MCAT Biology : Substitution and Elimination Mechanisms

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1 : Substitution And Elimination Mechanisms

Which of the following reactions is the nucleophile potassium tert-butoxide often used for?

Possible Answers:

SN1           

SN2

E1

E2

Correct answer:

E2

Explanation:

Tert-butoxide is a large, sterically hindered, strong nucleophile that is often used in E2 reactions. Strong nucleophiles usually undergo the SN2 or E2 pathway, but tert-butoxide is much too large to undergo a substitution reaction.

Example Question #2 : Substitution And Elimination Mechanisms

Which of the following factors do NOT favor an SN2 reaction of an alkyl halide?

Possible Answers:

A good nucleophile 

A polar aprotic solvent

A primary halide

A tertiary carbocation

Correct answer:

A tertiary carbocation

Explanation:

The way the question is phrased, three answer choices must favor an SN2 reaction, while the "correct" answer is a factor that does not favor, or disfavors an SN2 reaction.

SN2 reactions are bimolecular, and thus their rate of reaction depends on both the substrate and the nucleophile, forming a high energy transition state in which the nucleophile will displace the substate's leaving group at an angle of 180o. The more sterically hindered the compound is, the higher in energy the transition state will be, and the slower the rate of reaction will be. Consequently, SN2 reactions are favored when the leaving group (a halogen in this case) is on a primary carbon center. Additionally, because the reaction is bimolecular, step two of the reaction will NOT occur without a good nucleophile to displace the leaving group. Finally, all SN2 reactions are favored by polar aprotic solvents.  

Because SN2 reactions proceed via a transition state, no carbocation intermediate is formed (that happens in SN1 reactions) and therefore the formation of any carbocation favors an SN1 reaction, not an SN2 reaction.

Example Question #1 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

The reaction depicted in reaction 1 takes place in solution with a solvent. What type of solvent is most likely to be preferred for the reaction to occur as written?

Possible Answers:

Polar, aprotic solvent

Nonpolar, protic solvent

Nonpolar, aprotic solvent

This reaction requires water as a solvent

Polar, protic solvent

Correct answer:

Polar, aprotic solvent

Explanation:

Reaction 1 is an SN2 reaction. This type of substitution reaction prefers a polar, aprotic solvent. The polarity helps to solvate the nucleophile. Aprotic solvents help mediate the transition state and increase reaction rate.

Example Question #4 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

In reaction 1, a scientist is trying to modify the reaction by using a weaker nucleophile. Which of the following is a weaker nucleophile than what is used above (hydroxide ions)?

Possible Answers:

Correct answer:

Explanation:

Nucleophilicity increases to the left on the periodic table.  Nucleophilicity will also generally increase with charge. The only equally charged ion in the answers that is present to the right of oxygen on the periodic table is the fluoride ion.

Example Question #5 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

Using the product of reaction 2, a scientist adds bromine gas to the reaction chamber. After the bromine and the alkene react, he finds that his product consists entirely of single bonds, with two bromine atoms on the carbon chain. What kind of reaction most likely took place?

Possible Answers:

Addition reaction

Substitution reaction

Elimination reaction

Oxymercuration/demercuration reaction

Halogenation reaction

Correct answer:

Addition reaction

Explanation:

The addition of bromine gas () to the reaction vessel would likely result in the addition of one half of the diatomic bromine to each carbon, eliminating the double bond and resulting in an alkyl halide chain.

Halogenation reactions refer to reactions between a halogen and an alkane, while addition reactions occur between a halogen and an alkene (such as the product in reaction 2).

Example Question #6 : Substitution And Elimination Mechanisms

Cyclohex

Students in an organic chemistry lab perform two E2 elimination reactions, using compounds 1 and 2. The students observe that while compound 2 undergoes elimination with reagent X (not shown), compound 1 is unreactive. What is the best explanation for this discrepancy?

Possible Answers:

Bromine is not a good enough leaving group

Compound 1 is too sterically hindered

Compound 1 has no hydrogen molecules anti-periplanar to the bromine leaving group

Reagent X is more selective for compound 2 than for compound 1

Correct answer:

Compound 1 has no hydrogen molecules anti-periplanar to the bromine leaving group

Explanation:

This question tests your understanding on E2 elimination reactions. Because E2 reactions have no carbocation intermediates, the reaction proceeds through a transition state. The bromine atom (Br) must be displaced by the bond between an adjacent carbon and hydrogen, and that hydrogen must be anti-periplanar to the bromine. Compound 2, unlike compound 1 has one hydrogen that is anti-periplanar to the bromine, and thus undergoes an E2 elimination.

If compound 2 reacts, it is unlikely the reason that compound 1 would not react is because of steric hindrance. Additionally, aside from stereochemistry, there is nothing different enough about the two compounds that would suggest any reagent would be more favorable for one over the other.

Example Question #3 : Substitution And Elimination Mechanisms

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

Possible Answers:

Bromoethane

Cyclopentylbromide

3-methyl-3-iodopentane

Cis-2-bromo-1-methylcyclohexane

Benzylbromide

Correct answer:

Benzylbromide

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.

Example Question #7 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

In reaction 2, which of the following describe the rate limiting step?

I. It involves the formation of carbocation

II. It is favored by the presence of substituents on the central carbon

III. It involves a transition state, but no intermediate

Possible Answers:

II, only

I and II

III, only

II and III

I, only

Correct answer:

I and II

Explanation:

Reaction 2 represents an E1 reaction. The rate limiting step of reaction 2 involves the formation of a carbocation, whose stability is favored by the presence of substituents on the carbon involved. Carbocations are considered intermediates due to their relative stability compared to transition states.

Example Question #8 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

If reaction 1 were modified and a water molecule was used in place of the hydroxide ion, which of the following would likely be true?

Possible Answers:

The reaction would proceed more slowly, as water is a weaker nucleophile than hydroxide

The reaction would proceed more quickly, as water is a stronger nucleophile than hydroxide

The reaction would proceed more slowly, as water is a stronger nucleophile than hydroxide

The reaction would proceed more quickly, as water is a weaker nucleophile than hydroxide

The reaction would only proceed if the methyl group of the reactant's central carbon were changed to a hydrogen

Correct answer:

The reaction would proceed more slowly, as water is a weaker nucleophile than hydroxide

Explanation:

Reaction 1 represents an SN2 reaction. Such reactions depend, in part on the presence of strong nucleophiles, such as the hydroxide ion. Water can be a nucleophile as well, but it is weaker. Using water in place of hydroxide would cause reaction 1 to proceed far more slowly.

Example Question #8 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

Investigating reaction 2, you find that the reaction is initiated when a carbocation forms. Which of the following is likely true?

I. Concentration of the halide is the main determinant of reaction rate

II. The carbocation forms when the hydroxide removes the chlorine atom

III. The carbocation is planar

 

Possible Answers:

I, only

I and III

II and III

I and II

III, only

Correct answer:

I and III

Explanation:

The carbocation forms spontaneously with the loss of the chlorine atom. This is the rate determining step, thus, the concentration of the halide is the most important determinant of reaction rate. Carbocations form spontaneously in these reactions, and do not use the strong base to remove the halogen.

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