ruthenium with ligands.
Syn Addition (Sec 7.14A, 7.15, 8.6 – 8.8)
An addition reaction that places both parts of the adding reagent on the same face of the reactant.
Anti Addition (Sec 7.14A, 7.15, 8.6C, 8.13)
An addition reaction that places the parts of the adding reagent on opposite faces of the reactant.