Typical acid-base reactions involve a proton transfer mechanistic step. The arrows show how the hydrogen atom breaks off the acid and generates a conjugate base. Proton transfer reactions are very fast.
The main mechanistic step in all acid-base reactions are proton transfer steps. Proton transfer mechanistic steps involve two arrows. The first arrow has the tail of the curved arrow originating at the lone pair (or bond) of a Lewis base, and the head of the arrow terminates at the hydrogen atom of an acid. A second arrow originates at the bond between the hydrogen atom of the acid and terminates at the atom attached to the hydrogen atom. This arrow shows how the hydrogen atom breaks off the acid and generates a conjugate base. Proton transfer reactions are very fast.
Proton Transfer in Acid-Base Reaction
A proton transfer is the first step in many mechanisms. In a proton transfer step, a base removes the proton from an acid. The first curved arrow starts at the base and ends at the acidic hydrogen to show the base attacking and adding the hydrogen. A second curved arrow starts at the bond between the hydrogen and terminates at the atom attached to the hydrogen. This shows the breaking of the hydrogen-atom bond.
Not all bases in organic chemistry are the stereotypical Arrhenius base that releases hydroxide (OH–) ions when dissolved in water, such as NaOH and Ca(OH)2. If a molecule contains an atom with an unshared pair of electrons, it can also act as a base. Ammonia (NH3) is an organic compound that acts as a base and is referred to as an organic base. Because of the electron pair of the nitrogen, ammonia specifically acts as a Lewis base, which is an electron-pair donor in a Lewis acid-base reaction, by donating its unshared pair of electrons. Alcohols can also act as organic bases, where the oxygen acts as a Lewis base by donating its unshared pair of electrons.
Examples of Acid-Base Mechanisms
Acid-base reaction mechanisms involve a proton transfer step. Curved arrows are used to indicate the movement of electrons in the proton transfer step, which occurs via an attack of a base on an acid. Molecules, such as alcohols, can act as a Lewis acid, an electron pair acceptor, in the presence of a stronger base. Alcohols can also act as a Lewis base, an electron pair donor, in the presence of a stronger acid.
In the reaction of ammonia (NH3) and ethanol (C2H5OH), the reaction equilibrium shifts to the left because even though the ammonia acts as a base in this scenario, the conjugate acid (NH4+, pKa=9.2) generated is not weaker than the ethanol (pKa=16.2). This is a reaction where the starting compounds are the preferred species.
Many acid-base reactions must be set up in a nonaqueous solution. The primary reason for this is that very strong bases, such as sodium amide, will react with water, resulting in a solution of hydroxide ions and ammonia. This occurs because the hydroxide ion and ammonia are the weaker base and acid respectively and therefore are favored by the reaction equilibrium. A solvent with a very large pKa, such as hexane or liquid ammonia (very weak acid), is generally used as an alternative to water because it does not react with even very strong bases.
Acid-Base Reaction in a Nonaqueous Solvent
Acid-base reactions in nonaqueous solvents allow the use of a stronger base, such as sodium amide, which is needed to deprotonate acetylene.
