Question

Many moderately large organic molecules containing basic nitrogen atoms are not very soluble in water as neutral molecules, but they are frequently much more soluble as their acid salts. Assuming that \(\mathrm{pH}\) in the stomach is \(2.5\), indicate whether each of the following compounds would be present in the stomach as the neutral base or in the protonated form: nicotine, \(K_{b}=7 \times 10^{-7} ;\) caffeine, \(K_{b}=4 \times 10^{-14} ;\) strychnine, \(K_{b}=1 \times 10^{-6} ;\) quinine, \(K_{b}=1.1 \times 10^{-6}\)

Short answer

In the stomach (pH 2.5), nicotine, strychnine, and quinine will be present in their protonated forms, while caffeine will be present as the neutral base.

Step-by-step solution

Calculate the \(pK_b\) for nicotine

\[\mathrm{p}K_b=-\log(7\times 10^{-7})\approx 6.15\]

Calculate the \(pK_a\) for nicotine

\[\mathrm{p}K_a=14-\mathrm{p}K_b=14-6.15\approx 7.85\]

Determine the form of nicotine

Since \(pK_a > pH\) (7.85 > 2.5), nicotine will be present in the protonated form in the stomach. ##Caffeine##

Calculate the \(pK_b\) for caffeine

\[\mathrm{p}K_b=-\log(4\times 10^{-14})\approx 13.40\]

Calculate the \(pK_a\) for caffeine

\[\mathrm{p}K_a=14-\mathrm{p}K_b=14-13.40\approx 0.60\]

Determine the form of caffeine

Since \(pK_a < pH\) (0.60 < 2.5), caffeine will be present as the neutral base in the stomach. ##Strychnine##

Calculate the \(pK_b\) for strychnine

\[\mathrm{p}K_b=-\log(1\times 10^{-6})\approx 6.00\]

Calculate the \(pK_a\) for strychnine

\[\mathrm{p}K_a=14-\mathrm{p}K_b=14-6.00\approx 8.00\]

Determine the form of strychnine

Since \(pK_a > pH\) (8.00 > 2.5), strychnine will be present in the protonated form in the stomach. ##Quinine##

Calculate the \(pK_b\) for quinine

\[\mathrm{p}K_b=-\log(1.1\times 10^{-6})\approx 5.96\]

Calculate the \(pK_a\) for quinine

\[\mathrm{p}K_a=14-\mathrm{p}K_b=14-5.96\approx 8.04\]

Determine the form of quinine

Since \(pK_a > pH\) (8.04 > 2.5), quinine will be present in the protonated form in the stomach. To summarize, nicotine, strychnine, and quinine will be present in their protonated forms in the stomach, while caffeine will be present as the neutral base.