Organic Chemistry: Alkenes and Alkynes (Chapters 7–9)

Vocabulary flashcards covering the structure, properties, reactions, and synthesis of alkenes and alkynes based on Chapters 7, 8, and 9 of John E. McMurry's Organic Chemistry.

Alkene

A hydrocarbon containing a carbon–carbon double bond, sometimes referred to as an olefin.

Degree of Unsaturation

The number of multiple bonds or rings present in a molecule, where each multiple bond or ring replaces two hydrogen atoms compared to a saturated acyclic compound ($C_n H_{2n+2}$).

Cis-Trans Isomers

Stereoisomers in alkenes where substituents are positioned either on the same side (cis) or opposite sides (trans) of a double bond; rotation is prevented by a barrier of about $350\,kJ/mol$.

E,Z System

A system for designating alkene stereochemistry using Cahn–Ingold–Prelog sequence rules, assigning Z geometry when higher-ranked groups on each double-bond carbon are on the same side and E geometry when they are on opposite sides.

Hyperconjugation

A stabilizing electronic interaction wherein electrons in neighboring filled $\sigma$ orbitals interact with a vacant antibonding $\pi$ orbital in alkenes or a vacant $p$ orbital in carbocations.

Regiospecific

A characteristic of an addition reaction in which only one of two possible orientations of addition occurs to yield a single product.

Markovnikov's Rule

In the addition of HX to an alkene, the more highly substituted carbocation intermediate forms preferentially, causing the hydrogen atom to add to the double-bond carbon with fewer alkyl substituents and the halogen X to add to the carbon with more alkyl substituents.

Carbocation

A planar, trivalent, $sp^2$-hybridized intermediate containing six valence electrons distributed among three $\sigma$ bonds and an unoccupied $p$ orbital.

Hammond Postulate

A principle stating that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product of that step.

Hydride Shift

A carbocation rearrangement involving the migration of a hydrogen atom and its electron pair ($:H^-$) from a neighboring carbon to the positively charged carbon, generating a more stable carbocation.

Dehydrohalogenation

An elimination reaction that prepares alkenes through the loss of HX from an alkyl halide upon treatment with a strong base.

Dehydration

An elimination reaction wherein an alcohol loses water ($H_2O$) upon treatment with a strong acid to yield an alkene.

Halogenation (of Alkenes)

The addition of halogens ($Cl_2$ or $Br_2$) across an alkene double bond to yield 1,2-dihalides via anti stereochemistry.

Bromonium Ion

A three-membered cyclic halonium ion intermediate ($R_2Br^+$) formed by electrophilic addition of $Br^+$ to an alkene; its formation blocks one face, directing nucleophilic attack by $Br^-$ to the opposite face to produce trans/anti stereochemistry.

Halohydrin

A 1,2-halo alcohol synthesized by reacting an alkene with a hypohalous acid ($HO-Cl$ or $HO-Br$), or with $X_2$ in aqueous solution.

Oxymercuration–Demercuration

A two-step hydration process initiated by electrophilic addition of $Hg^{2+}$ to form a mercurinium ion, followed by nucleophilic attack of water and subsequent reduction with $NaBH_4$ to give Markovnikov alcohols without carbocation rearrangement.

Hydroboration

A single-step addition of a B–H bond of borane ($BH_3$) across an alkene double bond to yield an organoborane intermediate, which upon alkaline $H_2O_2$ oxidation affords alcohols with non-Markovnikov regiochemistry and syn stereochemistry.

Hydrogenation

The addition of molecular hydrogen ($H_2$) across double or triple bonds in the presence of a catalyst (such as $PtO_2$ or $Pd/C$) to yield a saturated hydrocarbon via syn stereochemistry.

Reduction (Organic Chemistry)

A reaction that increases electron density on a carbon atom, either by forming a bond between carbon and a less electronegative atom or by breaking a bond between carbon and a more electronegative atom.

Oxidation (Organic Chemistry)

A reaction that decreases electron density on a carbon atom, either by forming a bond between carbon and a more electronegative atom or by breaking a bond between carbon and a less electronegative atom.

Epoxide

A cyclic ether consisting of an oxygen atom in a three-membered ring, synthesized by treating an alkene with a peroxyacid ($RCO_3H$).

Carbene

A neutral, highly reactive intermediate ($R_2C{:}$) containing a divalent carbon atom with only six valence electrons, which adds to alkenes to synthesize cyclopropanes.

Simmons–Smith Reaction

A cyclopropane synthesis method that reacts an alkene with diiodomethane ($CH_2I_2$) and zinc-copper alloy ($Zn(Cu)$) to generate nonhalogenated cyclopropanes via a carbenoid intermediate without generating a free carbene.

Chain-Growth Polymer

A high-molecular-weight polymer formed through radical addition of alkene monomers across a repeating active center via initiation, propagation, and termination steps.

Alkyne

A hydrocarbon containing a carbon–carbon triple bond consisting of one $\sigma$ bond formed by $sp$ hybrid orbitals and two $\pi$ bonds formed by unhybridized $2p_y$ and $2p_z$ orbitals, with linear $180^{\circ}$ bond angles.

Vinylic Carbocation

An intermediate containing a positively charged $sp$-hybridized carbon atom on a double bond; it forms less readily and is less stable than an alkyl carbocation.

Tautomers

Isomeric compounds that rapidly interconvert via the migration of a proton, such as the rearrangement of an enol intermediate to a ketone or aldehyde.

Lindlar Catalyst

A deactivated palladium-on-carbon ($Pd/C$) catalyst that allows the selective catalytic hydrogenation of alkynes to cis alkenes.

Acetylide Anion

A strongly nucleophilic carbanion formed by deprotonating a terminal alkyne with a strong base (such as $NaNH_2$), stabilized by the $50\%$ $s$ character of its $sp$-hybridized carbon.