Alkyl Halides and Alkene Reactions: Substitution, Elimination, and Addition

Vocabulary practice flashcards covering nucleophilic substitution ($$S_N1$$, $$S_N2$$), elimination ($$E1$$, $$E2$$), and electrophilic/oxidative additions to alkenes from organic chemistry lecture notes.

Substitution Reaction

A chemical reaction in which a leaving group (such as a halogen) attached to an $\alpha$-carbon of an alkyl halide substrate is replaced by a nucleophile.

Elimination Reaction

A reaction in which an alkyl halide substrate loses a hydrogen atom from the $\beta$-carbon and a leaving group to form an alkene ($\pi$ bond).

Substrate

The alkyl halide molecule undergoing a nucleophilic substitution or elimination reaction.

Nucleophilicity

The kinetic rate at which a nucleophile attacks an electrophile, primarily governed by the nucleophile's polarizability and net electrical charge.

Polarizability

The ability of an atom to distribute its electron density unevenly; larger atoms with more electrons are more polarizable (e.g., $\text{SH}^- > \text{OH}^-$).

Protic Solvent

A solvent containing a hydrogen atom bonded directly to an electronegative atom.

Polar Aprotic Solvent

A solvent that lacks hydrogen atoms bonded directly to electronegative atoms, accelerating $S_N2$ reactions because it does not solvate nucleophilic anions effectively.

$S_N2$ Reaction

A bimolecular nucleophilic substitution characterized by second-order kinetics ($\text{rate} = k[\text{R-X}][\text{Nuc}]$) and a concerted mechanism where bond breaking and bond forming occur simultaneously in a single step.

Inversion of Stereochemistry

The stereochemical consequence of an $S_N2$ mechanism resulting from the nucleophile attacking strictly from the opposite face of the leaving group due to steric and electrostatic repulsion.

E2 Reaction

A bimolecular elimination reaction characterized by second-order kinetics ($\text{rate} = k[\text{R-X}][\text{Base}]$) and a concerted mechanism requiring the proton and leaving group to adopt an antiperiplanar geometry.

Antiperiplanar Configuration

The specific coplanar conformation required in an E2 reaction where the $\beta$-hydrogen and the leaving group reside on opposite faces of the molecule ($180^\circ$ dihedral angle).

Alkene Stability

The thermodynamic property indicating that alkenes become increasingly stable as the number of alkyl substituents attached to the $C=C$ bond increases.

Regioselective Reaction

A chemical reaction that preferentially yields one constitutional isomer over another possible product.

Zaitsev Product

The more substituted alkene isomer formed during an elimination reaction, favored when using small, sterically unhindered bases such as $\text{NaOEt}$.

Hofmann Product

The less substituted alkene isomer formed during an elimination reaction, favored when using sterically hindered bases such as potassium tert-butoxide ($\text{KO}t\text{Bu}$).

$S_N1$ Reaction

A unimolecular nucleophilic substitution governed by first-order kinetics ($\text{rate} = k[\text{R-X}]$) that proceeds through a carbocation intermediate and forms a racemic mixture.

E1 Reaction

A unimolecular elimination reaction governed by first-order kinetics ($\text{rate} = k[\text{R-X}]$) that proceeds via a stepwise carbocation intermediate to form an alkene; it competes with $S_N1$ and is favored at elevated temperatures.

Acid-Catalyzed Dehydration of Alcohols

An E1 pathway wherein an alcohol is treated with concentrated acid ($\text{H}_2\text{SO}_4$ or $\text{H}_3\text{PO}_4$) and heat to yield an alkene through a carbocation intermediate while regenerating the acid catalyst.

Hydrohalogenation

An electrophilic addition reaction across an alkene double bond using $\text{HCl}$, $\text{HBr}$, or $\text{HI}$, proceeding through a carbocation intermediate.

Markovnikov Addition

A regiochemical pattern where an electrophilic addition attaches the halogen (or heteroatom) to the more substituted carbon and the hydrogen to the double-bond carbon bearing more hydrogens.

Anti-Markovnikov Addition

A regiochemical outcome wherein a substituent adds to the less substituted carbon of a $C=C$ bond, as observed in the radical addition of $\text{HBr}$ in the presence of peroxides ($\text{ROOR}$).

Acid-Catalyzed Hydration of Alkenes

A reversible addition of water across an alkene in the presence of dilute $\text{H}_2\text{SO}_4$ to form a Markovnikov alcohol via a carbocation intermediate.

Oxymercuration-Demercuration

A two-step synthetic method utilizing 1) $\text{Hg(OAc)}_2$, $\text{H}_2\text{O}$ and 2) $\text{NaBH}_4$ to achieve Markovnikov addition of $\text{OH}$ and $\text{H}$ without carbocation rearrangement.

Hydroboration-Oxidation

A two-step addition sequence utilizing 1) $\text{BH}_3\cdot\text{THF}$ and 2) $\text{H}_2\text{O}_2$, $\text{NaOH}$ yielding an anti-Markovnikov, syn-addition of $\text{H}$ and $\text{OH}$ across an alkene without carbocation rearrangement.

Catalytic Hydrogenation

The syn-addition of molecular hydrogen ($\text{H}_2$) across an alkene double bond mediated by a transition metal catalyst such as $\text{Pt}$, $\text{Pd}$, or $\text{Ni}$.

Heterogeneous Catalyst

A solid, insoluble metal catalyst (such as $\text{Pt}$, $\text{Pd}$, or $\text{Ni}$) that exists in a different physical phase than the reactants, facilitating reaction directly at its surface.

Homogeneous Catalyst

A catalyst that dissolves completely in the same liquid phase as the reactants, exemplified by Wilkinson's catalyst ($\text{RhCl(PPh}_3)_3$).

Halogenation

The anti-addition of molecular chlorine ($\text{Cl}_2$) or bromine ($\text{Br}_2$) across an alkene occurring via a three-membered cyclic halonium ion intermediate.

Halohydrin Formation

An anti-addition reaction of a halogen ($\text{Cl}_2$ or $\text{Br}_2$) across an alkene in the presence of water, resulting in the halogen adding to the less substituted carbon and $\text{OH}$ adding to the more substituted carbon.

Anti-Dihydroxylation

A multi-step reaction converting an alkene to a trans-1,2-diol via an epoxide intermediate created with a peracid ($\text{RCO}_3\text{H}$), followed by acid-catalyzed ring opening.

Syn-Dihydroxylation

An oxidation process adding two hydroxyl groups to the same face of an alkene to form a cis-1,2-diol using reagents such as $\text{OsO}_4$ with $\text{Na}_2\text{SO}_3/\text{H}_2\text{O}$ or $\text{NMO}$.

Ozonolysis

The oxidative cleavage of a $C=C$ bond using ozone ($\text{O}_3$) followed by dimethyl sulfide ($\text{Me}_2\text{S}$ or DMS) to generate aldehydes or ketones via molozonide and ozonide intermediates.

Retrosynthetic Analysis

A synthetic planning approach denoted by $\Rightarrow$ that works backward from a target molecule to simple starting materials, aiming to minimize steps and maximize yield.