Glycine has pK_a values of 2.3 and 9.6. Do you expect the pK_a values of glycylglycine to be higher or lower than these values?

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

a. The first pK_a (for the COOH group in the center of the molecule) is lower than the pK_a of acetic acid.

Which of the following amino acid side chains can help remove a proton from the α-carbon of an aldehyde?

Rank the following compounds from easiest to hardest at removing a proton from its methyl substituent:

Explain the relative acidities.

Fosamax (shown on the previous page) has six acidic groups. The active form of the drug, which has lost two of its acidic protons, is shown in the box. 

(Notice that the phosphorus atom in Fosamax and the sulfur atom in [Problem 36] can be surrounded by more than eight electrons because P and S are below the second row of the periodic table.)

a. Why are the OH groups bonded to phosphorus the strongest acids of the six groups?

b. Which of the remaining four groups is the weakest acid?

Protonated cyclohexylamine has a Ka = 1 * 10-11. Using the same sequence of steps as in Problem 94, determine which is a stronger base: cyclohexylamine

or aniline.

e. Which has a greater Ka: cyclohexylammmonium ion or anilinium ion?

f. Which is a stronger acid: cyclohexylamine or aniline?

The pK_a values of a few ortho-, meta-, and para-substituted benzoic acids are shown below:

The relative pK_a values depend on the substituent. For chloro-substituted benzoic acids, the ortho isomer is the most acidic and the para isomer is the least acidic; for nitro-substituted benzoic acids, the ortho isomer is the most acidic and the meta isomer is the least acidic; and for amino-substituted benzoic acids, the meta isomer is the most acidic and the ortho isomer is the least acidic. Explain these relative acidities.

c. NH₂: meta > para > ortho

Explain why, when the guanidino group of arginine is protonated, the double-bonded nitrogen is the nitrogen that accepts the proton.