Organic Chemistry - Reaction Mechanisms: SN1, SN2, E1 & E2 Reactions Flashcards

Question 1

What is the rate law and kinetic order of an $S_N2$ reaction?

Accepted Answer

$	ext{Rate} = k[	ext{substrate}][	ext{nucleophile}]$. It is a second-order, bimolecular reaction where the rate depends on the concentration of both the electrophilic substrate and the nucleophile in a single concerted step.

Question 2

What is the stereochemical outcome of an $S_N2$ reaction, and why?

Accepted Answer

**Complete inversion of configuration** (Walden inversion). The nucleophile performs a back-side attack on the $C-LG$ $\sigma^*$ antibonding orbital opposite the leaving group, causing a umbrella-like flip of the stereocenter.

Question 3

What is the order of substrate reactivity for $S_N2$ reactions?

Accepted Answer

**Methyl > $1^\circ$ > $2^\circ$ >> $3^\circ$ (unreactive)**. Steric hindrance around the electrophilic carbon slows down back-side attack. Tertiary ($3^\circ$) substrates are too sterically hindered for $S_N2$ to occur.

Question 4

Why do polar aprotic solvents favor $S_N2$ reactions?

Accepted Answer

Polar aprotic solvents (e.g., acetone, DMSO, DMF, acetonitrile) solvate cations well but do not form hydrogen bonds with anions. This leaves the nucleophile **unsolvated ('naked')** and highly reactive, lowering the activation energy barrier.

Question 5

What is the rate law and rate-determining step of an $S_N1$ reaction?

Accepted Answer

$	ext{Rate} = k[	ext{substrate}]$. It is a first-order, unimolecular reaction. The rate-determining step (RDS) is the slow heterolytic cleavage of the substrate to form a carbocation intermediate.

Question 6

What is the stereochemical outcome of an $S_N1$ reaction at a chiral center?

Accepted Answer

**Racemization** (mixture of inversion and retention). Heterolytic cleavage forms a planar, $sp^2$-hybridized carbocation intermediate that can be attacked with equal probability from either face.

Question 7

What is the order of substrate reactivity for $S_N1$ reactions?

Accepted Answer

**$3^\circ$ > $2^\circ$ >> $1^\circ$ / Methyl (unreactive)**. Reactivity mirrors carbocation stability, which is stabilized by hyperconjugation and inductive electron donation from alkyl groups.

Question 8

Why do polar protic solvents favor $S_N1$ and $E1$ mechanisms?

Accepted Answer

Polar protic solvents (e.g., $H_2O$, $CH_3OH$, $EtOH$) contain acidic hydrogens that stabilize both the leaving group anion (via hydrogen bonding) and the carbocation intermediate, lowering the transition state energy of the rate-determining step.

Question 9

What triggers a 1,2-hydride or 1,2-alkyl shift in carbocation rearrangements?

Accepted Answer

Occurs whenever a carbocation intermediate can rearrange to a **more stable carbocation** (e.g., $2^\circ ightarrow 3^\circ$, or $2^\circ ightarrow$ resonance-stabilized allylic/benzylic) by shifting a adjacent hydrogen ($H^-$) or alkyl group ($-	ext{CH}_3$) with its bonding pair.

Question 10

What are the rate law and geometry requirements for an $E2$ reaction?

Accepted Answer

$	ext{Rate} = k[	ext{substrate}][	ext{base}]$. It is a concerted bimolecular elimination requiring an **anti-periplanar** arrangement ($180^\circ$ dihedral angle) between the $\beta$-hydrogen and the leaving group for optimal orbital overlap.

Question 11

How do base steric effects dictate Zaitsev vs. Hofmann product selectivity in $E2$?

Accepted Answer

- **Small, unhindered bases** (e.g., $NaOCH_3$, $NaOH$) give the **Zaitsev product** (more substituted, thermodynamically stable alkene).
- **Bulky, hindered bases** (e.g., potassium $t$-butoxide, $t	ext{-BuOK}$) give the **Hofmann product** (less substituted alkene) due to steric access to the $\beta$-hydrogen.

