Lewis Acid-Base Interaction in Ph₃P-BH₃

Source: Tamara M. Powers, Department of Chemistry, Texas A&M University 

One of the goals of chemistry is to use models that account for trends and provide insights into the properties of reactants that contribute to reactivity. Substances have been classified as acids and bases since the time of the ancient Greeks, but the definition of acids and bases has been modified and expanded over the years.¹

The ancient Greeks would characterize substances by taste, and defined acids as those that were sour-tasting, such as lemon juice and vinegar. The term "acid" is derived from the Latin term for "sour-tasting." Bases were characterized by their ability to counteract or neutralize acids. The first bases characterized were those of ashes from a fire, which were mixed with fats to make soap. In fact, the term "alkaline" is derived from the Arabic word for "roasting." Indeed, it has been known since ancient times that acids and bases can be combined to give a salt and water.

The first widely-used description of an acid is that of the Swedish chemist, Svante Arrhenius, who in 1894 defined acids as substances which dissociate in water to give hydronium ions, and bases as substances which dissociate in water to give hydroxide ions. This definition is thus limited to aqueous acids and necessitates that an acid contribute a proton.² For example, in water, HCl is an acid, as it dissociates to give the hydronium ion (H₃O)⁺ and the chloride ion. Boron trichloride would not be considered an acid, as in water it hydrolyzes to give B(OH)₃ and 3 HCl; the product HCl though is an Arrhenius acid.

In 1923, Johannes Nicolaus Brønsted and Martin Lowry independently defined acids and bases on their ability to donate and accept hydrogen ions, or protons. Thus came the concept of acid-base conjugate pairs, and the expansion of the definition of acids and bases in solvents other than water. For example, ammonium is an acid, as it can donate a proton and generate ammonia. Ammonia can accept a proton, to give ammonium. Thus, ammonia is the conjugate base of ammonium. This acid-base reaction can occur in water, ammonia, or other solvents.

This video deals with the acid-base definition of the American chemist, Gilbert N. Lewis, who also defined acids and bases in 1923. Indeed, this is the same Lewis from Lewis-dot structures in General Chemistry. His approach focuses not on the ability of acids and bases to donate and accept protons, but rather on their ability to accept and donate electron pairs, respectively. This encompasses the Brønsted-Lowry definition, as H⁺ accepts an electron pair from a Brønsted base during protonation. However, it greatly expands the definition of an acid, now encompassing metal ions and main-group compounds. Here, we compare the ³¹P NMR of the Lewis acid-base adduct Ph₃P-BH₃ to free triphenylphosphine.