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

Question 1

Radical Addition of HBr (Peroxide Effect)

Accepted Answer

In the presence of peroxides ($	ext{ROOR}$), $	ext{HBr}$ adds to alkenes with **Anti-Markovnikov regioselectivity** via a free-radical chain mechanism, because the bromine radical ($	ext{Br}^\bullet$) adds first to form the more stable carbon radical.

Question 2

$S_N2$ Mechanism Overview & Rate Law

Accepted Answer

A concerted, bimolecular nucleophilic substitution reaction occurring in a single step.

- **Rate Law**: $	ext{Rate} = k[	ext{Substrate}][	ext{Nucleophile}]$ (2nd order)
- **Intermediate**: None (proceeds via a single pentacoordinate transition state).

Question 3

$S_N2$ Stereochemical Outcome

Accepted Answer

Results in complete **inversion of configuration** (Walden inversion) at the electrophilic carbon due to back-side attack by the nucleophile opposite the leaving group.

Question 4

$S_N2$ Substrate Reactivity Trend

Accepted Answer

$	ext{Methyl} > 1^\circ > 2^\circ \gg 3^\circ$ (unreactive).

**Reason**: Steric hindrance at the reaction center slows or completely blocks back-side nucleophilic attack.

Question 5

$S_N1$ Mechanism Overview & Rate Law

Accepted Answer

A stepwise, unimolecular nucleophilic substitution reaction.

- **Rate Law**: $	ext{Rate} = k[	ext{Substrate}]$ (1st order)
- **Rate-Determining Step (RDS)**: Heterolytic cleavage of the carbon-leaving group bond to form a carbocation intermediate.

Question 6

$S_N1$ Stereochemical Outcome

Accepted Answer

**Racemization** (mixture of enantiomers with partial inversion excess).

The planar, $sp^2$-hybridized carbocation intermediate can be attacked equally from either the front or back face by the nucleophile.

Question 7

$S_N1$ Substrate Reactivity Trend

Accepted Answer

$3^\circ > 2^\circ \gg 1^\circ > 	ext{Methyl}$.

**Reason**: Governed by carbocation stability (hyperconjugation and inductive effect). Allylic and benzylic substrates are also highly reactive due to resonance stabilization.

Question 8

$E2$ Mechanism Overview & Rate Law

Accepted Answer

A concerted, bimolecular elimination reaction where a strong base abstracts a $\beta$-proton while the leaving group departs simultaneously.

- **Rate Law**: $	ext{Rate} = k[	ext{Substrate}][	ext{Base}]$ (2nd order)

Question 9

$E2$ Stereospecificity & Conformation

Accepted Answer

Requires an **anti-periplanar** transition state where the $\beta$-hydrogen and the leaving group lie in the same plane at a dihedral angle of $180^\circ$ (anti to each other).

Question 10

Zaitsev's Rule vs. Hofmann's Rule

Accepted Answer

- **Zaitsev's Product**: The more substituted, thermodynamically stable alkene. Favored by small, unhindered bases (e.g., $	ext{NaOCH}_3$).
- **Hofmann's Product**: The less substituted alkene. Favored by bulky, sterically hindered bases (e.g., potassium $t$-butoxide, $t	ext{-BuOK}$).

Question 11

$E1$ Mechanism Overview & Rate Law

Accepted Answer

A stepwise, unimolecular elimination reaction.

- **Rate Law**: $	ext{Rate} = k[	ext{Substrate}]$ (1st order)
- **Steps**: (1) Loss of leaving group to form a carbocation (RDS), (2) Deprotonation by a weak base to yield an alkene (predominantly Zaitsev product).

Question 12

Carbocation Rearrangements (1,2-Shifts)

Accepted Answer

If a carbocation intermediate ($S_N1$ or $E1$) can rearrange to a more stable state, a **1,2-hydride shift** or **1,2-alkyl shift** occurs before nucleophilic attack or deprotonation.

*Example*: $2^\circ$ carbocation adjacent to a $3^\circ$ or $4^\circ$ carbon shifts to become a $3^\circ$ carbocation.

