Organic Chemistry: Substitution, Elimination, and Alkene Addition Reactions

Comprehensive vocabulary flashcards covering nucleophilic substitution, elimination reactions, and alkene addition reactions.

Leaving Group

An atom or group that leaves with the bonding electron pair during a reaction; good leaving groups are weak bases that form stable species after leaving.

Halide Leaving Group Ability

The relative ease with which halide ions leave in a reaction, ranked from best to worst as $I^- > Br^- > Cl^- > F^-$.

Tosylate ($-\text{OTs}$)

A good leaving group formed by reacting an alcohol with $\text{TsCl}$, converting the $-\text{OH}$ group while preserving the original carbon–oxygen bond.

Mesylate ($-\text{OMs}$)

A good leaving group formed by reacting an alcohol with $\text{MsCl}$, converting the $-\text{OH}$ group while preserving the original carbon–oxygen bond.

Nucleophile

An electron-pair donor that attacks an electron-poor atom, commonly carbon; its reactivity is characterized by nucleophilicity (kinetic rate of attack).

Base

A species that accepts a proton ($H^+$); its reactivity is characterized by basicity (thermodynamic equilibrium tendency to accept $H^+$).

Sodium Acetate ($\text{CH}_3\text{CO}_2\text{Na}$)

A reagent containing acetate ($\text{CH}_3\text{CO}_2^-$) that acts as a good nucleophile but a weak base in substitution reactions.

Sodium Hydride ($\text{NaH}$)

A strong base utilized in substitution and elimination reactions specifically to promote E2 elimination.

Methoxide ($\text{CH}_3\text{O}^-$)

A small, unhindered reagent that behaves as both a strong nucleophile and a strong base.

tert-Butoxide ($t\text{-BuO}^-$)

A sterically hindered, strong bulky base and relatively poor nucleophile that favors elimination over substitution.

DBU and DBN

Strong, bulky, non-nucleophilic bases used in E2 reactions to favor the formation of less substituted elimination products.

Zaitsev's Rule

A rule predicting that an elimination reaction will favor the formation of the more substituted, thermodynamically more stable alkene.

Hofmann Product

The less substituted alkene formed in an elimination reaction, preferentially favored when sterically hindered, bulky bases such as tert-butoxide, DBU, or DBN are used.

$$\text{S}_\text{N}2\text{ Reaction}$$

A bimolecular nucleophilic substitution proceeding in one concerted step with backside attack and inversion of configuration, following the substrate order: methyl > primary ($1^\circ$) > secondary ($2^\circ$).

Polar Aprotic Solvent

A solvent lacking hydrogen atoms bonded to heteroatoms (such as acetonitrile, THF, and DMSO) that favors $\text{S}_\text{N}2$ reactions by leaving anionic nucleophiles free and unencumbered by hydrogen bonding.

Polar Protic Solvent

A solvent containing hydrogen atoms bonded directly to heteroatoms like oxygen or nitrogen (e.g., water, methanol, and ethanol) capable of hydrogen bonding.

E2 Reaction

A bimolecular elimination occurring in one concerted step requiring a strong base and an anti-periplanar geometry between the leaving group and beta hydrogen, following the substrate order: tertiary ($3^\circ$) > secondary ($2^\circ$) > primary ($1^\circ$).

Anti-periplanar Geometry

The required E2 transition-state alignment in which the beta hydrogen and the leaving group lie in the exact same plane pointing in opposite directions ($180^\circ$ dihedral angle).

Trans-diaxial Arrangement

The specific cyclohexane conformation required for E2 elimination, wherein the leaving group and beta hydrogen are both positioned axially and trans to one another.

$$\text{S}_\text{N}1\text{ Reaction}$$

A unimolecular nucleophilic substitution proceeding through at least two steps via a carbocation intermediate, following first-order kinetics ($\text{Rate} = k[\text{alkyl halide}]$) and leading to racemization at a stereocenter.

E1 Reaction

A unimolecular elimination proceeding via a carbocation intermediate under weak-base conditions to yield predominantly the more substituted Zaitsev trans alkene, favored by heating.

Hydrohalogenation

The addition of HX ($X = \text{Cl}, \text{Br}, \text{I}$) across an alkene double bond via a carbocation intermediate to yield an alkyl halide with Markovnikov regiochemistry.

Markovnikov Addition

A regiochemical outcome in alkene additions where the electrophilic hydrogen attaches to the less substituted carbon (bearing more hydrogens), and the nucleophile attaches to the more substituted carbon.

Acid-Catalyzed Hydration

An electrophilic addition of H and OH across an alkene using $H_3O^+ / H_2O$ or dilute aqueous $H_2SO_4$ to form a Markovnikov alcohol via a carbocation intermediate.

Oxymercuration–Demercuration

A two-step hydration process using $\text{Hg(OAc)}_2$ and $H_2O$ followed by $\text{NaBH}_4$ reduction to produce a Markovnikov alcohol via anti addition without carbocation rearrangements.

Hydroboration–Oxidation

A two-step reaction using $\text{BH}_3\cdot\text{THF}$ followed by $H_2O_2 / \text{NaOH}$ that converts an alkene into an alcohol with anti-Markovnikov regiochemistry and syn stereochemistry.

Alkene Halogenation

The addition of $Br_2$ or $Cl_2$ across an alkene in nonpolar or polar aprotic solvent to yield a vicinal dihalide via anti addition through a cyclic halonium intermediate.

Halonium Ion

A bridged, three-membered cationic intermediate (e.g., a bromonium ion) formed during alkene halogenation that prevents carbocation rearrangement and directs backside nucleophilic attack.

Halohydrin Formation

The reaction of an alkene with $Br_2$ or $Cl_2$ in water, resulting in anti addition where the halogen adds to the less substituted carbon and $-\text{OH}$ adds to the more substituted carbon.

Catalytic Hydrogenation

The syn addition of $H_2$ across a carbon–carbon double bond catalyzed by transition metals such as Pt, Pd, or Ni to form an alkane.

Anti-Dihydroxylation

A reaction sequence involving the conversion of an alkene to an epoxide using a peroxy acid, followed by aqueous acid- or base-catalyzed ring opening to yield a trans vicinal diol.

Syn-Dihydroxylation

The addition of two $-\text{OH}$ groups to the same face of an alkene using either $\text{OsO}_4$ followed by aqueous $\text{Na}_2\text{SO}_3$/$\text{NaHSO}_3$, or cold $\text{KMnO}_4$, producing a cis vicinal diol.

Ozonolysis

The oxidative cleavage of an alkene double bond using $O_3$ followed by a reductive workup (DMS or $\text{Zn}/H_2O$) to cleave the alkene into aldehydes and/or ketones.