BASES

Bases are proton acceptors. We have alread y seen two, OH^- and H₂O.   When we compare the strength of these two, it should be clear that OH^- is a stronger base since with a negative charge it would be more likely to "accept" a positive proton.

Consider an aqueuous solution of Na₂O. Such a solution, which consists of sodium and oxide ions, would react with water as shown below:

O^2- + H₂O --> OH^- + OH^-

Oxide is a stronger base than OH^- since it has two negative charges which should attract a proton more strongly than even OH^-. Therefore we can rank these bases from stronger to weaker bases as follows:

O^2- > OH^- > H₂O.

Notice also that these bases can all react with water to form OH^- in solution.

Bases can be strong (like O^2- & OH^- ) - analogous to strong acids (nitric, sulfuric, hydrochloric) or weak (like H₂O and ammonia - NH₃).

NH₃ is a common weak base as shown below. Ammonia accepts a proton from water to form another base, OH^-, which is clearly a stronger base than ammonia. Hence, from a stability point of view, the reverse reaction is favored (going from stronger acid and base to weaker acid and base).:

NH₃ + H₂O --> NH₄⁺ + OH^-

Base strength can be detemined by writing relevant acid/base reaction, and knowing which acids are strong or weak. For example, rank the following from least basic to most basic:

Cl^-, NO₃^-, H₂O, and CH₃COO^.

These bases are very different in structure. To rank them, write the reactions of acid which produce these bases when they react in water and determine the relative strength of the bases in each reactions (s = strong, w = weak) :

HCl(sa) + H₂O(sb) <==> H₃O⁺(wa) + Cl^-(wb)

HNO₃(sa) + H₂O(sb) <==> H₃O⁺(wa) + NO₃^-(wa)

CH₃COOH(wa) + H₂O(wb) <==> H₃O⁺(sa) + CH₃COO ^-(sa)

From the list, clearly, in terms of base strength:

Cl^- and NO₃^- < (less basic) < H₂O < CH₃COO ^-

For bases that are similar in structure, the chemical reactivity can be directly compared. Using the following reactions to rank the following from least basic to most basic: F-, Cl-, Br-, I-.

HF + H₂O --> H₃O⁺ + F^-

HCl + H₂O --> H₃O+ + Cl^-

HBr + H₂O --> H₃O⁺ + Br^-

Hl + H₂O --> H₃O⁺ + I^-

We studied this example earlier, where we ranked HI the strongest acid and HF the weakest in the series (due to dilution of the negative charge on the product). Therefore, I^- is most stable, and least basic, while F^- is least stable and most basic.

Strong bases interact completely with strong acids like HCl to form a salt and water in a neutralization reaction.

Ex: HCl + NaOH ----------> H₂O + NaCl

Other strong bases are soluble oxides, like Na₂O and soluble hydroxide salts. Strong bases are strong electrolytes.  Weak bases interact incompletely in water (ionize) to form OH^- and a positive ion.

Ex. NH₃ + H₂O ----------> NH₄⁺ + OH^-

 Weak bases interact completely with a strong acid like HCl to form a salt.

Ex: NH₃ + HCl ----------> NH₄Cl

Weak bases are weak electrolytes.   Bases have one or more pairs of nonbonded electrons which they use to form a bond to a proton (i.e. to accept a proton).  Note that ammonia is our prototypical example of a weak base (analogous to acetic acid as a weak acid).   The amine functional group is simliar in structure to ammonia, with one, two, or three Hs on ammonia replaced with an R group. Amines, unless they have four R- groups attached (a quaternary amine) have a lone pair of electrons, which just like the lone pair in ammonia, can accept a proton.  Hence amines, like ammonia, are weak bases.  Carboxylic acids biochemistry's weak acids.