Question 12

Describe the two-step mechanism and product selectivity of an $E1$ reaction.

Accepted Answer

1. Loss of leaving group to form a carbocation intermediate (RDS).
2. Weak base deprotonates the $\beta$-position to form a $\pi$ bond.
- Product selectivity follows **Zaitsev's rule** (favors the more substituted, stable alkene) with predominant $E$-stereochemistry.

Question 13

What characteristics determine a good leaving group, and how do halides rank?

Accepted Answer

Good leaving groups are **weak bases** (stable anions, conjugate bases of strong acids). Halide leaving group ability: $I^- > Br^- > Cl^- >> F^-$. Excellent non-halide leaving groups include tosylate ($	ext{OTs}^-$) and mesylate ($	ext{OMs}^-$).

Question 14

What is the key distinction between nucleophilicity and basicity?

Accepted Answer

- **Basicity**: A *thermodynamic* property measuring a species' affinity for a proton ($H^+$).
- **Nucleophilicity**: A *kinetic* property measuring the rate of attack on an electrophilic carbon. Steric hindrance drastically reduces nucleophilicity without significantly reducing basicity.

Question 15

What structural condition must be met for an $E2$ elimination on a cyclohexane ring?

Accepted Answer

The leaving group and the $\beta$-hydrogen **must be trans-diaxial** (both in axial positions on adjacent carbons). If the leaving group is equatorial, a ring flip must occur before $E2$ elimination can take place.

Question 16

How do primary ($1^\circ$) substrates behave under substitution vs elimination conditions?

Accepted Answer

- Strong nucleophile/base (e.g., $NaOH$, $NaOCH_3$): **$S_N2$** (major).
- Bulky strong base (e.g., $t	ext{-BuOK}$): **$E2$** (major).
- Unreactive via $S_N1$ or $E1$ due to unstable primary carbocation formation.

Question 17

How do tertiary ($3^\circ$) substrates behave under substitution vs elimination conditions?

Accepted Answer

- **$S_N2$ is IMPOSSIBLE** due to steric hindrance.
- Strong base (e.g., $NaOCH_3$, $t	ext{-BuOK}$): **$E2$** (rapid).
- Weak base/nucleophile (e.g., $H_2O$, $EtOH$): **$S_N1$ + $E1$** mixture (solvolysis).

Question 18

How does solvent choice alter halide nucleophilicity order in $S_N2$?

Accepted Answer

- **Polar Protic**: $I^- > Br^- > Cl^- > F^-$ (smaller ions are tightly caged by H-bonding, lowering reactivity).
- **Polar Aprotic**: $F^- > Cl^- > Br^- > I^-$ (unsolvated ions reflect intrinsic basicity/charge density).

Question 19

What is a ring-expansion carbocation rearrangement and when does it occur?

Accepted Answer

Occurs when a carbocation forms adjacent to a small, strained ring (e.g., cyclobutyl or cyclopentyl). A ring $\sigma$-bond breaks and shifts to the carbocation center, expanding a 4-membered ring to 5, or 5 to 6, relieving ring strain.

Question 20

How do secondary ($2^\circ$) substrates choose between $S_N2$, $S_N1$, $E2$, and $E1$?

Accepted Answer

- Strong base / Strong nucleophile ($HO^-$, $RO^-$): **$E2$ major, $S_N2$ minor**.
- Weak base / Strong nucleophile ($I^-$, $RS^-$): **$S_N2$**.
- Strong bulky base ($t	ext{-BuOK}$): **$E2$ (Hofmann)**.
- Weak base / Weak nucleophile ($H_2O$, $ROH$): **$S_N1$ / $E1$** (solvolysis with heating).

Organic Chemistry - Reaction Mechanisms: SN1, SN2, E1 & E2 Reactions

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