3. Acids and Bases

Acid–base reactions are reversible. Show a mechanism for the reverse of the reactions in Assessment 4.18.

Identify the nucleophile and the electrophile in each of the following reactions.

(a) <IMAGE>

Show an arrow-pushing mechanism that forms the product on the right from the reactant at left. Two arrows are necessary in each reaction.

(b) <IMAGE>

Identify the arrows shown by type and predict the product that should result.

(e) <IMAGE>

Identify the arrows shown by type and predict the product that should result.

(a) <IMAGE>

Would you expect the following species to be electrophiles or nucleophiles? Some may be both. Explain your answer.

(e) <IMAGE>

We discuss the following reactions in subsequent chapters. Given the mechanisms shown, draw the mechanism of the reverse reaction.

(b) <IMAGE>

Provide a reasonable arrow-pushing mechanism for the following Lewis acid–Lewis base reactions.

(a) <IMAGE>

Provide an arrow-pushing mechanism for the following hypothetical base half-reactions. [These are only intended to help you learn about arrow pushing in acid–base reactions.]

(b) <IMAGE>

(••••) THINKING AHEAD BCl₃ has an empty p orbital, so it is a strong Lewis acid. Would you expect an amide to react with BCl₃ at nitrogen or oxygen?

<IMAGE>

In each case, circle the stronger nucleophile.

(b) <IMAGE> vs. <IMAGE>

Based on the analysis you used in Assessment 17.3, which carbonyl would you expect to react most quickly with a nucleophile?

(c) <IMAGE> vs. <IMAGE>

Classify the following nucleophiles as strong, weak, or intermediate. Would you expect each to add to a carbonyl directly or wait for a carbocation to form?

b) <IMAGE>

Would you expect a ketone or an ester to be more reactive with a strong nucleophile? Justify your answer.

<IMAGE>

Rank the reactivity of the following carbonyls with nucleophiles, from least reactive to most reactive.

<IMAGE>

The following reaction steps are shown using conventional electron pushing. (a) Draw the second product whose formation would have been rationalized with this same arrow.

(a) <IMAGE> + ?

The following reaction steps are shown using conventional electron pushing. (b) Use the bouncing arrow formalism to illustrate the formation of only the product shown.

(a) <IMAGE> + ?

(••) For the following acid–base pairs,

(v) show a mechanism for the reaction;

(e) <IMAGE>

Identify the electrophile and the nucleophile in each of the following reaction steps and then draw curved arrows to illustrate the bond-making and bondbreaking processes.

a. <IMAGE>

Identify the electrophile and the nucleophile in each of the following reaction steps and then draw curved arrows to illustrate the bond-making and bondbreaking processes.

b. <IMAGE>

Identify the electrophile and the nucleophile in each of the following reaction steps and then draw curved arrows to illustrate the bond-making and bondbreaking processes.

c. <IMAGE>

Label the reactants in these acid–base reactions as Lewis acids (electrophiles) or Lewis bases (nucleophiles). Use curved arrows to show the movement of electron pairs in the reactions.

i. -BF3CH2(+CH2) <IMAGE> + CH2CH2 <IMAGE> —> -BF3CH2CH2CH2(+CH2) <IMAGE>

In each reaction, label the reactants as Lewis acids (electrophiles) or Lewis bases (nucleophiles).

Use curved arrows to show the movement of electron pairs in the reactions.

Draw any nonbonding electrons to show how they participate in the reactions.

c. CH3COH <IMAGE> + HCl —> CH3C(+OH)H <IMAGE> + Cl-

Each of these compounds can react as a nucleophile. In each case, use curved arrows to show how the nucleophile would react with the strong electrophile BF3.

e. CH3CH2OH

Each of these compounds can react as a nucleophile. In each case, use curved arrows to show how the nucleophile would react with the strong electrophile BF3.

f. (CH3)2S

Which atoms in the enol ether would you expect to react with Br⁺ ?

<IMAGE>