Question

For each of the following pairs, predict which substance is more soluble in water. a. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) or \(\mathrm{NH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{CN}\) or \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) d. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) e. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\) f. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) or \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\)

Short answer

a. \(\mathrm{NH}_{3}\) is more soluble. b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) is more soluble. c. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) is more soluble. d. \(\mathrm{CH}_{3} \mathrm{OH}\) is more soluble. e. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\) is more soluble. f. \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) is more soluble.

Step-by-step solution

a. Comparing \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) and \(\mathrm{NH}_{3}\)

Both of these substances are polar and will engage in hydrogen bonding with water. While both will be soluble, \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) has an additional methyl group which is nonpolar and may cause a slight decrease in solubility. Therefore, \(\mathrm{NH}_{3}\) is more soluble in water.

b. Comparing \(\mathrm{CH}_{3} \mathrm{CN}\) and \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)

The polarity of \(\mathrm{CH}_{3} \mathrm{CN}\) is due to the cyano group, while the polarity of \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) is due to hydrogen bonding with the oxygen. In this case, \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) is more soluble in water due to stronger hydrogen bonding.

c. Comparing \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) is polar due to the hydroxyl group, which can engage in hydrogen bonding with water. On the other hand, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) is nonpolar and has only weak van der Waals forces. Therefore, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) is more soluble in water.

d. Comparing \(\mathrm{CH}_{3} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)

Both of these substances are polar due to the hydroxyl group. However, \(\mathrm{CH}_{3} \mathrm{OH}\) has a smaller nonpolar region than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), making it slightly more soluble in water.

e. Comparing \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) and \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\)

Both of these substances have a polar hydroxyl group capable of hydrogen bonding. However, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) has a larger nonpolar region than \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\), which decreases its solubility in water. Therefore, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{OH}\) is more soluble in water.

f. Comparing \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\)

Both of these substances are polar. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) engages in hydrogen bonding with water, while \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) can form hydrogen bonds and also dissociate into ions. In this case, \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) is more soluble in water due to its ability to dissociate into ions, which further increases the solubility.