Elimination Mechanisms

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Organic Chemistry › Elimination Mechanisms

Questions 1 - 10
1

Which of the following compounds could NEVER undergo an E2 reaction when treated with potassium tert-butoxide?

Benzylbromide

Bromoethane

Cyclopentylbromide

3-methyl-3-iodopentane

Cis-2-bromo-1-methylcyclohexane

Explanation

For an E2 reaction to occur, there must be a hydrogen on the carbon adjacent to the carbon with the leaving group. Benzyl bromide contains no hydrogens on the carbon next to the carbon with the bromide, and would therefore undergo only a substitution reaction.

2

Which of the following compounds could NEVER undergo an E2 reaction when treated with potassium tert-butoxide?

Benzylbromide

Bromoethane

Cyclopentylbromide

3-methyl-3-iodopentane

Cis-2-bromo-1-methylcyclohexane

Explanation

For an E2 reaction to occur, there must be a hydrogen on the carbon adjacent to the carbon with the leaving group. Benzyl bromide contains no hydrogens on the carbon next to the carbon with the bromide, and would therefore undergo only a substitution reaction.

3

If the following disubstituted cyclohexane is refluxed (reacted in boiling solvent) in THF and sodium methoxide, which of the following will be the major product?

Q11

I

II

III

IV

V

Explanation

This is an elimination reaction because refluxing conditions indicate high heat, an essential component for these reactions. We can eliminate answer choices that include substitution products, namely those containing methoxy groups, thus addressing answers IV and V.

Because the leaving group, bromide, is bonded to a tertiary carbon, this reaction will undergo an E2 mechanism. This means a carbocation will be formed at carbon one, and a subsequent deprotonation of an adjacent hydrogen will form the alkene. As answer choice III cannot be formed via deprotonation of an adjacent hydrogen, we can eliminate it.

By Zaitsev's rule_,_ we know that the most substituted alkene product of an elimination reaction will be the most stable, and thus most favorable, product. This allows us to see that the product formed in reaction I will be the most favorable.

4

If the following disubstituted cyclohexane is refluxed (reacted in boiling solvent) in THF and sodium methoxide, which of the following will be the major product?

Q11

I

II

III

IV

V

Explanation

This is an elimination reaction because refluxing conditions indicate high heat, an essential component for these reactions. We can eliminate answer choices that include substitution products, namely those containing methoxy groups, thus addressing answers IV and V.

Because the leaving group, bromide, is bonded to a tertiary carbon, this reaction will undergo an E2 mechanism. This means a carbocation will be formed at carbon one, and a subsequent deprotonation of an adjacent hydrogen will form the alkene. As answer choice III cannot be formed via deprotonation of an adjacent hydrogen, we can eliminate it.

By Zaitsev's rule_,_ we know that the most substituted alkene product of an elimination reaction will be the most stable, and thus most favorable, product. This allows us to see that the product formed in reaction I will be the most favorable.

5

Which of the following statements concerning substitution and elimination reactions is true?

All of these are true statements

SN1 reactions follow a 2-step mechanism; SN2 reactions follow a 1-step mechanism

Acetate is a better nucleophile than acetic acid

The more hindered a strong base is, the more likely it is to produce an E2 reaction

In the absence of heat, strong bases, and good nucleophiles, tertiary alkyl halides will react via the SN1 mechanism

Explanation

All of the statements are correct.

SN1 substitution reactions take place in 2 steps, and SN2 substitution reactions take place on one step. Acetate is a better nucleophile than acetic acid because acetate is a negative ion, and therefore donates electrons as a nucleophile. The more hindered a strong base is, the more likely it is to produce an E2 reaction because the base will more easily remove a good leaving group to become more stable (done through elimination in one step via E2). In the absence of heat, strong bases, and good nucleophiles, tertiary alkyl halides will react via the SN1 mechanism because strong bases/good nucleophiles will always undergo SN1 mechanisms (substitution in two steps), with the exception of alkyl halides.

6

Which of the following statements concerning substitution and elimination reactions is true?

All of these are true statements

SN1 reactions follow a 2-step mechanism; SN2 reactions follow a 1-step mechanism

Acetate is a better nucleophile than acetic acid

The more hindered a strong base is, the more likely it is to produce an E2 reaction

In the absence of heat, strong bases, and good nucleophiles, tertiary alkyl halides will react via the SN1 mechanism

Explanation

All of the statements are correct.

SN1 substitution reactions take place in 2 steps, and SN2 substitution reactions take place on one step. Acetate is a better nucleophile than acetic acid because acetate is a negative ion, and therefore donates electrons as a nucleophile. The more hindered a strong base is, the more likely it is to produce an E2 reaction because the base will more easily remove a good leaving group to become more stable (done through elimination in one step via E2). In the absence of heat, strong bases, and good nucleophiles, tertiary alkyl halides will react via the SN1 mechanism because strong bases/good nucleophiles will always undergo SN1 mechanisms (substitution in two steps), with the exception of alkyl halides.

7

What is the product of the following reaction?

Screen shot 2015 10 24 at 10.13.33 am

Screen shot 2015 10 24 at 10.13.49 am

Screen shot 2015 10 24 at 10.13.42 am

Screen shot 2015 10 24 at 10.13.46 am

No reaction.

More than one of these.

Explanation

Because both hydrogens neighboring the are anti-periplanar, both elimination products would be formed.

8

What is the product of the following reaction?

Screen shot 2015 10 24 at 10.13.33 am

Screen shot 2015 10 24 at 10.13.49 am

Screen shot 2015 10 24 at 10.13.42 am

Screen shot 2015 10 24 at 10.13.46 am

No reaction.

More than one of these.

Explanation

Because both hydrogens neighboring the are anti-periplanar, both elimination products would be formed.

9

Which series of carbocations is arranged from most stable to least stable?

Tertiary, secondary, primary, methyl

Methyl, primary, secondary, tertiary

Primary, secondary, tertiary, methyl

Methyl, tertiary, secondary, primary

The stability of a carbocation depends only on the solvent of the solution

Explanation

Tertiary carbocations are stabilized by the induction of nearby alkyl groups. Since the carbocation is electron deficient, it is stabilized by multiple alkyl groups (which are electron-donating). Less substituted carbocations lack stability.

10

E1

The above image undergoes an E1 elimination reaction in a lab. The researchers note that the major product formed was the "Zaitsev" product. Which of the following compounds did the observers see most abundantly when the reaction was complete?

E4

E2

E3

E7

None of these

Explanation

The Zaitsev product is the most stable alkene that can be formed. This is the major product formed in E1 elimination reactions, because the carbocation can undergo hydride shifts to stabilize the positive charge. The most stable alkene is the most substituted alkene, and thus the correct answer.

E4

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