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

Question 1

What are the four main reaction mechanisms covered in this topic?

Accepted Answer

$S_N1$, $S_N2$, $E1$, and $E2$, representing unimolecular/bimolecular nucleophilic substitutions and eliminations.

Question 2

What is the primary characteristic of an $S_N1$ reaction?

Accepted Answer

A two-step, unimolecular nucleophilic substitution reaction involving a carbocation intermediate.

Question 3

Describe the rate law for an $S_N1$ reaction.

Accepted Answer

Rate $= k[RX]$, where $[RX]$ is the concentration of the alkyl halide. It's unimolecular because the rate-determining step (carbocation formation) only involves the alkyl halide.

Question 4

What is the stereochemical outcome of an $S_N1$ reaction?

Accepted Answer

Racemization, as the nucleophile can attack the planar carbocation from either face, leading to a mixture of enantiomers (or diastereomers if other chiral centers exist).

Question 5

How does carbocation stability influence $S_N1$ reactions?

Accepted Answer

$S_N1$ reactions are favored by more stable carbocations, following the trend: $3^\circ > 2^\circ > 1^\circ$ (methyl carbocations are generally too unstable).

Question 6

What type of solvent favors $S_N1$ reactions?

Accepted Answer

Polar protic solvents (e.g., water, alcohols) stabilize the carbocation intermediate and the leaving group through solvation, facilitating its formation.

Question 7

What is the primary characteristic of an $S_N2$ reaction?

Accepted Answer

A one-step, concerted, bimolecular nucleophilic substitution reaction with no intermediate.

Question 8

Describe the rate law for an $S_N2$ reaction.

Accepted Answer

Rate $= k[RX][Nu^-]$, where $[RX]$ is the alkyl halide and $[Nu^-]$ is the nucleophile. It's bimolecular because both reactants are involved in the rate-determining step.

Question 9

What is the stereochemical outcome of an $S_N2$ reaction?

Accepted Answer

Walden inversion (inversion of configuration) at the reacting chiral center, as the nucleophile attacks from the backside, opposite to the leaving group.

Question 10

What type of solvent favors $S_N2$ reactions?

Accepted Answer

Polar aprotic solvents (e.g., DMSO, acetone, DMF) solvate cations but leave anions (nucleophiles) 'naked' and highly reactive, without hindering backside attack.

Question 11

How does steric hindrance affect $S_N2$ reactions?

Accepted Answer

$S_N2$ reactions are highly sensitive to steric hindrance. Reactivity decreases with increasing substitution at the $\alpha$-carbon: $CH_3 > 1^\circ > 2^\circ$ (tertiary substrates rarely undergo $S_N2$).

Question 12

What is the primary characteristic of an $E1$ reaction?

Accepted Answer

A two-step, unimolecular elimination reaction involving a carbocation intermediate, often competing with $S_N1$.

Question 13

Describe the rate law for an $E1$ reaction.

Accepted Answer

Rate $= k[RX]$, identical to $S_N1$, as the rate-determining step is the formation of the carbocation.

Question 14

What type of base is typically used in $E1$ reactions?

Accepted Answer

Weak bases (often the solvent itself, e.g., water, alcohol) are sufficient to deprotonate the carbocation, as the carbocation formation is the rate-limiting step.

Question 15

What is Zaitsev's Rule, and which elimination reactions generally follow it?

Accepted Answer

Zaitsev's Rule states that the major product in an elimination reaction is the most substituted (most stable) alkene. Both $E1$ and $E2$ reactions generally follow Zaitsev's Rule unless a bulky base is used in $E2$.

Question 16

What is the primary characteristic of an $E2$ reaction?

Accepted Answer

A one-step, concerted, bimolecular elimination reaction requiring an anti-periplanar arrangement of the leaving group and the $\beta$-hydrogen.

Question 17

Describe the rate law for an $E2$ reaction.

Accepted Answer

Rate $= k[RX][Base]$, as both the alkyl halide and the base are involved in the single, concerted step.

Question 18

What type of base is typically used in $E2$ reactions?

Accepted Answer

Strong bases (e.g., $OH^-$, $RO^-$, DBN, DBU) are required to deprotonate the $\beta$-hydrogen in the concerted mechanism.

Question 19

When does Hofmann's Rule apply in $E2$ reactions?

Accepted Answer

Hofmann's Rule states that the major product is the least substituted alkene. This occurs when a bulky base (e.g., $t$-BuOK) is used, favoring the removal of a sterically accessible $\beta$-hydrogen.

Question 20

How do polar protic and polar aprotic solvents differ in their impact on nucleophiles?

Accepted Answer

Polar protic solvents hydrogen bond with nucleophiles, decreasing their reactivity. Polar aprotic solvents solvate cations but leave nucleophiles 'naked' and highly reactive.

Question 21

What is the role of a good leaving group in all four mechanisms?

Accepted Answer

A good leaving group (e.g., halides $I^- > Br^- > Cl^-$, tosylates) is essential for all four mechanisms as it must depart with its electron pair, forming a stable anion.

Question 22

How can you differentiate between $S_N1/E1$ and $S_N2/E2$ based on substrate?

Accepted Answer

$S_N1/E1$ are favored by $3^\circ > 2^\circ$ substrates due to carbocation stability. $S_N2$ is favored by $CH_3 > 1^\circ > 2^\circ$ due to steric hindrance. $E2$ can occur with $1^\circ, 2^\circ, 3^\circ$ substrates.

Question 23

Under what conditions might $S_N1$ and $E1$ reactions compete?

Accepted Answer

With $2^\circ$ or $3^\circ$ alkyl halides, a weak nucleophile/base, and a polar protic solvent, both $S_N1$ (substitution) and $E1$ (elimination) products are typically observed.

Question 24

Under what conditions might $S_N2$ and $E2$ reactions compete?

Accepted Answer

With $1^\circ$ or $2^\circ$ alkyl halides and a strong nucleophile/base, $S_N2$ and $E2$ can compete. Bulky bases favor $E2$, while strong, non-bulky nucleophiles favor $S_N2$.

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

Cards in this deck

Related Flashcard Sets

Organic Chemistry - SN1, SN2, E1, and E2 Reaction Mechanisms

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

German B1 - Intermediate Conversational Phrases & Structures

French A1/A2 - Essential Grammar & Vocab