3. Acids and Bases

Rank the following organic compounds in the order of increasing pKa.

Rank the following compounds in the order of increasing acidity.

Like nitrogen and carbon, oxygen also shows this same hybridization effect on acidity. Both of the following compounds can lose a proton from a positively charged oxygen with three bonds to give a conjugate base containing a neutral oxygen with two bonds. One of these structures has pKa = −2.4, while the other has pKa = −8.0.

a. Show the reaction of each compound with water.

b. Match each structure with its pKa, and explain your choice.

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Acetic acid can also react as a very weak base (pKb = 20). Two different sites on acetic acid might become protonated to give the conjugate acid.

1. Draw both of these possible conjugate acids, and explain (resonance) why the correct one is more stable.

2.Calculate the pKa of this conjugate acid.

Given the data in Problem 47:

What pH would you make the water layer to cause the carboxylic acid to dissolve in the water layer and the amine to dissolve in the ether layer?

Butyric acid, the compound responsible for the unpleasant odor and taste of sour milk, has a pKa value of 4.82. What is its Ka value? Is it a stronger acid or a weaker acid than vitamin C?

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?

Using the table of pKa values given in [Appendix I] <IMAGE>, answer the following:

a. Which is the most acidic organic compound in the table?

b. Which is the least acidic organic compound in the table?

c. Which is the most acidic carboxylic acid in the table?

For the bases shown, draw the conjugate acid and identify a pKₐ or the bases shown, draw the conjugate acid and identify a value from Table 4.5 that would help you accurately estimate its stability. value from Table 4.5 that would help you accurately estimate its stability.

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For each indicated proton, suggest an approximate pKₐ value from Table 4.5. Rationalize your choice.

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For the bases shown, draw the conjugate acid and identify a pKₐ or the bases shown, draw the conjugate acid and identify a value from Table 4.5 that would help you accurately estimate its stability. value from Table 4.5 that would help you accurately estimate its stability.

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a. Which is a stronger base: CH3COO− or HCOO−? (The pKa of CH3COOH is 4.8; the pKa of HCOOH is 3.8.)

b. Which is a stronger base: HO− or -NH2? (The pKa of H2O is 15.7; the pKa of NH3 is 36.)

c. Which is a stronger base: H2O or CH3OH? (The pKa of H3O+ is −1.7; the pKa of CH3O+H2 is −2.5.)

(•••) THINKING AHEAD A medicinal chemist needed to deprotonate acetylene ( HC≡CH ) for use in a coupling reaction. Among the options given, which base(s) could be used for this process?

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For each compound, indicate the atom that is most apt to be protonated.

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Rank the following from strongest base to weakest base:

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Citrus fruits are rich in citric acid, a compound with three COOH groups. Explain the following:

b. The third pKa is greater than the pKa of acetic acid.

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Given the structure of ascorbic acid (vitamin C):

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(a) Is ascorbic acid a carboxylic acid?

(b) Compare the acid strength of ascorbic acid (pKa = 4.71) with acetic acid.

Which is a stronger base: ethoxide ion or acetate ion? Give pKb values (without looking them up) to support your choice.

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Given the structure of ascorbic acid (vitamin C):

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(c) Predict which proton in ascorbic acid is the most acidic.

(d) Draw the form of ascorbic acid that is present in the body (aqueous solution, pH = 7.4)

Arrange the following compounds in order of decreasing acidity.

CH₃COOH, CH₃OH, CH₃CH₃, CH₃SO₃H, CH₃NH₂, CH₃SH, CH₃C≡CH

Which of the following bases can remove a proton from a terminal alkyne in a reaction that favors products?

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You are planning to carry out a reaction that produces protons. The reaction will be buffered at pH=10.5. Would it be better to use a protonated methylamine/methylamine buffer or a protonated ethylamine/ethylamine buffer? (pKa of protonated methylamine=10.7; pKa of protonated ethylamine=11.0)

Any base whose conjugate acid has a pKa greater than ______ can remove a proton from a terminal alkyne to form an acetylide ion (in a reaction that favors products).