Organic Chemistry - Reaction Mechanisms: Substitution & Elimination Flashcards

Question 1

$S_N1$ Mechanism: Rate Law and Order

Accepted Answer

Unimolecular Nucleophilic Substitution. Stepwise mechanism via a carbocation intermediate.

**Rate Law:** $	ext{Rate} = k[	ext{R-X}]$
**Order:** First-order overall (depends strictly on substrate concentration).

Question 2

$S_N2$ Mechanism: Rate Law and Order

Accepted Answer

Bimolecular Nucleophilic Substitution. Concerted mechanism (occurs in a single step).

**Rate Law:** $	ext{Rate} = k[	ext{R-X}][	ext{Nu}^-]$
**Order:** Second-order overall (1st order in substrate, 1st order in nucleophile).

Question 3

$S_N2$ Stereochemistry: Walden Inversion

Accepted Answer

Occurs via an mandatory **backside attack**, leading to **complete stereochemical inversion** (Walden inversion). An $(R)$ reactant yields an $(S)$ product, provided priority order remains unchanged.

Question 4

$S_N1$ Stereochemistry: Racemization

Accepted Answer

Forms a planar, $sp^2$-hybridized **carbocation intermediate**. The nucleophile can attack equally from either the top or bottom face, resulting in **racemization** (a 50:50 mixture of enantiomers).

Question 5

Substrate Reactivity Trends: $S_N2$ vs. $S_N1$

Accepted Answer

- **$S_N2$ Rate:** $	ext{Methyl} > 1^\circ > 2^\circ \gg 3^\circ$ (unreactive due to steric hindrance).
- **$S_N1$ Rate:** $3^\circ > 2^\circ \gg 1^\circ > 	ext{Methyl}$ (driven by carbocation stability).

Question 6

Polar Protic Solvents

Accepted Answer

Solvents containing $	ext{O-H}$ or $	ext{N-H}$ bonds (e.g., $H_2O$, $CH_3OH$, $EtOH$).
- Solvate both cations and anions via hydrogen bonding.
- Stabilize carbocations and weaken nucleophiles.
- **Favors $S_N1$ and $E1$ mechanisms.**

Question 7

Polar Aprotic Solvents

Accepted Answer

Solvents lacking hydrogen-bond donors (e.g., DMSO, DMF, acetone, acetonitrile).
- Solvate cations well, but leave nucleophiles 'naked' and unencumbered.
- **Favors $S_N2$ and $E2$ mechanisms.**

Question 8

Nucleophilicity vs. Basicity

Accepted Answer

- **Nucleophilicity:** A kinetic property measuring the rate at which a species attacks an electrophilic carbon atom.
- **Basicity:** A thermodynamic property measuring how strongly a species binds a proton ($H^+$) at equilibrium.

Question 9

Halide Nucleophilicity Trends by Solvent Type

Accepted Answer

- **Polar Protic:** $I^- > Br^- > Cl^- > F^-$ (larger ions are less solvated and more polarizable).
- **Polar Aprotic:** $F^- > Cl^- > Br^- > I^-$ (nucleophilicity mirrors basicity as hydrogen bonding is absent).

Question 10

$E2$ Mechanism: Key Features & Kinetics

Accepted Answer

Bimolecular Elimination. Concerted, single-step mechanism requiring a strong base.

**Rate Law:** $	ext{Rate} = k[	ext{R-X}][	ext{Base}]$
**Order:** Second-order overall.

Question 11

$E2$ Stereospecificity: Anti-Periplanar Requirement

Accepted Answer

The $\beta$-hydrogen and the leaving group must be arranged **anti-periplanar** (dihedral angle of $180^\circ$) in the transition state to allow optimal orbital overlap for $\pi$-bond formation.

Question 12

Zaitsev's Rule

Accepted Answer

Elimination reactions typically favor the formation of the **more substituted, thermodynamically more stable alkene** as the major product when using small, unhindered bases (e.g., $NaOCH_3$, $NaOH$).

Question 13

Hofmann's Rule

Accepted Answer

Elimination reactions yield the **less substituted, less stable alkene** as the major product when using sterically hindered (bulky) bases (e.g., potassium tert-butoxide, $t	ext{-BuOK}$).

Question 14

$E1$ Mechanism: Key Features & Kinetics

Accepted Answer

Unimolecular Elimination. Two-step mechanism where the leaving group departs first, forming a carbocation, followed by base deprotonation.

**Rate Law:** $	ext{Rate} = k[	ext{R-X}]$
Follows Zaitsev's rule.

Question 15

Carbocation Stability Trend

Accepted Answer

$	ext{Allylic/Benzylic} \approx 3^\circ > 2^\circ > 1^\circ > 	ext{Methyl}$

Stabilized by **hyperconjugation** (donation of adjacent $\sigma$-electron density) and **inductive effects** or **resonance**.

Question 16

1,2-Hydride Shift

Accepted Answer

A carbocation rearrangement where a hydride ion ($H^-$) migrates from an adjacent carbon to transform a less stable carbocation (e.g., $2^\circ$) into a more stable carbocation (e.g., $3^\circ$).

Question 17

1,2-Methyl (Alkyl) Shift

Accepted Answer

A carbocation rearrangement where a methyl group ($	ext{-CH}_3$) with its electron pair migrates from a quaternary adjacent carbon to a secondary carbocation, creating a more stable $3^\circ$ carbocation.

Question 18

Leaving Group Ability Trend

Accepted Answer

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

**Trend:** $I^- > Br^- > Cl^- \gg F^-$
*Super leaving groups:* Tosylate ($TsO^-$), Triflate ($TfO^-$), Mesylate ($MsO^-$).

Question 19

Reaction Matrix: Strong Base / Strong Nucleophile

Accepted Answer

Reagents: $OH^-$, $MeO^-$, $EtO^-$
- **$1^\circ$ Substrate:** Major $S_N2$, Minor $E2$
- **$2^\circ$ Substrate:** Major $E2$, Minor $S_N2$
- **$3^\circ$ Substrate:** $E2$ exclusively

Question 20

Reaction Matrix: Strong Base / Weak Nucleophile

Accepted Answer

Reagents: $t	ext{-BuOK}$, $NaH$, $LDA$
- **$1^\circ$ Substrate:** $E2$
- **$2^\circ$ Substrate:** $E2$
- **$3^\circ$ Substrate:** $E2$

Question 21

Reaction Matrix: Weak Base / Strong Nucleophile

Accepted Answer

Reagents: $I^-$, $Br^-$, $RS^-$, $HS^-$, $CN^-$
- **$1^\circ$ Substrate:** $S_N2$
- **$2^\circ$ Substrate:** $S_N2$
- **$3^\circ$ Substrate:** $S_N1$

Question 22

Reaction Matrix: Weak Base / Weak Nucleophile

Accepted Answer

Reagents: $H_2O$, $ROH$
- **$1^\circ$ & $2^\circ$ Substrates:** No reaction (too slow at room temp).
- **$3^\circ$ Substrate:** Mixture of $S_N1$ and $E1$ (heat shifts preference toward $E1$).

Organic Chemistry - Reaction Mechanisms: Substitution & Elimination

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