Acid-Base Measurements in Solution

1 ACIDITY AND BASICITY OF ORGANIC COMPOUNDS Acid-Base Measurements in Solution 

Acidity and basicity are fundamental properties of organic compounds, and acid-base reactions are essential steps in many organic transformations. Although there are several definitions of acidity and basicity, the Bransted theory and the Lewis theory are used most often in organic chemistryIn Lewis theory, an acid is an electron pair acceptor and a base is an electron pair donor, as in the reaction of a trialkylamine as Lewis base with boron trifluoride as Lewis acid (equation 7.1). 

In Bronsted theory, an acid is a proton donor and a base is a proton acceptor, as in the reaction of an amine with HCl in equation 7.2. There is a conceptual error in the latter definition, however. As Hawkes noted, it makes no more sense to speak of HCl donating a proton than of donating your purse to a mugger. It is more appropriate to speak of a Bronsted acid as 

Perspectives on Structure and Mechanism in Organic Chemistry, Second Edition By Felix A. Carroll Copyright 2010 John Wiley Sons, Inc. 

More precisely, a Bronsted acid is a substance from which a hydron can be removed and a Bronsted base is a substance that can remove a hydron from an acid. Hydron is a genered term for H+, irrespective of isotope, and it includes the proton ( H ), deuteron ), and triton ). This distinction is useful in discussions of isotope effects, but it is seldom made otherwise. The term proton has long been used to represent H in general as well as to represent the specific isotope H +. Because of this familiar usage, a mechanistic discussion that uses the terms protonation and deprotonation may be clearer than an analysis that uses the terms hydronation and dehydronation. The discussion here will therefore retain the term proton as a general term for H. Use of the term proton to mean the specific isotope H+ can be rmderstood in context. 

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