Question 1

How does Zn²⁺ catalyze the hydrolysis of trifluoroacetate esters?

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Question 2

Provide the two general ways that metal ions catalyze organic reactions.

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Question 3

In the hydrolysis of methyl trifluoroacetate,give two purposes of the zinc catalyst.You may choose more than one answer.

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Question 4

The compound CH₃OCH₂CH₂CH₂CH₂Br undergoes a solvolysis reaction in aqueous ethanol solution.Which of the following is evidence that a neighboring group effect is occurring?

Accepted Answer

Choice A shows evidence of a neighboring group effect, as the compound with the sulfur neighboring group (CH₃) reacts more slowly than the compound with the oxygen neighboring group (CH₃O). This suggests that the neighboring group is affecting the reactivity of the molecule, as expected in a neighboring group participation mechanism. Choices B, C, and D do not necessarily provide evidence of a neighboring group effect. The fact that the reaction is faster at higher concentrations of water (choice B) could be explained by other factors, such as increased solubility of the reactant or changes in the solvent environment. The reaction being first order (choice C) could also be consistent with other types of mechanisms. Lastly, the formation of HBr (choice D) is expected from the breaking of the carbon-bromine bond, but does not provide specific evidence of a neighboring group effect.

Question 5

Rank the following alkyl chlorides in increasing order of reactivity in aqueous ethanol,and explain why one of the compounds is 70,000 times more reactive than the other two.

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Question 6

Which of the following is an intermediate in the metal-ion catalyzed reaction shown below?

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Question 7

Hydrolysis of esters occurs more rapidly in the presence of catalytic imidazole.Explain this phenomenon and provide the structure of the acyl imidazole intermediate which is formed in the catalyzed process.

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Question 8

Which of the following are affected when an intramolecular catalyst is used?

Accepted Answer

All of the above are affected when an intramolecular catalyst is used. Intramolecular catalysts bring the two reactants together in a specific orientation, which increases the fraction of collisions with proper orientation. This also increases the number of collisions between the two reactants per unit time. Additionally, the intramolecular catalyst can lower the activation energy of the reaction, which increases the fraction of collisions with sufficient energy. All of these factors contribute to the increased rate of the reaction.

Question 9

Which of the following is not true about metal-ion catalysts?

Accepted Answer

Metal-ion catalysts act as Lewis acids, not Lewis bases. They are electron acceptors which coordinate with atoms with nonbonded electrons to stabilize intermediate species during a reaction.

Question 10

Offer an explanation of how iodide catalyzes the hydrolysis of 1-chloropropane in aqueous base.

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Question 11

Give the halide that is the most reactive as a nucleophilic catalyst.

Accepted Answer

Iodide is the most reactive as a nucleophilic catalyst due to its larger size and lower electronegativity compared to the other halides, which allows it to more easily donate its lone pair of electrons.

Question 12

How does a metal catalyst increase the rate of reaction?

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Question 13

Show why typical metal-bound water has a considerably lower pKa than that of water.

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Question 14

How does Cu²⁺ catalyze the decarboxylation of dimethyloxaloacetate?

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Question 15

What is the main purpose for using metal-ions as catalysts in the following hydrolysis reaction?

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Question 16

Explain why an intramolecular reaction resulting in the formation of five- or six-membered rings occurs more readily than the corresponding intermolecular reaction.

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Question 17

Rank the following compounds in increasing order of reactivity in the intramolecular displacement of p-bromophenolate to form a cyclic anhydride.Explain your ranking.

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Question 18

Which of the following is a step in the mechanism of the reaction shown below?

Accepted Answer

The mechanism of the given reaction involves the formation of a carbocation intermediate. The step II involves the formation of a carbocation by protonation of the alcohol with sulfuric acid.

Question 19

Describe the first step of a general-base-catalyzed dehydration of a hydrate.

Accepted Answer

In the first step of a general-base-catalyzed dehydration of a hydrate, the base (generally hydroxide ion) abstracts a proton-electron pair from the OH group of the hydrate. This results in the formation of a double bond between the two adjacent carbon atoms and the departure of a hydroxide ion as a leaving group. Therefore, the correct option is C where base abstracts a proton-electron pair from the O-H bond and forms a double bond, and hydroxide departs as the leaving group.

Question 20

"Being in the right place...can be the most powerful factor of all in determining how fast a reaction goes." In terms of reaction rate theory,this statement refers to the ________ for a reaction.

Accepted Answer

The probability factor, or collision frequency, is affected by the concentration of reactants and their physical properties. Being in the right place would increase the likelihood of successful collisions and therefore increase the reaction rate. The other factors mentioned (energy factor, resonance effect, inductive effect) have some impact on reaction rates but are not as directly affected by being in the right place.

Quiz 22: Catalysis in Organic Reactions and in Enzymatic Reactions

How does Zn2+ catalyze the hydrolysis of trifluoroacetate esters?