Pictures of the Day CH320M328M

Polar protic solvents have polar bonds (usually C=O, OH, or NH polar bonds) and a proton that can take part in hydrogen bonding. A good example is methanol. As the electrostatic potential surface indicates, there is a center of high electron density (indicated by the red color) associated with the lone pairs on oxygen, and a center of low electron density (indicated by the blue color) associated with the hydroxyl group proton. As a result, methanol and other polar protic solvents are good a solvating, and thus dissolving ionic compounds. Here is shown in schematic form how BOTH an anion (red sphere) and cation (blue sphere) are well solvated by methanol. The anion is surrounded by the protons of several methanol molecules, while the cation is surrounded by the lone pairs. If the anion were going to take part as a nucleophile in an SN2 process, then this strong solvation would decrease its nucleophilic character, since all of these methanol molecules would have to move out of the way prior to reaction.

Shown above are shown solvation in schematic form of how methanol would solvate the tert-butyl bromide starting material (the tert-butyl bromide is shown colored by atom type, not electrostatic potential), then on the right how the tert-butyl cation intermediate and bromide anion leaving group would both be well solvated by methanol (here the carbocation intermediate and bromide anion are shown colored by electrostatic potential surface to emphasize how charges dictate solvation). In SN1 and E1 reactions, the slow step is formation of a carbocation and leaving group anion. Conditions that favor formation of these ions will also help facilitate the SN1 and E1 processes. Polar protic solvents like carboxylic acids, alcohols, and water accelerate SN1/E1 reactions because they are so good at solvating both the carbocations and leaving group anions that they actually assist in the process. Because these three solvents are also very weak bases, they do not promote E2 processes for tertiary alkyl halides. Thus the combined effects of acceleration of the slow bond cleavage step and the lack of basic character mean that SN1/E1 processes predominate for tertiary alkyl halides in carboxylic acids, alcohols, and water.

Alcohols have relatively high boiling points because the molecules stick together due to hydrogen bonding. Hydrogen bonding occurs when the hydrogen atoms attached to N, O, or S atoms interact with the lone pairs of electrons on N, O, or S atoms. This is really an electrostatic interaction in that the hydrogen atoms possess a partial positive charge are thus attracted to the partial negative charge of the electronegative N, O, or S atoms. For calibration, hydrogen bonds are worth about 1-4 kcals/mol. Hydrogen bonds are extremely important in almost all aspects of biochemistry and biology. In the above 2-dimensional illustration, notice how the alcohol molecules are arranged to produce the maximum amount of hydrogen bonding. Hydrogen bonds are indicated as red dashed lines in the schematic on the right.

Hydrocarbons, like pentane are not polar and cannot hydrogen bond. They therefore do not have any way to disrupt the strong hydrogen bonding between water molecules. Since they cannot disrupt the strong attractions between water molecules, the non-polar pentane molecules are kept out of the water and are left to interact with themselves through much weaker dispersion forces. As a result, “oil and water” do not mix, and being less dense (carbon has a lower atomic weight than oxygen), hydrocarbons such as crude oil float on top of water. Again notice the extensive hydrogen bonding taking place between water molecules in this 2-dimensional illustration.

Solvents dissolve molecules like themselves (“like dissolves like”. Thus, hydrogen bonding solvents like water will dissolve hydrogen bonding substances like alcohols with small carbon chains because the hydrogen bonds between water molecules is replaced with hydrogen bonds between the alcohol and water molecules. Notice how each OH group of the “dissolved” alcohol molecules in this 2-dimensional illustration is taking part in hydrogen bonding with water molecules.