Question 1

Which of the following statements best explains why proton transfers between heteroatoms (such as O, N, S) are typically much faster than nucleophilic substitution reactions at carbon centers?

Accepted Answer

Proton transfers are diffusion-controlled and often occur in a single collision due to favorable linear hydrogen bonding.

Question 2

Given the following pKa values: $$\mathrm{CH_3CH_3} (50$$), $$\mathrm{CH_3NH_2} (36$$), $$\mathrm{CH_3OH} (16$$), which compound has the most acidic hydrogen?

Accepted Answer

$$\mathrm{CH_3OH}$$

Question 3

Why is the acetylide anion ($$\mathrm{RC \equiv C^-}) $$much more nucleophilic than the hydronium ion ($$\mathrm{H_3O^+}$$)?

Accepted Answer

Because the acetylide anion has a much higher pKa, indicating greater basicity and nucleophilicity.

Question 4

Which factor most increases the acidity of a hydrogen atom attached to carbon?

Accepted Answer

Increasing the resonance stabilization of the conjugate base.

Question 5

Consider the reaction: $$\mathrm{RC \equiv CH + H_2O \rightleftharpoons RC \equiv C^- + H_3O^+}. If $$the pKa of $$\mathrm{RC \equiv CH} is 24 $$and the pKa of $$\mathrm{H_3O^+} is -1.7$$, what is the equilibrium constant ($$K_{eq}) $$for this reaction?

Accepted Answer

$$K_{eq} = 10$$^{-1.7 - 24}$$ = 10$$^{-25.7}$$

Question 6

Which of the following best explains why proton transfers involving five- or six-membered ring intermediates are more favorable than those involving tetrahedral intermediates?

Accepted Answer

Five- or six-membered rings allow for more linear hydrogen bonding, which facilitates proton transfer.

Question 7

In the context of acid-base equilibria, what does a low pKa value indicate about a compound?

Accepted Answer

The compound is a strong acid.

Question 8

Why is it easier to deprotonate an alcohol ($$\mathrm{CH_3OH}$$, pKa 16) than an amine ($$\mathrm{CH_3NH_2}$$, pKa 36)?

Accepted Answer

Because the oxygen atom in alcohol is more electronegative and stabilizes the negative charge better than nitrogen.

Question 9

Which of the following statements about the effect of electronegative atoms on acidity is correct?

Accepted Answer

An electronegative atom adjacent to the anion increases acidity by stabilizing the conjugate base.

Question 10

In the Fischer esterification reaction, why is HCl used as a catalyst instead of protonated methanol?

Accepted Answer

HCl is a much stronger acid and can protonate the carbonyl oxygen more effectively, driving the reaction forward.

Question 11

If the difference in pKa ($$\Delta$$pKa) between an acid and a base is greater than 3, what does this imply about the rate of proton transfer?

Accepted Answer

The proton transfer is diffusion-controlled and occurs rapidly with every collision.

Question 12

Which of the following best describes the relationship between pKa and nucleophilicity in organic chemistry?

Accepted Answer

A higher pKa of the conjugate acid generally means the conjugate base is a stronger nucleophile.

Question 13

Which of the following is the best explanation for why the rate constant for the backward reaction in a proton transfer can be as high as $$10^{11}$$ \ \mathrm{$$M^{-1}$$ $$s^{-1}$$}$$?

Accepted Answer

The backward reaction is diffusion-controlled, so every collision results in a proton transfer.

Question 14

Which of the following compounds would be most difficult to deprotonate based on their pKa values?

Accepted Answer

$$\mathrm{CH_3CH_3} ($$pKa 50)

Question 15

Which of the following statements about the effect of hybridization on acidity is correct?

Accepted Answer

A carbon atom with more s-character (e.g., sp) forms a more stable anion, increasing acidity.

Question 16

In the reaction of piperidine with NaOH and an acyl chloride, which species acts as the nucleophile?

Accepted Answer

The neutral piperidine molecule

Question 17

Which of the following best explains why the equilibrium for the reaction $$\mathrm{H_2O + F_3CCO_2H \rightleftharpoons HOCH_2CH_2OH + F_3CCO_2^-}$$ favors the starting materials?

Accepted Answer

The trifluoromethyl group stabilizes the conjugate base, making the acid much stronger.

Question 18

Which of the following is a correct statement about the role of resonance in acidity?

Accepted Answer

Resonance stabilization of the conjugate base increases the acidity of the parent compound.

Question 19

If the equilibrium constant ($$K_{eq}) $$for a proton transfer reaction is $$10^{-5}$$ and the rate constant for the backward reaction ($$k_{-1}) is$$ $$10^{11}$$ \ \mathrm{$$M^{-1}$$ $$s^{-1}$$}$$, what is the rate constant for the forward reaction (k_1$)?

Accepted Answer

$$k_1$$ = $$10^{6}$$ \ \mathrm{$$M^{-1}$$ $$s^{-1}$$}$$

Acids and Bases in Organic Chemistry: Mechanisms, pKa, and Equilibria

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