Question 13

Solvent Effects: Polar Protic vs. Polar Aprotic

Accepted Answer

- **Polar Protic** (e.g., $	ext{H}_2	ext{O}$, $	ext{EtOH}$): Solvates cations and anions via hydrogen bonding. Stabilizes carbocations (favors $S_N1/E1$) but lowers nucleophilicity.
- **Polar Aprotic** (e.g., $	ext{DMSO}$, $	ext{DMF}$, acetone): Solvates cations only, leaving nucleophiles 'naked' and highly reactive (favors $S_N2$).

Question 14

Nucleophilicity vs. Basicity

Accepted Answer

- **Nucleophilicity**: Kinetic property measuring how fast a atom attacks an electrophilic carbon.
- **Basicity**: Thermodynamic property measuring affinity for a proton ($	ext{H}^+$).

*Note*: Steric bulk reduces nucleophilicity significantly without decreasing basicity (e.g., $t	ext{-BuO}^-$ is a strong base but poor nucleophile).

Question 15

Leaving Group Ability Trend

Accepted Answer

Good leaving groups are weak bases (conjugate bases of strong acids) that stably hold negative charge.

**Trend**: $	ext{I}^- > 	ext{Br}^- > 	ext{Cl}^- \gg 	ext{F}^-$
*Other good leaving groups*: $	ext{TsO}^-$ (tosylate), $	ext{TfO}^-$ (triflate), $	ext{H}_2	ext{O}$.

Question 16

Decision Matrix: Primary ($1^\circ$) Alkyl Halides

Accepted Answer

- Strong Base / Strong Nucleophile (e.g., $	ext{NaOH}$): **$S_N2$** (major)
- Bulky Base (e.g., $t	ext{-BuOK}$): **$E2$** (major, Hofmann product)
- Weak Base / Weak Nucleophile: No reaction.

Question 17

Decision Matrix: Secondary ($2^\circ$) Alkyl Halides

Accepted Answer

- Strong Base / Strong Nucleophile (e.g., $	ext{NaOEt}$): **$E2$** (major) + $S_N2$ (minor)
- Weak Base / Strong Nucleophile (e.g., $	ext{NaI}$, $	ext{NaRS}$): **$S_N2$**
- Weak Base / Weak Nucleophile (e.g., $	ext{EtOH}$, solvolysis): **$S_N1 / E1$**

Question 18

Decision Matrix: Tertiary ($3^\circ$) Alkyl Halides

Accepted Answer

- Strong Base (e.g., $	ext{NaOEt}$ or $t	ext{-BuOK}$): **$E2$** ($S_N2$ impossible due to steric hindrance)
- Weak Base / Weak Nucleophile (e.g., $	ext{MeOH}$ solvolysis): **$S_N1 / E1$** mixture

Question 19

Markovnikov's Rule

Accepted Answer

In electrophilic addition of $	ext{H-X}$ to an unsymmetrical alkene, the $	ext{H}^+$ adds to the carbon with more hydrogens to form the most stable carbocation intermediate, followed by attack of $	ext{X}^-$.

Question 20

Anti-Markovnikov Hydroboration-Oxidation Mechanism

Accepted Answer

1. **Hydroboration**: Concerted syn-addition of $	ext{BH}_3$ across the alkene; boron attaches to the less substituted carbon.
2. **Oxidation**: $	ext{H}_2	ext{O}_2 / 	ext{NaOH}$ replaces boron with $-	ext{OH}$ with retention of stereochemistry, yielding an anti-Markovnikov alcohol.

Question 21

Temperature Effect on Substitution vs. Elimination

Accepted Answer

**Higher temperatures favor elimination ($E1/E2$) over substitution ($S_N1/S_N2$)**.

Elimination cleaves two bonds to form three molecules ($\Delta S > 0$). In $\Delta G = \Delta H - T\Delta S$, the $-T\Delta S$ term becomes more dominant at higher temperatures.

Question 22

Ring Expansion Rearrangements

Accepted Answer

A special 1,2-alkyl shift where a 4- or 5-membered ring adjacent to a carbocation expands into a 5- or 6-membered ring to relieve thermodynamic ring strain.

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

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