Organic Chemistry - Reaction Mechanisms, SN1/SN2 & E1/E2 Pathways Flashcards

Question 1

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

Accepted Answer

$Rate = k[	ext{Substrate}][	ext{Nucleophile}]$. It is a **bimolecular** ($2^{nd}$ order) reaction where the rate depends on concentrations of both the substrate and the nucleophile.

Question 2

What stereochemical outcome occurs during an $S_N2$ reaction and why?

Accepted Answer

**Inversion of configuration** (Walden inversion). The nucleophile attacks from the backside of the $C-LG$ bond into the $\sigma^*$ antibonding orbital, turning the stereocenter inside out.

Question 3

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

Accepted Answer

$Rate = k[	ext{Substrate}]$. The **rate-determining step** is unimolecular heterolytic cleavage of the $C-LG$ bond to form a carbocation intermediate.

Question 4

What stereochemical outcome is expected for an $S_N1$ reaction at a chiral center?

Accepted Answer

**Racemization** (formation of a mixture of enantiomers). The intermediate carbocation is planar ($sp^2$ hybridized), allowing the nucleophile to attack equally from either the top or bottom face.

Question 5

How do polar protic solvents affect $S_N1$ vs $S_N2$ reactions?

Accepted Answer

Polar protic solvents (e.g., $H_2O$, $EtOH$) hydrogen-bond with nucleophiles, forming a solvation shell that lowers nucleophilicity (**inhibiting $S_N2$**), but stabilize carbocation intermediates and leaving groups (**favoring $S_N1$/$E1$**).

Question 6

How do polar aprotic solvents accelerate $S_N2$ reactions?

Accepted Answer

Polar aprotic solvents (e.g., DMSO, DMF, Acetone) solvate cations well but cannot hydrogen-bond with anions, leaving nucleophiles "naked" and highly reactive to perform backside attacks.

Question 7

What is the substrate preference order for $S_N2$ vs $S_N1$ pathways?

Accepted Answer

- **$S_N2$**: $	ext{Methyl} > 1^\circ > 2^\circ \gg 3^\circ$ (unhindered for backside attack; $3^\circ$ blocked by steric hindrance).
- **$S_N1$**: $3^\circ > 2^\circ \gg 1^\circ > 	ext{Methyl}$ (requires stable carbocation).

Question 8

What is the fundamental difference between Nucleophilicity and Basicity?

Accepted Answer

- **Nucleophilicity**: A **kinetic** parameter measuring the rate at which an electron pair attacks an electrophilic carbon atom.
- **Basicity**: A **thermodynamic** parameter measuring the affinity of a species for a proton ($H^+$) at equilibrium.

Question 9

How does steric hindrance affect nucleophilicity compared to basicity?

Accepted Answer

Steric hindrance **significantly decreases nucleophilicity** because bulky groups block access to carbon centers, but it has **minimal impact on basicity** because protons ($H^+$) are tiny and unhindered. Example: Potassium $tert$-butoxide ($t	ext{-BuOK}$) is a strong base but poor nucleophile.

Question 10

What geometric orientation is strictly required for an $E2$ mechanism?

Accepted Answer

The $\beta$-hydrogen and leaving group must be **anti-periplanar** (dihedral angle of $180^\circ$). This allows optimal parallel alignment of the breaking $\sigma$ bonds to form the new $\pi$ bond concerted launch.

Question 11

What is Zaitsev's Rule in elimination reactions?

Accepted Answer

In an elimination reaction, the major product is the **more substituted, thermodynamically stable alkene**, which forms when using a small, unhindered base (e.g., $NaOCH_3$, $KOH$).)

Question 12

What is Hofmann's Rule in elimination reactions?

Accepted Answer

In an $E2$ elimination, using a **sterically hindered base** (e.g., $t	ext{-BuOK}$, LDA) yields the **less substituted alkene** as the major product because the base abstracts the most accessible $\beta$-proton.

Question 13

What is the rate law and mechanism of an $E1$ reaction?

Accepted Answer

$Rate = k[	ext{Substrate}]$. A two-step process: (1) Departure of the leaving group to form a carbocation, followed by (2) Deprotonation at a $\beta$-carbon by a weak base to yield an alkene.

Question 14

What is the rate law and mechanism of an $E2$ reaction?

Accepted Answer

$Rate = k[	ext{Substrate}][	ext{Base}]$. A single-step concerted mechanism where a strong base removes a $\beta$-proton at the same time the $\pi$ bond forms and the leaving group leaves.

Question 15

What characteristics define a good leaving group?

Accepted Answer

Good leaving groups are **weak bases** (conjugate bases of strong acids) that stably hold a negative charge or neutral state upon departure. Order: $I^- > Br^- > Cl^- \gg F^-$, $H_2O > OH^-$.

Question 16

What is carbocation rearrangement and when does it occur?

Accepted Answer

A 1,2-hydride shift or 1,2-alkyl shift that converts a less stable carbocation into a more stable one (e.g., $2^\circ \rightarrow 3^\circ$). It occurs exclusively in pathways with carbocation intermediates ($S_N1$ and $E1$).

Question 17

How do primary ($1^\circ$) alkyl halides react with strong nucleophiles vs bulky bases?

Accepted Answer

- **Strong unhindered nucleophile/base** (e.g., $NaOH$, $NaOCH_3$): Major **$S_N2$**, minor $E2$.
- **Strong bulky base** (e.g., $t	ext{-BuOK}$): Major **$E2$** (Hofmann alkene).
- **Good nucleophile/weak base** (e.g., $I^-$, $CN^-$): Exclusively **$S_N2$**.

Question 18

How do tertiary ($3^\circ$) alkyl halides react under strong base vs weak nucleophile/base conditions?

Accepted Answer

- **Strong base** (e.g., $NaOCH_3$, $KOH$): Exclusively **$E2$** ($S_N2$ is blocked by steric hindrance).
- **Weak nucleophile/base** (e.g., $H_2O$, $EtOH$): Mixture of **$S_N1$ and $E1$** via solvolysis.

Question 19

What is solvolysis?

Accepted Answer

A substitution ($S_N1$) or elimination ($E1$) reaction where the solvent acts as the nucleophile or base (e.g., hydrolysis in $H_2O$ or alcoholysis in $CH_3OH$).

Question 20

Why are allylic and benzylic substrates highly reactive in both $S_N1$ and $S_N2$ pathways?

Accepted Answer

- **$S_N1$**: The adjacent $\pi$ system stabilizes the carbocation intermediate via **resonance**.
- **$S_N2$**: The adjacent $p$-orbitals lower the transition state energy through **conjugation** with the incoming nucleophile.

Question 21

How do electron push arrows represent a nucleophilic attack in mechanism drawing?

Accepted Answer

A curved arrow starts from the **lone pair or $\pi$ bond of the nucleophile** (electron donor) and points directly to the **electrophilic carbon** (electron acceptor).

Organic Chemistry - Reaction Mechanisms, SN1/SN2 & E1/E2 Pathways

Cards in this deck

Related Flashcard Sets

TypeScript - Advanced Type Systems, Generics & Design Patterns

Harvard ECON1011 - Intermediate Microeconomics: Game Theory & Market Structures

MIT 18.03 - Differential Equations: Ordinary Differential Equations, Laplace & Fourier

CPA Exam FAR - Financial Accounting & Reporting: US GAAP & Revenue Recognition