3. Acids and Bases

Hydrogen gas (H₂) has a relatively high pKₐ value. Is it a stable or unstable acid? Do you expect it to participate in acid–base reactions?

Calculate K_eq for these acid–base reactions.

(d) <IMAGE>

(•••) THINKING AHEAD Using pKₐ values for the conjugate acids of the bases on each side of the reaction arrow, identify which side of the equilibrium would be favored in the following hypothetical reactions.

(c) <IMAGE>

Calculate Kₑq for the acid–base reaction shown. Which side is favored and why?

<IMAGE>

Write the K_eq expression for the following acid–base reactions. [You don't need to calculate K_eq here.]

(c) <IMAGE>

(••••) Parts (a)–(d) of this assessment assist in the development of what will become a common theme in organic reactions and should be worked in order. [Think carefully about how each question relates to the others.]

(c) Without worrying about the mechanism of the reaction, estimate an equilibrium constant for the following carbonyl addition reaction based on the relative stability of the Lewis bases.

<IMAGE>

Calculate the equilibrium constant for the reaction of ethylamine and propanoic acid. Which side is favored?

<IMAGE>

How do you know that a proton with a low pKₐ value is acidic (besides 'I just remember')?

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

(iv) calculate Keq;

(e) <IMAGE>

Given that the indicated pKₐ values correspond to the acid dissociation reactions shown, calculate the ratio of acid to conjugate base for the reactions shown.

(a) <IMAGE>

Using pKₐ values, calculate an approximate K_eq value for the following substitution reaction. <IMAGE>

Amino acids exist predominantly in one of the following forms. Which is it? Explain your answer.

<IMAGE>

At what pH is the concentration of a compound, with a pKa=4.6, 10 times greater in its basic form than in its acidic form?

b. At what pH does 80% of the acid exist in its acidic form?

The pKa of a protonated alcohol is about -2.5, and the pKa of an alcohol is about 15. Therefore, as long as the pH of the solution is greater than _______ and less than _______, more than 50% of 2-propanol (the product of the reaction on p. 244) will be in its neutral, nonprotonated form.

The of protonated acetone is about and the of protonated hydroxylamine is 6.0.

a. In a reaction with hydroxylamine at pH 4.5 (Figure 16.2) <IMAGE>, what fraction of acetone is present in its acidic, protonated form? (HINT: See Section 2.10. )

The pKa of acetic acid in water is 4.76. What effect will a decrease in the polarity of the solvent have on the pKa? Why?

Calculate the equilibrium constant for the acid–base reaction between the reactants in each of the following pairs:

c. CH3NH2+H2O

d. CH3N+H3+H2O

Which reaction in Problem 23 has the smallest equilibrium constant?

Given that pH+pOH=14 and that the concentration of water in a solution of water is 55.5 M, show that the pKa of water is 15.7. (Hint: pOH=−log [HO−])

(••••) Parts (a)–(d) of this assessment assist in the development of what will become a common theme in organic reactions and should be worked in order. [Think carefully about how each question relates to the others.]

(b) Calculate K_eq for the following acid–base reaction.

<IMAGE>

For each of the following compounds (here shown in their acidic forms), write the form that ­predominates in a solution with a pH=5.5:

g. HNO2 (pKa=3.4)

h. HNO3 (pKa=−1.3)

As long as the pH is not less than ___________, at least 50% of a protonated amine with a pKa value of 10.4 will be in its neutral, nonprotonated form.

a. Indicate whether a protonated amine (RN+H3) with a pKa value of 9 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

For each of the following compounds, indicate the pH at which

a. 50% of the compound is in a form that possesses a charge.

1. CH3CH2COOH (pKa=4.9)

2. CH3N+H3 (pKa=10.7)

For each of the following compounds, draw the form that predominates at pH=3, pH=6, pH=10, and pH=14:

A. CH3COOH

pKa=4.8

For each of the following compounds (here shown in their acidic forms), write the form that ­predominates in a solution with a pH=5.5:

e. +NH4 (pKa=9.4)

f. HC≡N (pKa=9.1)

b. Indicate whether an alcohol (ROH) with a pKa value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

1. pH = 1

2. pH = 3

3. pH = 5

b. Indicate whether an alcohol (ROH) with a pKa value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

3. pH = 7

4. pH = 10

5. pH = 13

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pKa values of the two groups are shown.

<IMAGE>

d. Draw the structure of alanine in a solution at pH=12.