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

Question 1

What does $S_N1$ stand for?

Accepted Answer

Unimolecular Nucleophilic Substitution.

Question 2

Describe the mechanism of an $S_N1$ reaction.

Accepted Answer

A two-step mechanism: 
1. The leaving group departs, forming a planar carbocation intermediate (rate-determining step).
2. A nucleophile attacks the carbocation from either face.

Question 3

What is the rate law for an $S_N1$ reaction?

Accepted Answer

The rate law is $\	ext{Rate} = k[R-X]$, where $[R-X]$ is the concentration of the alkyl halide. The reaction rate depends only on the concentration of the substrate because carbocation formation is the slow, rate-determining step.

Question 4

How does carbocation stability influence $S_N1$ reaction rates?

Accepted Answer

More stable carbocations form faster, leading to faster $S_N1$ reactions. The stability order is: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$) > Methyl. Resonance stabilization (e.g., allylic, benzylic carbocations) also significantly increases stability.

Question 5

What is the stereochemical outcome of an $S_N1$ reaction at a chiral center?

Accepted Answer

Racemization occurs, meaning a mixture of enantiomers (typically close to 50:50) is formed. This is because the planar carbocation intermediate can be attacked by the nucleophile from either its top or bottom face with equal probability.

Question 6

What type of substrates favor $S_N1$ reactions?

Accepted Answer

Tertiary ($3^\circ$) and secondary ($2^\circ$) alkyl halides are favored, especially those that can form resonance-stabilized carbocations (e.g., allylic, benzylic). Primary and methyl halides do not react via $S_N1$ due to unstable carbocation formation.

Question 7

How do solvents affect $S_N1$ reactions?

Accepted Answer

Polar protic solvents (e.g., water, alcohols, carboxylic acids) strongly favor $S_N1$ reactions. They stabilize the developing carbocation intermediate and the departing leaving group through solvation (hydrogen bonding and dipole-dipole interactions), lowering the activation energy.

Question 8

What does $S_N2$ stand for?

Accepted Answer

Bimolecular Nucleophilic Substitution.

Question 9

Describe the mechanism of an $S_N2$ reaction.

Accepted Answer

A one-step, concerted mechanism where the nucleophile attacks the carbon bearing the leaving group from the backside simultaneously as the leaving group departs. This forms a single, pentavalent transition state where both the nucleophile and leaving group are partially bonded to the carbon.

Question 10

What is the rate law for an $S_N2$ reaction?

Accepted Answer

The rate law is $\	ext{Rate} = k[R-X][Nu^-]$, where $[R-X]$ is the concentration of the alkyl halide and $[Nu^-]$ is the concentration of the nucleophile. The reaction rate depends on the concentrations of both the substrate and the nucleophile.

Question 11

What is the stereochemical outcome of an $S_N2$ reaction at a chiral center?

Accepted Answer

Inversion of configuration (also known as Walden inversion) occurs. Because the nucleophile attacks from the backside, the stereochemistry at the chiral center is completely inverted relative to the starting material.

Question 12

What type of substrates favor $S_N2$ reactions?

Accepted Answer

Methyl and primary ($1^\circ$) alkyl halides are highly favored due to minimal steric hindrance around the electrophilic carbon. Secondary ($2^\circ$) alkyl halides can react but are slower, while tertiary ($3^\circ$) alkyl halides are generally unreactive via $S_N2$ due to significant steric hindrance.

Question 13

How do solvents affect $S_N2$ reactions?

Accepted Answer

Polar aprotic solvents (e.g., DMSO, acetone, DMF, acetonitrile) strongly favor $S_N2$ reactions. They solvate cations effectively but leave anions (nucleophiles) relatively 'naked' and highly reactive, enhancing their nucleophilicity.

Question 14

What does $E1$ stand for?

Accepted Answer

Unimolecular Elimination.

Question 15

Describe the mechanism of an $E1$ reaction.

Accepted Answer

A two-step mechanism: 
1. The leaving group departs, forming a carbocation intermediate (rate-determining step).
2. A weak base abstracts a proton from an adjacent $\beta$-carbon, forming a $\pi$ bond and an alkene product.

Question 16

What is the rate law for an $E1$ reaction?

Accepted Answer

The rate law is $\	ext{Rate} = k[R-X]$, identical to $S_N1$. The reaction rate depends only on the concentration of the alkyl halide because carbocation formation is the slow, rate-determining step.

Question 17

What is the regioselectivity rule for $E1$ reactions?

Accepted Answer

E1 reactions generally follow Zaitsev's Rule (also known as Saytzeff's Rule). The major product is the more substituted (more stable) alkene, formed by removing a proton from the $\beta$-carbon with fewer hydrogens.

Question 18

What type of substrates and conditions favor $E1$ reactions?

Accepted Answer

Tertiary ($3^\circ$) and secondary ($2^\circ$) alkyl halides are favored (due to carbocation stability). Weak bases and polar protic solvents are typically used. High temperatures generally favor elimination (E1 or E2) over substitution (SN1 or SN2).

Question 19

What does $E2$ stand for?

Accepted Answer

Bimolecular Elimination.

Question 20

Describe the mechanism of an $E2$ reaction.

Accepted Answer

A one-step, concerted mechanism where a strong base abstracts a proton from a $\beta$-carbon simultaneously as the leaving group departs from the $\alpha$-carbon, forming an alkene. This requires an anti-periplanar arrangement of the $\beta$-hydrogen and the leaving group.

Question 21

What is the rate law for an $E2$ reaction?

Accepted Answer

The rate law is $\	ext{Rate} = k[R-X][Base]$, where $[R-X]$ is the concentration of the alkyl halide and $[Base]$ is the concentration of the base. The reaction rate depends on the concentrations of both the substrate and the base.

Question 22

What is the regioselectivity rule for $E2$ reactions?

Accepted Answer

Generally, Zaitsev's Rule (more substituted alkene) is followed with small, unhindered bases. However, the Hofmann Rule (less substituted alkene) applies with bulky bases (e.g., $t$-BuOK) or when the leaving group is bulky or charged (e.g., quaternary ammonium salts).

Question 23

What is the stereochemical requirement for an $E2$ reaction?

Accepted Answer

The $\beta$-hydrogen and the leaving group must be anti-periplanar (180$^\circ$ dihedral angle) to allow for the concerted transition state. This specific geometric requirement is crucial for the reaction to occur, especially in cyclic systems (e.g., cyclohexane chairs).

Question 24

How do solvents affect $E2$ reactions?

Accepted Answer

Polar aprotic solvents are generally preferred for E2 reactions as they do not solvate the strong base, leaving it more reactive. Polar protic solvents can decrease base strength by solvating the base, hindering its ability to abstract a proton.

Question 25

Differentiate between protic and aprotic solvents.

Accepted Answer

**Protic solvents** (e.g., $\	ext{H}_2\	ext{O}$, $\	ext{CH}_3\	ext{OH}$, $\	ext{CH}_3\	ext{CH}_2\	ext{OH}$, $\	ext{CH}_3\	ext{COOH}$) have O-H or N-H bonds, allowing them to donate hydrogen bonds. They can stabilize ions effectively.
**Aprotic solvents** (e.g., DMSO, acetone, DMF, THF, acetonitrile) lack these acidic hydrogens and cannot donate hydrogen bonds. They are good at solvating cations but leave anions relatively 'naked' and more reactive.

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

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