Organic Chemistry - S_N1, S_N2, E1, and E2 Mechanisms Flashcards

Question 1

$S_N2$ Rate Law & Kinetics

Accepted Answer

Second-order kinetics: $	ext{Rate} = k[	ext{Substrate}][	ext{Nucleophile}]$. It is a bimolecular, single-step (concerted) reaction with no reaction intermediate.

Question 2

$S_N2$ Stereochemical Outcome

Accepted Answer

Complete **inversion of configuration** (Walden inversion) at a chiral electrophilic center due to backside attack by the incoming nucleophile opposite the leaving group.

Question 3

$S_N2$ Substrate Reactivity Trend

Accepted Answer

$	ext{Methyl} > 1^\circ > 2^\circ \gg 3^\circ$. $3^\circ$ substrates do not undergo $S_N2$ due to extreme steric hindrance at the electrophilic carbon.

Question 4

$S_N1$ Rate Law & Kinetics

Accepted Answer

First-order kinetics: $	ext{Rate} = k[	ext{Substrate}]$. It is a unimolecular, multi-step mechanism where loss of the leaving group to form a carbocation is the rate-determining step (RDS).

Question 5

$S_N1$ Stereochemical Outcome

Accepted Answer

**Racemization** (a mixture of retention and inversion). The intermediate is a planar, $sp^2$-hybridized carbocation that can be attacked with equal probability from either the top or bottom face.

Question 6

$S_N1$ Substrate Reactivity Trend

Accepted Answer

$3^\circ > 2^\circ \gg 1^\circ > 	ext{Methyl}$. Reactivity depends directly on carbocation stability (stabilized by inductive effects and hyperconjugation).

Question 7

E2 Rate Law & Mechanism

Accepted Answer

Second-order kinetics: $	ext{Rate} = k[	ext{Substrate}][	ext{Base}]$. A concerted, single-step bimolecular elimination where a strong base pulls a $\beta$-proton as the double bond forms and the leaving group departs.

Question 8

E2 Stereochemical Requirement

Accepted Answer

Requires an **anti-periplanar** conformation ($180^\circ$ dihedral angle) between the $\beta$-hydrogen and the leaving group to ensure proper orbital overlap ($	ext{C-H}\ \sigma ightarrow 	ext{C-X}\ \sigma^*$) during the transition state.

Question 9

E1 Rate Law & Mechanism

Accepted Answer

First-order kinetics: $	ext{Rate} = k[	ext{Substrate}]$. Stepwise unimolecular elimination where leaving group departure forms a carbocation (RDS), followed by deprotonation by a weak base to form an alkene.

Question 10

Zaitsev's Rule

Accepted Answer

Elimination reactions preferentially yield the **more substituted, thermodynamically stable alkene** as the major product by removing a $\beta$-hydrogen from the carbon with fewer hydrogens.

Question 11

Hofmann's Rule

Accepted Answer

Elimination reactions yield the **less substituted alkene** as the major product. Preferred when using a sterically hindered base (e.g., $t$-BuOK) or a poor leaving group (e.g., $-	ext{F}$ or $-	ext{NR}_3^+$).

Question 12

Polar Protic Solvents & Their Role

Accepted Answer

Solvents containing $	ext{O-H}$ or $	ext{N-H}$ bonds (e.g., $	ext{H}_2	ext{O}$, $	ext{EtOH}$, $	ext{MeOH}$). They form strong hydrogen bonds, stabilizing both carbocations (favoring $S_N1$/E1) and nucleophiles (reducing nucleophilicity for $S_N2$).

Question 13

Polar Aprotic Solvents & Their Role

Accepted Answer

Solvents lacking $	ext{O-H}$ or $	ext{N-H}$ bonds (e.g., DMSO, DMF, acetone, acetonitrile). They solvate cations well but leave nucleophiles naked/unsolvated, greatly accelerating $S_N2$ rates.

Question 14

Carbocation Rearrangements ($S_N1$ and E1)

Accepted Answer

Occur via $1,2$-hydride shifts or $1,2$-alkyl shifts whenever a carbocation intermediate can convert into a more stable state (e.g., $2^\circ \rightarrow 3^\circ$ or allylic/benzylic).

Question 15

Leaving Group Ability Trend

Accepted Answer

Good leaving groups are weak conjugate bases of strong acids: $	ext{I}^- > 	ext{Br}^- > 	ext{Cl}^- \gg 	ext{F}^-$, as well as $	ext{TsO}^-$, $	ext{MsO}^-$, $	ext{TfO}^-$, and $	ext{H}_2	ext{O}$. Poor leaving groups ($-	ext{OH}$, $-	ext{NH}_2$, $-	ext{OR}$) must be protonated or modified to leave.

Question 16

Nucleophilicity vs. Basicity

Accepted Answer

**Nucleophilicity** is a kinetic property measuring the rate of attack on an electrophilic carbon. **Basicity** is a thermodynamic property measuring affinity for a proton ($H^+$).

Question 17

Temperature Effect on Elimination vs. Substitution

Accepted Answer

**High heat favors elimination** over substitution. Elimination splits one substrate molecule into two product molecules (alkene + conjugate acid + LG), resulting in a positive entropy change ($\Delta S^\circ > 0$), making $\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ$ more favorable at higher $T$.

Question 18

E2 Regioselectivity in Cyclohexane Rings

Accepted Answer

Elimination can only occur if the leaving group and the adjacent $\beta$-hydrogen are both in **trans-diaxial** (axial-axial) orientations in the ring chair conformation.

Question 19

Halide Nucleophilicity Trends in Different Solvents

Accepted Answer

In **polar protic** solvents: $	ext{I}^- > 	ext{Br}^- > 	ext{Cl}^- > 	ext{F}^-$ (larger ions are less solvated). In **polar aprotic** solvents: $	ext{F}^- > 	ext{Cl}^- > 	ext{Br}^- > 	ext{I}^-$ (follows basicity due to minimal solvation).

Question 20

Summary: $1^\circ$, $2^\circ$, and $3^\circ$ Alkyl Halide Pathways

Accepted Answer

- **$1^\circ$**: $S_N2$ (with good Nu), E2 (with strong hindered base like $t$-BuOK).
- **$2^\circ$**: $S_N2$ (weak base, good Nu in aprotic), E2 (strong base), $S_N1$/E1 (solvolysis with weak Nu/base).
- **$3^\circ$**: E2 (strong base), $S_N1$/E1 (weak base/solvolysis); **never $S_N2$**.

Organic Chemistry - S_N1, S_N2, E1, and E2 Mechanisms

Cards in this deck

Related Flashcard Sets

Organic Chemistry - Nucleophilic Substitution & Elimination Mechanisms

Organic Chemistry - SN1, SN2, E1, and E2 Reaction Mechanisms

Organic Chemistry - Reaction Mechanisms (SN1, SN2, E1, E2)

Series 7 Exam - Options Strategies & Municipal Securities