SN1/SN2 Mechanisms

Question 1

SN2 Mechanism (Characteristics)

Answer 1

• One-Step Mechanism (Bimolecular)
• Second-Order Kinetics (R=k[RX][:Nu])
• Rate: Methyl > 1 > 2 > 3
• Backside Attack of Nucleophile
• Stereochemical Inversion of Configuration
• Favors Strong Nucleophile (Strong Base, Anion)
• Better Leaving Group = Faster Reaction
• Favors Polar Aprotic Solvents

Question 2

Polar Protic Solvents

Answer 2

• Favored by SN1 Reactions
• Common Examples: H2O, CH3OH, CH3CH2OH, (CH3)3COH, CH3COOH

Note: Polar Protic Solvents have O-H or N-H bonds

Question 3

Polar Aprotic Solvents

Answer 3

• Favored by SN2 reactions
• Common Examples: Acetone, Acetonitrile, THF, DMSO, DMF, HMPA

Note: Polar Aprotic Solvents lack O-H and N-H bonds

Question 4

Rate Law for SN1 Mechanism

Answer 4

SN1 Mechanism:

• First-Order Kinetics
• R = k[RX]
• Dependent only on the concentration of the substrate (alkyl halide)

Question 5

Rate Law for SN2 Mechanism

Answer 5

SN2 Mechanism:

• Second-Order Kinetics
• R = k[RX][:Nu]
• Dependent on both the concentration of the substrate (alkyl halide) and the concentration of the Nucleophile (base)

Question 6

Stereochemistry of SN1 Mechanism

Answer 6

SN1 Stereochemistry:

• Trigonal Planar Carbocation Intermediate Produced
• Racemization at Stereogenic Center

Question 7

Stereochemistry of SN2 Mechanism

Answer 7

SN2 Stereochemistry:

• Backside attack of nucleophile
• Stereochemical inversion of configuration

Question 8

Good Leaving Group for Nucleophilic Substitution (SN1 and SN2)

Answer 8

• R-Cl -> Cl
• R-Br -> Br
• R-I -> I
• R-OH2 -> H2O

Note: Fluorine (F) is the only halogen that is not a good leaving group for nucleophilic substitution

Question 9

Poor Leaving Groups for Nucleophilic Substitution (SN1 and SN2)

Answer 9

• R-F -> F
• R-OH -> OH
• R-NH2 -> NH2
• R-H -> H
• R-R -> R

Note: These molecules do not undergo nucleophilic substitution

Question 10

Strong Nucleophiles

Answer 10

• Favored by SN2 Reactions

- Common Examples: OH, OR, CH3COO, N3, CN, Cl, Br, I, HS, RS

Question 11

Neutral Nucleophiles

Answer 11

• Favored by SN1 Reactions

- Common Examples: H2O, ROH, NH3, RNH2, H2S, RSH

Question 12

3 > 2 > 1 > Methyl is characteristic of:

Answer 12

-SN1 Reactions

Question 13

Methyl > 1 > 2 > 3 is characteristic of:

Answer 13

-SN2 Reactions

Question 14

SN1 Mechanism (Characteristics)

Answer 14

• Two-Step Mechanism (Unimolecular)
• First-Order Kinetics (R=k[RX])
• Rate: 3 > 2 > 1 > Methyl
• Trigonal Planar Carbocation Intermediate
• Racemization Stereogenic Center
• Favors Weak Nucleophile (Weak Base, Neutral)
• Better Leaving Group = Faster Reaction
• Favors Polar Protic Solvents

Question 15

Hammond Postulate:

Answer 15

• The transition state in endothermic reactions resemble the products
• The transition state in exothermic reactions resemble the reactants

Note: The transition state is higher in energy than both the reactants and products