Organic Chemistry: Alkenes, Dienes, and Alkynes

Flashcards covering the structure, properties, nomenclature, regioselectivity, and various addition and oxidation reactions for alkenes, dienes, and alkynes as presented in the lecture notes.

Alkenes

Unsaturated hydrocarbons that contain at least one carbon-carbon double bond ($C=C$).

$sp^2$ Hybridization

The electron configuration of carbon atoms in a double bond, resulting in a planar structure around the $C=C$ bond with $120^{\circ}$ bond angles.

Degree of Unsaturation ($\rho$)

A value representing the number of $\pi$ bonds and/or rings in a molecule, calculated using the formula $\rho = \frac{2 + 2C - H + N - X}{2}$.

Markovnikov's Rule

In the addition of $HX$ to an unsymmetrical alkene, the hydrogen bonds to the less substituted carbon (the one with more hydrogens initially).

Cis Isomer

A geometric isomer where two identical or similar groups attached to the double bond are on the same side.

Trans Isomer

A geometric isomer where two identical or similar groups attached to the double bond are on opposite sides; these are generally more stable than cis isomers.

E/Z Isomerism

A nomenclature system for alkenes based on Cahn-Ingold-Prelog (CIP) rules to prioritize groups; $Z$ ($Zusammen$) indicates high-priority groups on the same side, while $E$ ($Entgegen$) indicates opposite sides.

Hydrohalogenation

A two-step electrophilic addition reaction where $HX$ transforms an alkene into an alkyl halide via a carbocation intermediate.

Halogenation

A two-step addition of $X_2$ ($Cl_2$ or $Br_2$) to form a vicinal dihalide through a bridged halonium ion intermediate, resulting in $anti$-addition.

Hydration

A three-step mechanism involving the addition of $H_2O$ in the presence of an acid catalyst ($H_2SO_4$) to produce an alcohol from an alkene.

Halohydrin Formation

The reaction of an alkene with a halogen ($X_2$) and water ($H_2O$) to form a halohydrin ($OH$ and $X$ adjacent), proceeding via a bridged halonium ion and $anti$-addition.

$N$-bromosuccinimide ($NBS$)

A reagent used in aqueous $DMSO$ to form bromohydrins from alkenes through the same mechanism as $Br_2/H_2O$.

Oxymercuration-Demercuration

A two-step hydration method using $Hg(OAc)_2$ and $NaBH_4$ that produces a Markovnikov alcohol without carbocation rearrangements.

Hydroboration-Oxidation

A two-step reaction using $BH_3\text{-THF}$ followed by $H_2O_2/OH^{-}$ that results in $anti$-Markovnikov, $syn$-addition of $H$ and $OH$.

Syn Addition

An addition reaction where both substituents are added to the same face or side of the double bond.

Anti Addition

An addition reaction where substituents are added to opposite sides of the double bond.

Hydrogenation

The addition of $H_2$ across a multiple bond using a metal catalyst (such as $Pd/C$, $Pt$, or $Ni$), typically proceeding via $syn$-addition.

Monomers

Small molecules, often alkenes, that link covalently to form long, repeating chains called polymers.

Epoxidation

The formation of a three-membered cyclic ether (epoxide) by reacting an alkene with a peracid (such as $mCPBA$) via a concerted, $syn$-addition mechanism.

Ozonolysis

The oxidative cleavage of a $C=C$ bond using ozone ($O_3$), followed by a reductive workup ($Zn/H_2O$ or $DMS$) to yield aldehydes and/or ketones.

Conjugated Diene

A diene where two double bonds are separated by exactly one single bond ($C=C-C=C$), allowing for $\pi$-electron overlap and resonance stabilization.

Isolated Diene

A diene where the double bonds are separated by two or more single bonds, preventing $\pi$-electron interaction.

Cumulated Diene (Allene)

A diene where the double bonds share a single central carbon atom ($C=C=C$).

Thermodynamic Product

In diene addition, the 1,4-addition product that is most stable, favored at higher temperatures (e.g., $40^{\circ}C$).

Kinetic Product

In diene addition, the 1,2-addition product that is formed faster, favored at lower temperatures (e.g., $0^{\circ}C$).

Alkynes

Unsaturated hydrocarbons containing at least one carbon-carbon triple bond ($C \equiv C$).

$sp$ Hybridization

The electron configuration of carbon atoms in a triple bond, resulting in a linear geometry with $180^{\circ}$ bond angles.

Terminal Alkyne

An alkyne where the triple bond is at the end of the carbon chain ($RC \equiv CH$); these are more acidic than alkenes or alkanes.

Acetylide Ion

A nucleophile formed by the deprotonation of a terminal alkyne by a strong base like $NaNH_2$, used to form new carbon-carbon bonds via alkylation.

Lindlar's Catalyst

A poisoned palladium catalyst used for the selective partial reduction of an alkyne to a $cis$-alkene.

Tautomers

Constitutional isomers in rapid equilibrium that differ in the position of a double bond and a hydrogen atom, such as enol and keto forms.

Geminal Dihalide

A product formed by the addition of two equivalents of $HX$ to an alkyne, where both halogens are bonded to the same carbon atom.