Substitution and Elimination Mechanisms Lecture Review

Summary of Substitution (SN1, SN2) and Elimination (E1, E2) reaction mechanics, solvent effects, and common bases/nucleophiles.

$SN2$ Mechanism

A single-step, bimolecular reaction ($Nu^- + R-X \rightarrow Nu-R + X^-$) characterized by second-order kinetics and 100% inversion of stereochemistry.

$SN1$ Mechanism

A two-step, unimolecular reaction ($R-X \rightarrow R^+ + X^-$ followed by $R^+ + Nu^- \rightarrow R-Nu$) characterized by first-order kinetics and carbocation formation.

$SN2$ Rate Equation

$\text{Rate} = k[RX][Nu]$, where the rate depends on both the nature and concentration of the nucleophile.

$SN1$ Rate Equation

$\text{Rate} = k[RX]$, indicating the rate is independent of the nucleophile concentration.

$SN2$ Reactivity Order (Alkyl Halides)

$CH_3X > RCH_2X > R_2CHX > R_3CX$ (Methyl > Primary > Secondary > Tertiary).

$SN1$ Reactivity Order (Alkyl Halides)

$R_3CX > R_2CHX > RCH_2X > CH_3X$ (Tertiary > Secondary > Primary > Methyl).

Leaving Group Reactivity

The order of reactivity for leaving groups in both $SN1$ and $SN2$ is $RI > RBr > RCl > RF$.

Major Product in Eliminations

When eliminations give rise to more than one alkene, the more highly substituted alkene is typically the major product.

Elimination Promotion Factors

Substances and conditions that promote eliminations include strong bases, hindered bases, and high temperatures.

$SN2$ Solvent Preference

Polar aprotic solvents (such as $DMSO$, $DMF$, and $CH_3CN$) provide the fastest reaction rates.

$SN1$ Solvent Preference

Rate increases with the polarity of the solvent, typically utilizing polar protic solvents.

Strong Bases

Examples include $NaOH$, $NaOCH_3$, $NaOCH_2CH_3$, $NaNH_2$, $NaNR_2$, $NaH$, and $NaC\equiv{CH}$.

Strong Hindered Bases

Examples include $NaOCH(CH_3)_2$, $NaOC(CH_3)_3$, and $LDA$ (lithium diisopropylamide).

Weak Bases

Examples include $NaCN$, $NaN_3$, $NaSH$, $NaCl$, $NaBr$, $NaI$, $RCO_2Na$, and $NH_3$.

Very Weak Bases

Neutral compounds such as $H_2O$, $CH_3OH$, $CH_3CH_2OH$, and other general alcohols ($ROH$).

Polar Protic Solvents

Solvents capable of hydrogen bonding, including water, $MeOH$, $EtOH$, $PrOH$, and acetic acid.

Polar Aprotic Solvents

Solvents that lack acidic protons, including $DMSO$, $DMF$, acetone, and $CH_3CN$ (acetonitrile).

$E2$ Geometry Requirement

For secondary ($2^o$) and tertiary ($3^o$) alkyl halides reacting with a strong base, the leaving group ($X$) and the $\beta$ hydrogen ($H$) must be coplanar.

Solvolysis

A specific type of $SN1$ reaction occurring with $2^o$ or $3^o$ alkyl halides using $H_2O$ or $ROH$ as both the solvent and nucleophile.

$E1$ Reaction Conditions

Typically occurs with secondary ($2^o$) or tertiary ($3^o$) alkyl halides using weak or dilute bases in polar protic solvents, often with elevated temperatures.