Organic Chemistry: Substitution and Elimination Reactions

Comprehensive vocabulary flashcards generated from the substitution and elimination reaction lecture notes, detailing reaction types, mechanisms, rate laws, stereochemistry, and solvent effects.

Substitution Reaction

A reaction in which an atom or group (such as $\text{OH}$) takes the place of (substitutes) a leaving group ($\text{LG}$) on a carbon atom.

Elimination Reaction

A reaction in which a hydrogen atom ($\text{H}$) and a leaving group ($\text{LG}$) are removed (eliminated) from adjacent carbon atoms to form a double bond.

Halogen Leaving Group Ability

The trend in leaving group ability for halogens, which increases with atomic size ($\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-$) because larger halogen atoms form longer carbon-halogen bonds.

Sulfonate Leaving Groups

Common non-halogen leaving groups that include tosylates ($\text{TsO}$), triflates ($\text{TfO}$), and mesylates ($\text{MsO}$).

Substitution on $sp^2$-Hybridized Carbons

A rule stating that substitution reactions cannot occur when the leaving group is attached directly to an $sp^2$-hybridized carbon, such as in vinyl halides or aryl halides.

$S_N1$ Reaction

Substitution nucleophilic unimolecular; a substitution mechanism where the slow (rate-determining) step involves only one reactant, forming a carbocation intermediate.

Rate Law for $S_N1$

The rate equation for an $S_N1$ reaction, given by $\text{rate} = k[\text{electrophile}]$, which depends only on the concentration of the substrate.

Stereochemistry of $S_N1$

The stereochemical outcome of an $S_N1$ reaction, which yields a racemic mixture of enantiomers because the nucleophile can attack the planar carbocation intermediate from either side.

$S_N2$ Reaction

Substitution nucleophilic bimolecular; a concerted substitution mechanism where two reactants (electrophile and nucleophile) are involved in the single rate-determining step.

Rate Law for $S_N2$

The rate equation for an $S_N2$ reaction, given by $\text{rate} = k[\text{electrophile}][\text{nucleophile}]$.

$S_N2$ Transition State

A single pentacoordinate transition state in which the central carbon atom is semi-$sp^2$-hybridized as the nucleophile attacks from the backside while the leaving group leaves.

Stereochemistry of $S_N2$

The stereochemical outcome of an $S_N2$ reaction, which results in complete inversion of stereochemistry at the stereocenter due to backside attack by the nucleophile.

Strong Nucleophile

A species with a localized negative charge (e.g., $M\text{-}CN$, $M\text{-}OR$, $M\text{-}OH$, $M\text{-}SR$) that favors bimolecular ($S_N2$) substitution pathways.

Weak Nucleophile

A neutral or resonance-stabilized species (e.g., $\text{RCO}_2^-$; $\text{HOR}$, $\text{H}_2\text{O}$) that favors unimolecular ($S_N1$) substitution pathways.

$E1$ Reaction

Elimination unimolecular; an elimination mechanism where the rate-determining step involves only the electrophile forming a carbocation intermediate before deprotonation by a weak base.

Rate Law for $E1$

The rate equation for an $E1$ reaction, given by $\text{rate} = k[\text{electrophile}]$.

$E2$ Reaction

Elimination bimolecular; a concerted elimination reaction where a base removes a proton as the leaving group leaves in a single rate-determining step involving both reactants.

Rate Law for $E2$

The rate equation for an $E2$ reaction, given by $\text{rate} = k[\text{electrophile}][\text{base}]$.

Zaitsev's Rule

A principle stating that elimination reactions ($E1$ and $E2$) predominantly form the most-substituted double bond and favor the $E$-alkene over the $Z$-alkene.

Anti-Zaitsev Product via Conjugated Dienes

An exception to Zaitsev's rule in which elimination forms a conjugated diene as the major product because the stability of conjugation outweighs alkene substitution degree.

Anti-Zaitsev Product via Tert-Butyl Bases

An exception to Zaitsev's rule in which sterically bulky bases (e.g., tert-butoxide or tert-butanol) remove the most accessible proton to yield the less-substituted alkene as the major product.

Protic Solvent

A solvent with hydrogen atoms bonded to oxygen, nitrogen, or sulfur (e.g., $\text{CH}_3\text{OH}$, $\text{CH}_3\text{CH}_2\text{OH}$, $\text{H}_2\text{O}$) that stifles strong nucleophiles/bases and favors $S_N1$, $E1$, and $E2$ reactions.

Aprotic Solvent

A solvent lacking hydrogens bonded to electronegative heteroatoms (e.g., DMSO, acetone, DMF, acetonitrile) that does not stifle strong nucleophiles/bases and favors $S_N2$ and $E2$ reactions.

Anti-Periplanar Geometry

The geometric requirement in $E2$ reactions where the eliminated hydrogen atom and leaving group reside in the same plane pointing $180^\circ$ opposite to each other.

Axial Requirement in Cyclohexanes ($E2$)

The requirement in cyclohexane rings that both the eliminated hydrogen atom and the leaving group must be in axial positions (anti-coplanar) for an $E2$ elimination to occur.

Polarizability

The ability of an atom to distort its electron cloud; larger atoms are soft (highly polarizable), while smaller atoms are hard (low polarizability).

Hard/Soft Acid/Base (HSAB) Theory

A theory stating that soft acids prefer bonding with soft bases (highly polarizable) and hard acids (like the small $\text{H}^+$ ion) prefer bonding with hard bases (low polarizability).

Nucleophilicity Trend in Protic Solvents

The periodic trend where nucleophilicity increases going down a column because larger, softer atoms are less stifled by hydrogen bonding with protic solvent molecules.

Nucleophilicity Trend in Aprotic Solvents

The periodic trend where nucleophilicity increases going up a column because smaller atoms with higher charge density are not stifled by hydrogen bonding in aprotic solvents and act as stronger nucleophiles.