91Ó°ÊÓ

For each molecule, analyze the available hydrogen atoms and determine which one is the most acidic based on its location within the molecule. The hydrogen is more likely to be acidic if it is attached to a more electronegative atom.

Using pKa values, justify our choice of the most acidic hydrogen. Recall that a lower \(\mathrm{p} K_{\mathrm{a}}\) value denotes a stronger acid, so we'll be looking for the smallest pKa value associated with each molecule. (a) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{O}\) (alcohol), \(\mathrm{p} K_{\mathrm{a}} \approx 16\) (b) \(\mathrm{HSCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{S}\) (thiol), \(\mathrm{p} K_{\mathrm{a}} \approx 10\) (c) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{O}\) (alcohol), \(\mathrm{p} K_{\mathrm{a}} \approx 16\) (d) \(\mathrm{HOCCH}_{2} \mathrm{CH}_{2} \mathrm{SH}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{O}\) (carboxylic acid), \(\mathrm{p} K_{\mathrm{a}} \approx 4\) (e) \(\mathrm{CH}_{3} \mathrm{CHCOH}_{\mathrm{CH}}^{\mathrm{HO}}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{O}\) (alcohol), \(\mathrm{p} K_{\mathrm{a}} \approx 16\) (f) \(\mathrm{H}_{3} \mathrm{NCH}_{2} \mathrm{CH}_{2}\|_{\mathrm{COH}}^{\mathrm{O}}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{O}\) (carboxylic acid), \(\mathrm{p} K_{\mathrm{a}} \approx 4\) (g) \(\mathrm{H}_{3} \mathrm{NCH}_{2} \mathrm{CH}_{2} \|_{\mathrm{CO}^{-}}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{N}\) (amine, as carboxylate is already deprotonated), \(\mathrm{p} K_{\mathrm{a}} \approx 38\) (h) \(\mathrm{HSCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) - Most acidic H: \(\mathrm{H}\) on \(\mathrm{S}\) (thiol), \(\mathrm{p} K_{\mathrm{a}} \approx 10\) In each case, the most acidic hydrogen has been identified and justified using appropriate \(\mathrm{p} K_{\mathrm{a}}\) values.