Organic Chemistry : Substitution Mechanisms

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Help With Sn1 Reactions

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][base]

Rate=k[substrate]

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 #1 : Help With Sn1 Reactions

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 #1 : Reaction Mechanisms, Energetics, And Kinematics

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

Possible Answers:

Racemization of products

A strong nucleophile is required

All are true

Rearrangements are possible

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 #1 : Help With Sn1 Reactions

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

SN1

Either SN1 or SN2

E1

E2

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.

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

Which SN2 reaction would proceed the fastest?

Possible Answers:

1-chloropentane and sodium iodide

1-bromo-2-methylpentane and sodium iodide

1-bromopentane and sodium iodide

tert-butyl bromide and sodium iodide

sec-butyl bromide and sodium iodide

Correct answer:

1-bromopentane and sodium iodide

Explanation:

SN2 reactions involve a backside nucleophilic attack on an electrophilic carbon. As a result, less steric congestion for this backside attack results in a faster reaction, meaning that SN2 reactions proceed fastest for primary carbons. In addition, beta-branching next to a primary carbon results in a slower reaction, as does a poorer leaving group (i.e. chloride instead of bromide).

1-bromopentane has a good leaving group (bromine) attached to a primary carbon with no beta-branching, meaning it will proceed the fastest.

Example Question #1 : Reaction Types

In reactions involving the alkylation of acetylide ions, it is preferred that the alkyl halide be primary. What is the reason for this?

Possible Answers:

The reaction involves a carbocation as intermediate

The mechanism for these reactions is SN2

The reactions generally occur in two steps

The mechanism for these reactions is SN1

An answer cannot be determined without more information about the reaction conditions

Correct answer:

The mechanism for these reactions is SN2

Explanation:

The reason that the alkyl halide is preferred to be primary is because the mechanism for these reactions is SN2. SN2 indicates a substitution reaction that takes place in one step. A primary alcohol is preferred to prevent steric congestion caused by the simultaneous binding of the nucleophile and release of the leaving group. This reaction mechanism is faster because it omits the formation of a carbocation intermediate.

In contrast, SN1 reactions take place in two steps and involve the formation of a carbocation intermediate.

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

Predict the major product of the given SN1 reaction.

8

Possible Answers:

IV

I

III

II

Correct answer:

IV

Explanation:

SN1 reactions are characterized by two distinct steps. The first step, which determines the rate of the reaction, is the dissociation of the leaving group. This step leaves behind a carbocation intermediate.

As opposed to SN2 reactions, in which nucleophilic substitution occurs in one step, the temporary formation of a carbocation in SN1 reactions allows for carbocation rearrangement, which serves to stabilize the positive charge.

The major product, molecule IV, results from the shift of a hydrogen atom from the adjacent carbon, moving the positive charge to a carbon with greater alkyl substitution. Electron density is inducted to the secondary carbocation (bound to two alkyl groups), stabilizing the positive charge. Carbocation rearrangement occurs extremely fast, usually before a nucleophile (in this case water) may bind.

As a result, molecule IV is the major product instead of molecule II (the SN2 product).

Example Question #1 : Help With Sn2 Reactions

Sn2

What type of reaction is shown?

Possible Answers:

E1

SN1

SN2

None of these

E2

Correct answer:

SN2

Explanation:

This is an SN2 reaction. When there is a methyl halide with a strong nucleophile, the nucleophile will force the halide group to leave. Strong nucleophiles dictate SN2 reaction mechanisms.

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

Which of the following reagents would complete this reaction with the proper stereochemistry?

Screen shot 2015 10 24 at 10.13.55 am

Possible Answers:

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Screen shot 2015 10 27 at 10.59.49 pm

Screen shot 2015 10 27 at 10.59.46 pmand  Screen shot 2015 10 27 at 10.59.42 pm

Screen shot 2015 10 27 at 10.59.42 pm

Screen shot 2015 10 27 at 10.59.42 pmand Screen shot 2015 10 27 at 10.59.49 pm

Correct answer:

Screen shot 2015 10 27 at 10.59.46 pmand  Screen shot 2015 10 27 at 10.59.42 pm

Explanation:

The sterochemistry should be inverted for an SN2 reaction. The product is has S chirality Screen shot 2015 10 24 at 10.14.21 am, so the starting material should have R.

Screen shot 2015 10 24 at 10.14.13 am

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