Ch. 7 - Substitution ReactionsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

The SN2 reaction is a one-step bimolecular substitution that occurs between a nucleophile and a molecule with a methyl, primary, or secondary leaving group. The SN1 reaction is a two-step unimolecular substitution between a molecule with a secondary or tertiary leaving group. 

Characteristics

The SN2 reaction

The SN1 reaction

One step

Two steps

Has no intermediate

Has an intermediate

Prefers leaving groups that are not sterically hindered

Prefers leaving groups that are sterically hindered

Reaction rate depends on the concentrations of both the nucleophile and the substrate

Reaction rate depends solely on the concentration of the substrate

Prefers polar aprotic solvents

Prefers polar protic solvents

Inverts stereochemistry

Produces a racemic mixture


Leaving-groupsLeaving groupsThe SN2 reaction prefers leaving groups that aren’t sterically hindered because the substitution occurs through a nucleophilic backside attack; more R-groups means more steric hindrance. 

The SN1 reaction prefers leaving groups with more R-groups attached because the rate-determining factor is the carbocation produced by leaving-group dissociation; R-groups stabilize carbocations through hyperconjugation. 

Mechanisms

To illustrate the mechanisms, let’s use an achiral alkyl halide as our substrate and hydroxide as our nucleophile.

SN2-mechanism

SN2 mechanism

SN2-transition-stateSN2 transition stateIn an SN2 reaction, the nucleophile does a “backside attack” on the leaving group’s carbon, inverting chirality if present. SN2 reactions only have one transition state, and it has partial bonds between the nucleophile and carbon and between the carbon and leaving group. The nucleophile and carbon both have partial negative charges. 


SN1-mechanismSN1 mechanism

SN1-transition-statesSN1 transition statesIn an SN1 reaction, the leaving group dissociates first, and the nucleophile attacks the resulting electrophile (the carbocation). SN1 reactions have two different transition states. The first one (T.S. 1) shows the dissociation of the leaving group with a partial negative on the leaving group and partial positive on the carbon. The second (T.S. 2) shows the bonding of the nucleophile to the carbon with a partial negative on the nucleophile and a partial positive on the carbon. 

Reaction Coordinate

 SN2-energy-diagramSN2 energy diagram

SN2 reaction diagrams have one single peak because there is only one transition state. The more sterically hindered the leaving group, the greater the energy requirement is to reach the transition state. 

SN1-energy-diagramSN1 energy diagram

SN1 reactions have two transition states and an intermediate between them. Transition state 1 (TS1) is higher energy than TS2 because it leads to the formation of the carbocation. Carbocation formation is the rate-determining step. The less stable the carbocation, the higher in energy both TS1 and the intermediate are. 

P.S. Check out my video on how to determine if a mechanism will proceed through SN2, SN1, E2, or E1 using the BIG DADDY FLOWCHART.


Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.


Additional Problems
Which one of the following correctly describes the reaction below?
Each reaction shown below is a nucleophilic substitution reaction. Compare the mechanisms of the two reactions. Label each mechanism as Sn1 or Sn2.
Give the rate equation for the reaction occuring between NaCN and CH 3CH2Cl. Write your answer in the box below.
Does the following chart depict and S N1 or SN2 type mechanism?
For each of the following pairs of SN2 reactions, tick the box that corresponds to the SLOWEST reaction of the two:
Consider the following reaction. Assuming no other changes, what would happen to the rate of the reaction if the concetration of the nucleophile was doubled and if the concentration of the alkyl halide was tripled? a. There would be no effect b. The rate would double c. The rate would triple d. The rate would increase 6-fold e. The rate would increase 9-fold
Put a circle around the statements that relate to an SN 2 mechanism and put a square around the ones that talk about SN1.    CH3X > 1˚ > 2˚                                  No rearrangements   Rearranged products                        Rate = k [alkyl halide] [Nuc]   Polar protic solvent                            Racemization   Inversion at chiral carbon                  Rate = k [alkyl halide]   3˚ > 2˚                                                Polar aprotic solvent   Strong nucleophile                             Weak nucleophile (may also be solvent)
Compare the following reactions and decide which reaction in each group would occur faster. Write your answer and concisely defend your choice.  
What is the stereochemistry of the nitrile produced in the reaction shown?
Which of these structures best depicts the transition state for the reactions of CH  3I with CH3OK in CH3OH?
Predict the organic product of the following reaction. When appropriate, be sure to indicate stereochemistry. If more than one product is formed be sure to indicate the major product. Draw your answer in skeletal form. You will be graded on the product your draw from the reaction no other information is needed for this question.
Consider the reaction of I - with CH3CH2Cl.(a) Would you expect the reaction to be SN1 or SN2? The rate constant for the reaction at 60°C is 5 x 10 -5 L mol-1 s-1.  
Determine the product(s) for the following reaction a) A b) B c) C  d) a mixture of A and B
What is (are) the organic product(s) of the following reaction?
The following reaction will not occur. Explain why the reaction will fail. 
For a unimolecular nucleophilic substitution (________) reactions, bond breaking and bond formation occur _______, ________ is/are involved in the transition state of the rate-determinining step. However, for biomolecular nucleophilic substitution (_______) reactions, bond breaking and bond formation occur _______. A bimolecular reaction is one in which _______ is/are involved in the transition state of the rate determining step. SN2; in two distinct steps; One species; SN1; simultaneously; two species SN1; simultaneously; Two species; SN2; in two distinct steps; one species SN2; simultaneously; Two species; SN1; in two distinct steps; one species SN1; in two distinct steps; One species; SN2; simultaneously; two species
1-Bromobicyclo[2.2.1]heptane is extremely unreactive in either SN2 or SN1 reactions. Provide explanations for this behavior.  
When the alkyl bromides (listed here) were subjected to hydrolysis in a mixture of ethanol and water (80% EtOH/20% H2O) at 55°C, the rates of the reaction showed the following order: (CH3)3CBr ˃ CH3Br ˃ CH3CH2Br ˃ (CH3)2CHBr Provide an explanation for this order of reactivity.    
In contrast to SN2 reactions, SN1 reactions show relatively little nucleophile selectivity. That is, when more than one nucleophile is present in the reaction medium, SN1 reactions show only a slight tendency to discriminate between weak nucleophiles and strong nucleophiles, whereas SN2 reactions show a marked tendency to discriminate. (a) Provide an explanation for this behavior.  
In contrast to SN2 reactions, SN1 reactions show relatively little nucleophile selectivity. That is, when more than one nucleophile is present in the reaction medium, SN1 reactions show only a slight tendency to discriminate between weak nucleophiles and strong nucleophiles, whereas SN2 reactions show a marked tendency to discriminate. (b) Show how your answer accounts for the following:  
Identify the substitution product(s) in the following reaction. a) A b) B c) C  d) a mixture of A and B
For the following reaction a) Label the nucleophile, electrophile, and leaving group. b) Identify if it follows SN1 or SN2 mechanism. c) Provide a detailed curved arrow mechanism
Which of the following is  NOT a possible step in a substitution reaction?
Predict the major organic substitution product for the following reaction:
Give structures for the products of each of the following reactions: