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regions are on the outside exposed to water and form hydrogen bonds with water and the hydrophobic (dislikes water) regions are on the inside away from water. Such selective interactions with water are an important driving force that helps proteins to obtain the correct shapes necessary to function. It is because water can readily reform its network structure through the making and breaking of hydrogen bonds that it can accommodate the proteins’ structural transformation and a vast number of other chemical processes that occur in living organisms.

WATER IS A GREAT SOLVENT

One of water’s properties is its ability to dissolve a very wide variety of chemical compounds. We have discussed that NaCl will dissolve in water to give ions, Na+ and Cl-. The positive ions are surrounded by the slightly negative oxygens of water, and the negative ions are surrounded by the slightly positive hydrogens of water. Salt dissolves because of water’s ability to interact favorably with both cations and anions. Water can also dissolve a very wide variety of organic compounds. Water will not dissolve hydrocarbons such as ethane, but it will dissolve organic molecules like ethanol that have hydroxyl groups (—OH) or other groups that have slightly charged or fully charged portions. Water dissolves ethanol by forming hydrogen bonds to the hydroxyl group of ethanol. In pure ethanol, ethanol molecules hydrogen bond one to another to form chains, as shown in Figure 15.2. When ethanol is put in water, water can form hydrogen bonds to ethanol’s hydroxyl, incorporating ethanol molecules into water’s hydrogen bonding network. Vodka is essentially ethanol in water. Wine is water with less ethanol in it than vodka. Wine also has large organic molecules that give red wine its color and all wines their distinct aroma and taste.

ETHANOL WILL UNDERGO CHEMICAL REACTIONS WITH OXYGEN

If wine is exposed to air for too long, it will “go bad” and turn into vinegar. Vinegar is made by purposely making wine go bad. The chemical reactions that convert wine into vinegar are actually facilitated by a bacteria, acetobacter, which has the ability to convert ethanol into acetic acid when oxygen is present. The process requires two chemical reaction steps. The chemical reactions are written as

CH3CH2OH → CH3CHO + H2

2CH3CHO + O2 → 2CH3COOH

First ethanol (CH3CH2OH) is converted to acetaldehyde (CH3CHO) and hydrogen gas (top line), and then two molecules of acetaldehyde and one oxygen molecule (two oxygen atoms) are converted into two molecules of acetic acid (CH3COOH), which is vinegar. The structure of ethanol is shown in Figure 15.1. The structures of acetaldehyde (top) and acetic acid (bottom) are shown in Figure 15.4. In both acetaldehyde and acetic acid, the carbon labeled C1 forms a methyl group. C1 is bonded to three hydrogens and carbon C2. Acetaldehyde has C2 also bonded to a single hydrogen and double bonded to an oxygen. In general, an aldehyde has a carbon double bonded to an oxygen, bonded to a hydrogen, and bonded to another carbon. In formaldehyde (Figure 14.3), instead of bonding to another carbon, C2 is bonded to a second hydrogen. C2 uses three sp2 hybrid orbitals to form three σ bonds and an additional 2p orbital to combine with a 2p orbital on the oxygen to formaπbond to make the double bond. As shown in the top line of the chemical reaction equations and looking at the structure of ethanol in Figure 15.1, ethanol goes to acetaldehyde by eliminating two hydrogen atoms to give acetaldehyde and an H2 molecule. Two molecules of acetaldehyde each pick up one oxygen atom from O2 to yield two acetic acid molecules (bottom of Figure 15.4). Acetic acid has C2 bonded to two oxygen atoms. C2 is double bonded to one and single bonded to the oxygen of the hydroxyl group—OH.

FIGURE 15.4. Acetaldehyde (top) and acetic acid (bottom). Oxygens are the almost black spheres. Acetaldehyde C2carbon is bonded to C1, a hydrogen, and double bonded to an oxygen. Acetic acid C2is bonded to C1, double bonded to an oxygen and single bonded to another oxygen