Organic Chemistry - Reaction Mechanisms & Substitutions Flashcards

Question 1

$S_N1$ Stereochemical Outcome

Accepted Answer

**Racemization** (yielding a racemic mixture of inversion and retention). The nucleophile can attack the planar, $sp^2$-hybridized carbocation intermediate equally from either face.

Question 2

Curved Arrow Notation

Accepted Answer

Curved arrows illustrate the movement of **electron pairs** (not individual atoms). A full arrowhead represents a pair of electrons moving, while a single-barbed (fishhook) arrow represents a single electron moving in radical pathways.

Question 3

Carbocation Stability Order

Accepted Answer

Order of stability: $	ext{Benzylic} \approx 	ext{Allylic} > 3^\circ > 2^\circ > 1^\circ > 	ext{Methyl} (CH_3^+)$.

Stabilized by **hyperconjugation** (sigma bond electron donation into the empty $p$-orbital), **inductive effects** from alkyl groups, and **resonance delocalization** in allylic/benzylic systems.

Question 4

$S_N2$ Mechanism Characteristics

Accepted Answer

- **Kinetics**: Second-order (Rate = $k[	ext{substrate}][	ext{nucleophile}]$).
- **Mechanism**: One-step, concerted attack of nucleophile and leaving group departure.
- **Substrate Preference**: $	ext{Methyl} > 1^\circ > 2^\circ \gg 3^\circ$ (unreactive due to steric hindrance).

Question 5

$S_N2$ Stereochemical Outcome

Accepted Answer

**Inversion of configuration** (Walden inversion). The nucleophile performs a backside attack opposite the leaving group, causing a stereocenter to flip like an umbrella in a high wind ($R \rightarrow S$ or $S \rightarrow R$).

Question 6

$S_N1$ Mechanism Characteristics

Accepted Answer

- **Kinetics**: First-order (Rate = $k[	ext{substrate}]$).
- **Mechanism**: Two-step pathway via a rate-determining carbocation intermediate.
- **Substrate Preference**: $3^\circ > 2^\circ \gg 1^\circ, 	ext{Methyl}$ (governed by carbocation stability).

Question 7

Polar Protic Solvents

Accepted Answer

- **Definition**: Solvents containing hydrogen bond donors ($O-H$ or $N-H$), e.g., $	ext{H}_2	ext{O}$, $	ext{CH}_3	ext{OH}$, $	ext{EtOH}$.
- **Pathway Favored**: **$S_N1$ / $E1$** by solvating both cations and anions, stabilizing the carbocation intermediate and leaving group.

Question 8

Polar Aprotic Solvents

Accepted Answer

- **Definition**: Polar solvents lacking acidic protons ($O-H$ or $N-H$), e.g., Acetone, DMSO, DMF, Acetonitrile.
- **Pathway Favored**: **$S_N2$** by solvating cations while leaving nucleophiles unsolvated ("naked") and highly reactive.

Question 9

Nucleophilicity vs. Basicity

Accepted Answer

- **Nucleophilicity**: A **kinetic** measurement of how rapidly an electron pair attacks an electrophilic carbon atom.
- **Basicity**: A **thermodynamic** measurement of affinity for a proton ($	ext{H}^+$), quantified by $K_a$ / $pK_a$.

Question 10

Halide Nucleophilicity Trends in Solvents

Accepted Answer

- **Polar Protic**: $	ext{I}^- > 	ext{Br}^- > 	ext{Cl}^- > 	ext{F}^-$ (Larger, more polarizable ions break free from solvent hydrogen-bonding cages more easily).
- **Polar Aprotic**: $	ext{F}^- > 	ext{Cl}^- > 	ext{Br}^- > 	ext{I}^-$ (Nucleophilicity tracks basicity because hydration shells are absent).

Question 11

Good Leaving Group Properties

Accepted Answer

Good leaving groups are **weak bases** (conjugate bases of strong acids). They stably hold negative charge due to high electronegativity, large ionic size, or resonance. Examples: $	ext{I}^-$, $	ext{Br}^-$, $	ext{H}_2	ext{O}$, and Tosylate ($	ext{OTs}^-$).

Question 12

$E2$ Mechanism Characteristics

Accepted Answer

- **Kinetics**: Second-order (Rate = $k[	ext{substrate}][	ext{base}]$).
- **Mechanism**: Concerted abstraction of a $\beta$-proton while the leaving group departs.
- **Stereospecificity**: Requires an **anti-periplanar** dihedral geometry ($180^\circ$) between the $\beta$-hydrogen and leaving group.

Question 13

Zaitsev's Rule

Accepted Answer

In an elimination reaction ($E1$ or $E2$), the **more substituted, thermodynamically stable alkene** is formed as the major product when using small, unhindered bases (e.g., $	ext{NaOCH}_3$, $	ext{KOH}$).

Question 14

Hofmann Product

Accepted Answer

The **less substituted alkene** is formed as the major product in $E2$ eliminations when using a **bulky, sterically hindered base** (e.g., $	ext{K}^{+}{}^-	ext{OtBu}$) or when using poor leaving groups like fluoride.

Question 15

$E1$ Mechanism Characteristics

Accepted Answer

- **Kinetics**: First-order (Rate = $k[	ext{substrate}]$).
- **Mechanism**: Loss of leaving group forms a carbocation intermediate, followed by deprotonation by a weak base.
- **Regioselectivity**: Always yields the Zaitsev product; competes with $S_N1$ under neutral/heat conditions.

Question 16

Markovnikov's Rule

Accepted Answer

In the electrophilic addition of $	ext{H-X}$ to an asymmetric alkene, the proton ($	ext{H}^+$) adds to the carbon with **more hydrogens**, forming the more stable carbocation intermediate on the more substituted carbon.

Question 17

Anti-Markovnikov Addition Conditions

Accepted Answer

Addition of $	ext{HBr}$ in the presence of **peroxides** ($	ext{ROOR}$) and heat/light. Operates through a **free-radical mechanism**, placing the bromine radical on the less substituted carbon to generate the more stable alkyl radical.

Question 18

Carbocation Rearrangements

Accepted Answer

Occurs during $S_N1$ or $E1$ processes when a less stable carbocation ($1^\circ$ or $2^\circ$) can shift to form a more stable carbocation ($2^\circ$ or $3^\circ$).
- **1,2-Hydride Shift**: Migration of a hydride ion ($	ext{H}^-$).
- **1,2-Methide Shift**: Migration of a methyl group ($	ext{CH}_3^-$).

Question 19

Competition: Strong Base/Nucleophile ($1^\circ$ vs $2^\circ$ vs $3^\circ$ Substrate)

Accepted Answer

With strong nucleophiles/bases (e.g., $	ext{NaOEt}$):
- **$1^\circ$ Substrates**: $S_N2$ predominates ($E2$ only if base is bulky).
- **$2^\circ$ Substrates**: $E2$ predominates.
- **$3^\circ$ Substrates**: $E2$ exclusively ($S_N2$ blocked by steric hindrance).

Question 20

Solvolysis Reactions

Accepted Answer

A substitution reaction in which the **solvent acts as the nucleophile** (e.g., hydrolysis with $	ext{H}_2	ext{O}$, alcoholysis with $	ext{CH}_3	ext{OH}$). It generally proceeds via an **$S_N1$ mechanism**.

Organic Chemistry - Reaction Mechanisms & Substitutions

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