MCAT Biology : Organic Chemistry, Biochemistry, and Metabolism

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Reaction 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 stronger nucleophile than hydroxide

The reaction would proceed more slowly, 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

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

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

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 #21 : Organic Chemistry

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 and III

III, only

I and II

I, only

II and III

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.

Example Question #61 : Organic Chemistry, Biochemistry, And Metabolism

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 the rate limiting step of reaction 2, which of the following describe the intermediate chemical species?

I. It has sp2 hybridization

II. It is trigonal planar

III. It exhibits bond rigidity, limiting rotation

Possible Answers:

II and III

III, only

I and III

I, II, and III

I and II

Correct answer:

I and II

Explanation:

Reaction 2 is an E1 reaction, in which the rate limiting step is the formation of the carbocation intermediate. The carbocation intermediate has three single bonds and a positive charge on the central carbon; thus, it has sp2 hybridization, a planar structure, and free rotation about its single bonds. Bond rigidity is only observed with the presence of pi bonds.

Example Question #61 : Organic Chemistry, Biochemistry, And Metabolism

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

A scientist modifies reaction 1 by changing the reactant, removing a hydrogen from the central carbon and replacing it with a methyl group. The new reactant thus has two methyl groups and one hydrogen on the central carbon. What is true of reaction 1 following this modification? Assume the temperature remains constant and no catalyst is added.

Possible Answers:

Reaction 1 proceeds more slowly, owing to a higher activation energy

Reaction 1 proceeds more quickly, owing to a more stable carbocation

Reaction 1 only proceeds with a stronger nucleophile

Reaction 1 proceeds more quickly, owing to a decrease in steric hindrance

Reaction 1 proceeds more slowly, owing to a less stable carbocation

Correct answer:

Reaction 1 proceeds more slowly, owing to a higher activation energy

Explanation:

Reaction 1 will experience greater steric hindrance with the addition of a methyl group, in place of a hydrogen, on the central carbon of the reactant. The result of this is increased activation energy, and a reduced rate of reaction in unchanging temperature and with no addition of a catalyst.

Example Question #11 : 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

Which of the following describe the intermediate in reaction 1?

I. It is planar

II. It is uncharged (neutral)

III. It is a carbocation

IV. Reaction 1 does not involve an intermediate

Possible Answers:

IV, only

I and II

II, only

III, only

I, only

Correct answer:

IV, only

Explanation:

Intermediates are relatively stable, while transition states are unstable and transient. The transition state (not the intermediate) of reaction 1 is a planar uncharged structure; however, only relatively stable species such as carbocations are considered intermediates. Reaction 1 does not have an intermediate, and is an example of an SN2 reaction; only SN1 reactions use a carbocation intermediate.

Example Question #1 : Elimination Mechanisms

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?

Possible Answers:

E4

E2

E3

None of these

E7

Correct answer:

E4

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

Example Question #41 : Reactions

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

A scientist is studying the rate of reaction 1. He wants to double the rate of the reaction, but is unsure how to increase concentrations of the reactants. Which of the following is true?

Possible Answers:

Neither doubling the concentration of halide, nor doubling the concentration of hydroxide, will quadruple the reaction rate

Reaction rate in this reaction is not determined by concentration

Doubling the concentrations of both the hydroxide and the halide will quadruple the reaction rate

Doubling the concentration of the halide only will quadruple the reaction rate

Doubling the concentration of the hydroxide only will quadruple the reaction rate

Correct answer:

Doubling the concentrations of both the hydroxide and the halide will quadruple the reaction rate

Explanation:

Reaction 1 represents an SN2 reaction. The rate limiting step involves both reactants coming together to form a transition state. The rate of this reaction depends on the concentration of both the organic molecule and the nucleophile.

In contrast, reaction 2 is an E1 reaction, in which the rate limiting step is the removal of the leaving group to form a carbocation. In E1 and SN1 reactions, adjusting the concentration of the halide only is enough to affect the rate.

Example Question #1 : Nucleophilic Substitution

When exposed to a good nucleophile, which molecule will most readily undergo an  reaction?

Possible Answers:

Correct answer:

Explanation:

 reactions, also known as unimolecular nucleophilic substitution reactions, occur in two steps. Here, we are concerned with the first and second (rate-determining) steps, in which the leaving group breaks off of the molecule to form a carbocation. Alkanes that form the most stable carbocations are most likely to undergo  reactions. Tertiary carbocations are the most stable, followed by secondary. Primary and methyl carbocations are very unstable and unlikely to form at all. The tertiary alkane, , will form a very stable tertiary carbocation compared to the other answer choices.

Example Question #42 : Reactions

Which of the following compounds can be oxidized to form a ketone?

Possible Answers:

Methanol

Propyl alcohol

2-methyl-2-butanol

3-pentanol

Correct answer:

3-pentanol

Explanation:

An alcohol can be oxidized and form a ketone only if the alcohol is a secondary alcohol. Tertiary alcohols cannot be oxidized and primary alcohols are oxidized to form aldehydes and carboxylic acids.

3-pentanol is a secondary alcohol, while 2-methyl-2-butanol is a tertiary alcohol. Methanol and propyl alcohol are both primary alcohols.

Example Question #43 : Reactions

What reagent would be best suited to accomplish the transformation shown below?

Mcat_5

Possible Answers:

Methyl magnesium bromide,

Correct answer:

Explanation:

Reduction of an ester to an alcohol requires a strong reducing agent, such as .

is not a strong enough reducing agent for this reaction. Hydrogenation and reaction with zinc would reduce the nitro group to an amino group instead of affecting the carboxylic acid. is a Grignard reagent, and would not function as a reducing agent.

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