Substitution and Elimination Reactions – Key Vocabulary

35 vocabulary flashcards summarizing essential terms, trends, and mechanistic features of nucleophilic substitution and elimination reactions.

Nucleophile

An electron-rich species that donates electron density to an electrophile during a reaction.

Electrophile

An electron-poor species susceptible to attack by a nucleophile.

Nucleophilicity Trend Across a Period

Increases from right to left on the periodic table because lower electronegativity makes atoms more willing to donate electrons.

Nucleophilicity Trend Down a Group in Polar Protic Solvents

Increases down the group; larger ions are less solvated and therefore more reactive.

Nucleophilicity Trend Down a Group in Polar Aprotic Solvents

Decreases down the group because nucleophilicity parallels basicity, which drops with increasing atomic size.

Polar Protic Solvent

Solvent containing hydrogen-bond donors (e.g., H₂O, methanol, ethanol, acetic acid, NH₃) that stabilize anions by hydrogen bonding.

Polar Aprotic Solvent

Solvent lacking hydrogen-bond donors (e.g., acetone, DMSO, DCM, THF) that leaves anions unsolvated and highly nucleophilic.

Leaving Group (LG)

Atom or group that departs with an electron pair; best when it is a weak, stable base (e.g., I⁻, Br⁻, Cl⁻, tosylate).

SN2 Reaction

Bimolecular nucleophilic substitution in one concerted step where the nucleophile attacks and the leaving group departs simultaneously.

SN2 Rate Law

rate = k [nucleophile][electrophile] (second order, depends on both reactants).

Pentavalent Transition State

High-energy state in SN2 where the electrophilic carbon is weakly bonded to both nucleophile and leaving group.

Backside Attack

SN2 nucleophile approaches opposite the leaving group, causing inversion of configuration at a chiral center.

Walden Inversion

Stereochemical inversion produced by backside attack in SN2 reactions.

SN1 Reaction

Stepwise nucleophilic substitution involving leaving-group departure, carbocation formation, then nucleophilic attack.

SN1 Rate Law

rate = k [electrophile] (unimolecular; independent of nucleophile concentration).

Carbocation Intermediate

Positively charged sp² carbon species formed in SN1 and E1 mechanisms.

Carbocation Rearrangement

Hydride or methyl shift that converts a less stable carbocation into a more stable one.

Racemic Mixture

Equal mixture of enantiomers produced when nucleophile attacks planar carbocation from either side in SN1.

E2 Reaction

Concerted beta-elimination where base removes a proton while leaving group departs to form an alkene.

E2 Rate Law

rate = k [base][substrate] (second order, bimolecular).

Anti-Periplanar Requirement

In E2, the beta-hydrogen and leaving group must be 180° apart to allow π-bond formation.

Stereoselective E2

When two beta-hydrogens exist, the more stable E (trans) alkene is preferred over the Z (cis) product.

Stereospecific E2

When only one relevant beta-hydrogen exists, a single alkene stereoisomer forms, dictated by anti-periplanar geometry.

E1 Reaction

Two-step elimination: leaving group departs to form a carbocation, then base removes a beta-proton to give an alkene.

E1 Rate Law

rate = k [substrate] (unimolecular; depends only on substrate concentration).

Zaitsev Product

The more substituted, thermodynamically favored alkene produced in elimination reactions.

Hofmann Product

The less substituted alkene favored when a bulky base is used.

Bulky Base

Sterically hindered strong base (e.g., t-BuOK) that favors E2 elimination to give the Hofmann product.

Acid-Catalyzed Dehydration

Elimination of water from an alcohol using concentrated acid and heat; gives Zaitsev alkene (E1 for 2°/3°, E2 for 1°).

Hydride Shift

Migration of a hydrogen with its bonding electrons to a neighboring carbocation, increasing stability.

Methyl Shift

Migration of a CH₃ group with its bonding electrons to stabilize a carbocation.

Substrate Degree of Substitution

Primary favors SN2/E2; tertiary favors SN1/E1 (or E2); secondary may undergo any mechanism depending on other factors.

Base/Nucleophile Strength

Strong charged species promote SN2/E2; weak neutral species promote SN1/E1.

Steric Hindrance of Base

Bulky bases act mainly as bases leading to E2 over SN2.

Solvent Effect on Mechanism

Polar aprotic favors SN2; polar protic encourages E2 (and can stabilize carbocations for SN1/E1).

Temperature Effect

Higher temperature shifts competition toward elimination (E1/E2) over substitution (SN1/SN2).