Proton transfer Br0nsted-Lowry acid-base definition

PROTON TRANSFER AND THE BR0NSTED-LOWRY ACID-BASE DEFINITION... 

In 1923, Br0nsted and Lowry defined acids and bases on the basis of the transfer of protons. A Br0nsted-Lowry add is any species that can donate a proton, and a Br0nsted-Lowry base is any species that can accept a proton. These definitions also include all the Arrhenius acids and bases because compounds that dissociate to give H30 are proton donors, and compounds that dissociate to give OH are proton acceptors. (Hydroxide ion accepts a proton to form H2O.)... 

Although the concepts of specific acid and specific base catalysis were useful in the analysis of some early kinetic data, it soon became apparent that any species that could effect a proton transfer with the substrate could exert a catalytic influence on the reaction rate. Consequently, it became desirable to employ the more general Br0nsted-Lowry definition of acids and bases and to write the reaction rate constant as... 

According to the Br0nsted-Lowry definitions, any species that contains hydrogen can potentially act as an acid, and any compound that contains a lone pair of electrons can act as a base. Therefore, neutral molecules can also act as bases if they contain an oxygen, nitrogen or sulphur atom. Both an acid and a base must be present in a proton transfer reaction, because an acid cannot donate a proton unless a base is present to accept it. Thus, proton-transfer reactions are often called acid-base reactions. 

The Br0nsted-Lowry definition of acids and bases depends on the transfer of a proton from the acid to the base. The base uses a pair of nonbonding electrons to form a bond to the proton. G. N. Lewis reasoned that this kind of reaction does not need a proton. Instead, a base could use its lone pair of electrons to bond to some other electron-deficient atom. In effect, we can look at an acid-base reaction from the viewpoint of the bonds that are formed and broken rather than a proton that is transferred. The following reaction shows the proton transfer, with emphasis on the bonds being broken and formed. Organic chemists routinely use curved arrows to show the movement of the participating electrons. 

Understand the Br0nsted-Lowry definitions of an add and a base discuss how water can act as a base or as an acid and how an add-base reaction is a proton-transfer process involving two conjugate add-base pairs, with the stronger acid and base forming the weaker base and add ( 18.3) (SPs 18.4,18.5) (EPs 18.24-18.39)... 

The Br0nsted-Lowry definition states that an acid is a proton donor (Chapter 2, Section 2.1), but the proton does not fly off. In fact, the acidic proton in A H is pulled off by the base, and this reaction leads to cleavage of the covalent bond between A and H with transfer of those two electrons from the A-H bond to A. This reaction generates the electron-rich A (the conjugate base). Therefore, the acid-base reaction shown is simply a chemical reaction in which the electron-rich base donates two electrons to the electron-poor proton, forming a new covalent bond and breaking the covalent bond between A-H with transfer of those two electrons to A. 

We began our definitions of acids and bases with the Arrhenius model. We then saw how the Br0nsted-Lowry model, by introducing the concept of a proton donor and proton acceptor, expanded the range of substances that we consider acids and bases. We now introduce a third model, which further broadens the range of substances that we can consider acids. This third model is the Lewis model, named after G. N. Lewis, the American chemist who devised the electron-dot representation of chemical bonding (Section 9.1). While the Br0nsted-Lowry model focuses on the transfer of a proton, the Lewis model focuses on the transfer of an electron pair. Consider the simple acid-base reaction between the ion and NH3, shown here with Lewis structures ... 

The Arrhenius definition of an acid and a base attributed acidity to the presence of H" (aq), and alkalinity to OH (aq). Br0nsted-Lowry theory generalizes the acid-base concept by focusing on proton transfer, rather than on particular aqueous ions. Here, we discuss an attempt to generalize it further by focusing on the changes in electronic structure that occur when acid-base reactions take place, ideas introduced by G. N. Lewis. 

Br0nsted and Lowry proposed definitions of acids and bases in terms of their ability to transfer protons ... 

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