Solubility
To dissolve ionic compounds asolvent must also have a high dielectric constant,
thaf is, h b e high insulating properties to lower the attraction between oppositely
charged ions once they are solvated.
Water owes its shperiority as a solvent for ionic substances not only to its
polarity and its high dielectric constant but to another factor as well: it contains
the -OH group and thus can form hydrogen bonds. Water solvates both cations
and ixhions: cations, at its negative pole (its unshared electrons, essentially);
anions, through hydrogen bonding.
Now let us turn to the dissolution of non-ionic solutes.
The solubility characteristics of non-ionic compounds are determined chiefly
by their polarity. Non-polar or weakly polar compounds dissolve in non-polar or
weakly polar solvents; highly polar compounds dissolve in highly polar solvents.
"Like dissolves like" is an extremeiy useful rule of thumb. Methane dissolves in
carbon tetrachloride because the forces holding methane molecules to each other
and carbon tetrachloride molecules to each other-van der Waals interactionsare
replaced by very similar forces holding methane molecules to carbon tetrachloride
molecules.
Neither methane nor carbon tetrachloride is readily soluble in water. The
highly polar water molecules are held to each other by very strong dipole-dipole
interactions-hydrogen bonds; there could be only very weak attractive forces
between water molecules on the one hand and the non-polar methane or carbon
tetrachloride molecules on the other.
In contrast, the highly polar organic compound methanol, CH,OH, is quite .
soluble in water. Hydrogen bonds between water and methanol molecules can
readily replace the very similar hydrogen bonds between different methanol
molecules and different water molecules.
An understanding of the nature of solutions is fundamental to an understanding
of organic chemistry. Most organic reactions are camed out in solution and, different molecules together so that they can react with each other. The solvent is
iradFcd m the reactions that take place in it: just how much it is involved, and in
what ways, is only now being realized. In Chapter 7, when we know a little more
about organic reactions and how they take place, we shall return to this subjectwhich
we have barely touched upon here-and examine in detail the role played
by the solvent.
Acids and bases
Turning from physical to chemical properties, let us review briefly one familiar
topic that is fundamental to the understanding of organic chemistry: acidity and
basicity.
The terms acid and base have been defined in a number of ways, each definition
corresponding to a particular way of looking at the properties of acidity and
basicity. We shall find it useful to look at acids and bases from two of these
viewpoints; the one we select will depend upon the problem at hand.
According to the Lowry-Brensted definition, an acid is a substance t b t gives
up a proton, and a base is a substance that accepts a proton. When sulfuric acid
dissolves in water, the acid H2S04 gives up a proton (hydrogen nucleus) to the
base H20 to form the new acid H,O+ and the new base HS04-. When hydrogen
chloride reacts with ammonia, the acid HCl gives up a proton to the base NH, to
form the new acid NH4+ and the new base C1-According to the Lowry-Brmsted definition, the strength of an acid depends
upon its tendency to give up a proton, and the strength of a base depends upon its
tendency to accept a proton. Sulfuric acid and hydrogen chloride are strong acids
since they tend to give up a proton very readily; conversely, bisulfate ion,
HS04-, and chloride ion must necessarily be weak bases since they have little
tendency to hold on to protons. In each of the reactions just described, the
equilibrium favors the formation of the weaker acid and the weaker base.
If aqueous H2S04 is mixed with aqueous NaOH, the acid H,O+ (hydronium
ion) gives up a proton to the base OH- to form the new acid H20 and the new
base H20. When aqueous NH,Cl is mixed with aqueous NaOH, the acid NH, (ammonium ion) gives up a proton to the base OH- to form the new acid H20 and
the new base NH3. In each case the strong base, hydroxide ion, has accepted a
proton to form the weak acid H20. If we arrange these acids in the order shown,
we must necessarily arrange the corresponding (conjugate) bases in the oppositeOHLike
water, many organic compounds that contain oxygen can act as bases
and accept protons; ethyl alcohol and diethyl ether, for example, form the oxonium
ions I and 11. For convenience, we shall often refer to a structure like I as a
protonated alcohol and a structure like I1 as a protonated ether. According to the Lewis definition, a base is a substance that can furnish an
electron pair to form a covalent bond, and an acid is a substance that can take up an
electron pair to form a covalent bond. Thus an acid is an electron-pair acceptor and a
base is an electron-pair donor. This is the most fundamental of the acid-base
concepts, and the most general; it includes all the other concepts.
A proton is an acid because it is deficient in electrons, and needs an electron
pair to complete its valence shell. Hydroxide ion, ammonia, and water are bases
because they contain electron pairs available for sharing. In boron trifluoride, BF, ,
boron has only six electrons in its outer shell and hence tends to accept another
pair to complete its octet. Boron trifluoride is an acid and combines with such
bases as ammonia or diethyl ether.