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

Question 1

Nucleophilicity vs. Basicity

Accepted Answer

- **Basicity:** A *thermodynamic* property measuring affinity for a proton ($K_b$).
- **Nucleophilicity:** A *kinetic* property measuring attack rate on an electrophilic carbon atom.

Question 2

$S_N2$ Reaction Mechanism & Rate Law

Accepted Answer

A bimolecular nucleophilic substitution proceeding in a single concerted step without intermediates.

**Rate Law:** $	ext{Rate} = k[	ext{substrate}][	ext{nucleophile}]$

Question 3

$S_N2$ Stereochemical Outcome

Accepted Answer

Results in complete **inversion of stereochemical configuration** (Walden inversion) at the electrophilic carbon due to required $180^\circ$ backside attack by the incoming nucleophile.

Question 4

$S_N2$ Substrate Preference

Accepted Answer

Favored by unhindered substrates: $	ext{Methyl} > 1^\circ > 2^\circ \gg 3^\circ$.

$3^\circ$ substrates **cannot** undergo $S_N2$ due to extreme steric hindrance at the reaction center.

Question 5

$S_N1$ Reaction Mechanism & Rate Law

Accepted Answer

A unimolecular nucleophilic substitution proceeding in two steps via a carbocation intermediate.

**Rate Law:** $	ext{Rate} = k[	ext{substrate}]$ (rate-determining step is leaving group departure).

Question 6

$S_N1$ Stereochemical Outcome

Accepted Answer

Results in **racemization** (a mixture of retention and inversion) because the planar, $sp^2$-hybridized carbocation intermediate can be attacked equally from either face.

Question 7

$S_N1$ Substrate Preference

Accepted Answer

Favored by highly substituted substrates: $3^\circ > 2^\circ \gg 1^\circ > 	ext{Methyl}$.

$3^\circ$ carbocations are stabilized by hyperconjugation and inductive effects.

Question 8

Role of Polar Aprotic Solvents ($S_N2$)

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 unsolvated ('naked'), raising nucleophile energy and accelerating $S_N2$ reactions.

Question 9

Role of Polar Protic Solvents ($S_N1$ & $E1$)

Accepted Answer

Solvents with $	ext{O-H}$ or $	ext{N-H}$ bonds (e.g., $	ext{H}_2	ext{O}$, $	ext{MeOH}$, $	ext{EtOH}$). They form hydrogen bonds that stabilize carbocation intermediates and leaving groups, accelerating $S_N1$ and $E1$ rate-determining steps.

Question 10

$E2$ Reaction Mechanism & Rate Law

Accepted Answer

A bimolecular elimination proceeding in a single concerted step where a base removes a $\beta$-hydrogen as the leaving group departs to form a $\pi$ bond.

**Rate Law:** $	ext{Rate} = k[	ext{substrate}][	ext{base}]$

Question 11

$E2$ Stereochemical Requirement

Accepted Answer

Requires the $\beta$-hydrogen and the leaving group to be **anti-periplanar** ($180^\circ$ dihedral angle) to allow optimal orbital overlap for $\pi$-bond formation.

Question 12

Zaitsev's Rule

Accepted Answer

In elimination reactions ($E1$ or $E2$ with a small base), the major product is the **more substituted, thermodynamically more stable alkene** (e.g., trisubstituted > disubstituted).

Question 13

Hofmann's Rule & Bulky Bases

Accepted Answer

When an $E2$ reaction uses a sterically hindered base (e.g., potassium $t$-butoxide, $t	ext{-BuOK}$), the base abstracts the most accessible $\beta$-hydrogen, yielding the **less substituted alkene** as the major product.

Question 14

$E1$ Reaction Mechanism & Rate Law

Accepted Answer

A unimolecular elimination proceeding via a carbocation intermediate in two steps.

**Rate Law:** $	ext{Rate} = k[	ext{substrate}]$

Competes directly with $S_N1$ under neutral or weakly basic conditions.

Question 15

Carbocation Rearrangements in $S_N1$ / $E1$

Accepted Answer

$1,2$-hydride or $1,2$-alkyl shifts occur whenever a secondary ($2^circ$) carbocation can rearrange to a more stable tertiary ($3^circ$) or resonance-stabilized carbocation before nucleophilic attack or elimination.

Question 16

Distinguishing Reaction Path for $1^\circ$ Alkyl Halides

Accepted Answer

- **Strong Unhindered Nucleophile/Base** (e.g., $	ext{NaOEt}$): $S_N2$ (major), $E2$ (minor)
- **Strong Bulky Base** (e.g., $t	ext{-BuOK}$): $E2$
- **Weak Nucleophile/Base** (e.g., $	ext{H}_2	ext{O}$): Unreactive (too slow)

Question 17

Distinguishing Reaction Path for $2^\circ$ Alkyl Halides

Accepted Answer

- **Strong Base / Strong Nucleophile** (e.g., $	ext{OH}^-$, $	ext{RO}^-$): $E2$ (major), $S_N2$ (minor)
- **Weak Base / Strong Nucleophile** (e.g., $	ext{I}^-$, $	ext{CN}^-$, $	ext{N}_3^-$): $S_N2$
- **Weak Base / Weak Nucleophile** (e.g., $	ext{H}_2	ext{O}$, $	ext{EtOH}$): $S_N1$ / $E1$ mixture

Question 18

Distinguishing Reaction Path for $3^\circ$ Alkyl Halides

Accepted Answer

- **Strong Base** (e.g., $	ext{OH}^-$, $	ext{RO}^-$, $t	ext{-BuOK}$): $E2$ exclusively ($S_N2$ is sterically blocked).
- **Weak Base / Weak Nucleophile** (e.g., $	ext{H}_2	ext{O}$, $	ext{EtOH}$): $S_N1$ / $E1$ mixture.

Question 19

Leaving Group Ability Trends

Accepted Answer

Good leaving groups are weak, stable bases (conjugate bases of strong acids).

**Trend:** $	ext{I}^- > 	ext{Br}^- > 	ext{Cl}^- \gg 	ext{F}^-$

Sulfonate esters like tosylate ($	ext{OTs}^-$) and mesylate ($	ext{OMs}^-$) are exceptional leaving groups due to resonance stabilization.

Question 20

Temperature Effect on Substitution vs. Elimination

Accepted Answer

**High heat favors elimination ($E1$/$E2$) over substitution ($S_N1$/$S_N2$).**

Elimination increases entropy (1 molecule $ightarrow$ 2 or 3 molecules, $\Delta S > 0$). In $\Delta G = \Delta H - T\Delta S$, higher $T$ amplifies the $-T\Delta S$ term, making elimination more favorable.

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

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