Ch. 7 - Substitution ReactionsSee 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

SN1 reaction

See all sections
Sections
Nucleophilic Substitution
Good Leaving Groups
SN2 Reaction
SN1 reaction
Substitution Comparison
Additional Practice
Physical Properties of RX
Ion Pairing Effects
Additional Guides
Alkyl Halide

The SN1 mechanism is similar to SN2 in that you get a substitution product, but the path to get there is completely different. It’s important that we understand how it’s different from SN2

Concept #1: Drawing the SN1 Mechanism

Summary: A neutral nucleophile reacts with an inaccessible leaving group to produce substitution in two-step.

Concept #2: Why highly substituted leaving groups favor SN1

The slow step of this mechanism is the formation of a carbocation intermediate. These types of intermediates are unstable, so anything that we can do to stabilize them will help them form faster.

-R groups stabilize carbocations through a phenomenon called hyperconjugation. Meaning that the more substituted the carbocation, the more stable it is. 

Concept #3: Understanding the properties of SN1

You are a manager at Pepe and Son Carbocations Inc, and your boss has asked you to increase production of your product (carbocations in a box- these are awesome).

You have plenty of boxes, but it takes time for the conveyer belt to crank out these custom carbocations.

  • Will increasing the number of boxes increase the amount of product?
  • Will increasing the speed of the conveyer belt increase the amount of product? 

Properties of SN1 reactions:

  • Nucleophile =  Weak
  • Leaving Group =  Highly Substituted
  • Reaction coordinate = Intermediate
  • Reaction = Two-Step
  • Rate =  Unimolecular
  • Rate =  k[RX]
  • Stereochemistry = Racemic
  • Nickname = Solvolysis

Practice: Predict the product of the following reaction:

 

The product must be a racemate if the original leaving group is located on a chiral center.

NOTE: Substitution reactions with neutral nucleophiles require an additional deprotonation step. 

Practice: Predict the product of the following reaction:

 

Additional Problems
What set of reaction conditions would favor an SN1 reaction on 2-bromo-3-methylbutane? a)Strong nucleophile in a protic solvent. b) Strong nucleophile in an aprotic solvent. c) Weak nucleophile in a protic solvent. d) Weak nucleophile in an aprotic solvent.
Draw the reaction mechanism and the energy diagram for the following substitution reaction. Indicate if it follows an SN1 or an SN2 mechanism.
Draw the mechanism for the following SN1 reaction
Draw the mechanism of the following SN1 processes: 
Draw the mechanism of the following SN1 processes: 
Draw the mechanism of the following SN1 processes: 
Assuming that the rate-determining step in the reaction of 2-methyl-2-butanol with hydrogen chloride to give 2-chloro-2-methylbutane is unimolecular, write an equation for this step. Use curved arrows to show the flow of electrons. 
Draw the mechanism for each of the following reactions: 
Draw the mechanism for each of the following reactions: 
Draw the mechanism for the solvolysis of (R)-3-iodo-3-methylhexane in water, leading to a neutral organic product. Show all lone pairs, formal charges, and (where appropriate) stereochemistry.
Rank the following alkyl halides in order of increasing SN1 reactivity. (1 – least reactive, 3 – most reactive)
Rank the following molecules in increasing order of susceptibility to an SN1 reaction.
Which SN1 reaction of each pair would you expect to take place more rapidly? Explain your answer.  
Which SN1 reaction of each pair would you expect to take place more rapidly? Explain your answer.  
Complete the following mechanism for an SN1 reaction that occurs when 2-chloro-2-methylbutane is heated in water. NOTE, ALTHOUGH AN E1 PROCESS WILL ACCOMPANY THIS SN1 REACTION, WE ARE ONLY INTERESTED IN THE SN1 MECHANISM FOR THIS PROBLEM. Use appropriate arrows to show all movement of electrons, show all non-bonding electrons as dots and show any formal charges. If any of the species are really racemic mixtures of enantiomers, you only need to draw one stereoisomer and write "racemic". In the boxes provided, write which kind of mechanisitc element is being shown for that step, i.e. "make a bond", etc.
Listed below are several hypothetical nucleophilic substitution reactions. None is synthetically useful because the product indicated is not formed at an appreciable rate. In each case provide an explanation for the failure of the reaction to take place as indicated.  
How many atoms and electrons are directly involved in the bond-making and bond-breaking of the first step in an SN1 reaction? (A) three atoms, four electrons (B) two atoms, four electrons  (C) three atoms, two electrons  (D) two atoms, two electrons
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.
A) Provide structures in the box to complete the Lewis acid-base reaction.  B) Create the electron arrows needed for the reactants to produce the product.
 Provide the major product for the following reaction. 
Compare the following reactions and decide which reaction in each pair would occur faster. Write your answer and concisely defend your choice. No reasoning, no points.  
The hybridization state of the charged carbon in an S N1 reaction is: s sp sp2 sp3 sp4
Use A-G to label the graph with the structures given for the reaction:  
Draw the structure(s) of the product(s) that you would expect from the following reaction.
Energy Diagram and Features of SN1 Reactions
For each of the following sets of molecules, CIRCLE the set that will undergo an S N1 reaction faster. If both react equally fast, CIRCLE BOTH. 
Using arrows to show the flow of electrons, write a stepwise mechanism for the reaction given below. Furthermore, draw a structural representation of the latest (also known as the most product-like) transition state structure with consideration of each elementary steps in the reaction mechanism.
Identify the correct potential energy diagram for a secondary alkyl iodide (2°RI) versus a secondary alkyl chloride (2°RCl) undergoing an SN1 reaction. Provide rationale to support your choices
Rank the substrates below in terms of relative SN1 reaction rate. (1 = fastest, 4 = slowest)
(1) CIRCLE the correct energy diagram below that represents the variation of energy vs. reaction coordinate for the SN1 reaction of 2-bromo-2-methylpropane with azide ion, N 3-,(2) DRAW the structure of the three species present at the points indicated on that diagram(3) INDICATE in the box below each structure which point (A-R) you are drawing. Use dotted lines and partial charges for transition states. (Notes: You need not draw out all the bonds within the N3-. Stereochemistry is not necessary.)
Which of the following molecules is expected to selectively NOT participate in Sn1 reactions?
Draw the major SN1 product for the following reaction HL
 Rank the SN1 reaction rates for the following compounds: 
Draw the expected product(s) of the following solvolysis process: 
Draw a structural formula for the substitution product of the reaction shown below. 
Assuming no other changes, what is the effect of doubling only the concentration of the alkyl halide in the above SN1 reaction? a. quadruples the rate b. rate is halved c. no change d. triples the rate e. doubles the rate
Rank the relative rates of solvolysis for the following alkyl halides in aqueous tone in decreasing order.(Drag the structure to the appropriate box, fastest on the top to slowest on the bottom.)