Organic Chemistry : Help with SN1 Reactions

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Reaction Mechanisms, Energetics, And Kinematics

Which of the following substrates would have the fastest reaction rate for an SN1 mechanism?

Possible Answers:

Correct answer:

Explanation:

The SN1 mechanism involves the formation of a carbocation intermediate in the rate-determining step. The most stable carbocation will produce the fastest reaction. We can immediately eliminate any answer choices that will produce primary or secondary carbocations, since a tertiary carbocation will be much more stable. When comparing tertiary carbocations, larger and more electronegative substituents will allow for more charge stabilization.

Since the tertiary carbocation formed by the dissociation of iodide from will the be most stable, this substrate will react the fastest.

Example Question #2 : Reaction Mechanisms, Energetics, And Kinematics

Which of the following determines the general rate of an  reaction?

Possible Answers:

Rate=k[nucleophile]

Rate=k[substrate][nucleophile]

Rate=k[substrate]

Rate=k[substrate][base]

Correct answer:

Rate=k[substrate]

Explanation:

The rate of an  reaction is determined only by the concentration of the substrate. Unlike an  reaction, where the addition occurs in one step and requires the activity of the substrate and the nucleophile, an  reaction occurs in two steps and is only limited by the activity (i.e. leaving ability) of the substrate. Once the leaving group leaves the substrate, the nucleophile does not hesitate to attack the exposed carbocation.

Example Question #3 : Reaction Mechanisms, Energetics, And Kinematics

Img 0658

What is the final product of the pictured reaction?

1. Img 0660

2. Img 0661

3. Img 0659

Possible Answers:

No reaction

Correct answer:

Explanation:

Keep in mind that after the aldehyde is reduced into an alcohol, the molecule can undergo an intramolecular reaction, as alcohol is a good nucleophile and the halogen is a stellar leaving group.

Img 0662

Example Question #4 : Reaction Mechanisms, Energetics, And Kinematics

Which of the following is not true for an SN1 reaction?

Possible Answers:

A strong nucleophile is required

All are true

Rearrangements are possible

Racemization of products

Correct answer:

A strong nucleophile is required

Explanation:

A strong nucleophile is not required for SN1. A weak nucleophile may be used. Remember that the SN1 mechanism goes through a carbocation intermediate.

Rearrangements are possible for SN1 reactions (not SN2). A rearrangement will occur to create a more stable intermediate in the mechanism. For example, if the carbocation is secondary, a methyl shift may occur to make the carbocation intermediate tertiary.

A racemic mixture of products occurs when with the nucleophile may attack the carbocation from either the top face or bottom face. When a reaction goes through a carbocation intermediate, as in SN1, there may be a racemic mix of products.

SN1 is unimolecular, and the rate of the reaction is determined by the substrate and reaction constant.

Example Question #5 : Reaction Mechanisms, Energetics, And Kinematics

A student carried out a substitution reaction in the lab using ethanol as a solvent. The student began with an optically pure reactant (100% (R)-configuration) and finished with a racemic mixture of products (50% (R)-configuration, 50% (S)-configuration).

The reaction went through which of the following mechanisms?

Possible Answers:

SN2

E2

E1

Either SN1 or SN2

SN1

Correct answer:

SN1

Explanation:

SN1 reactions result in racemization when the nucleophile has a 50% chance of attacking the carbocation intermediate from the top face, and a 50% chance of attacking from the bottom face. SN1 reactions are favored in polar protic solvents, such as ethanol.

E2 and E1 are incorrect as they are elimination reaction mechanisms, and we are looking for a substitution mechanism. SN2 reactions result in inversion, not racemization. Additionally we know that SN2 is incorrect because SN2 is favored in polar aprotic solvents.

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