Electron-Pair Donation and the Lewis Acid-Base Definition

8 ELECTRON-PAIR DONATION AND THE LEWIS ACID-BASE DEFINITION 

The final acid-base concept we consider was developed by Gilbert N. Lewis, whose contribution to understanding the importance of valence electron pairs in molecular bonding we discussed in Chapter 9. Whereas the Brpnsted-Lowry concept focuses on the proton in defining a species as an acid or a base, the Lewis concept highlights the role of the electron pair. The Lewis acid-base definition holds that 

The product of any Lewis acid-base reaction is called an adduct, a single species that contains a new covalent bond  

the Lewis concept radically broadens the idea of acid-base reactions. What to Arrhenius was the formation of H2O from H and OH became, to Brpnsted and Lowry, the transfer of a proton from a stronger acid to a stronger base to form a weaker base and weaker acid. To Lewis, the same process became the donation and acceptance of an electron pair to form a covalent bond in an adduct. 

As we ve seen, the key feature of a Lewis base is a lone pair of electrons to donate. The key feature of a Lewis acid is a vacant orbital (or the ability to rearrange its bonds to form one) to accept that lone pair and form a new bond. In the upcoming discussion, you ll encounter a variety of neutral molecules and positively charged ions that satisfy this requirement. 

all Br0nsted-Lowry acids donate H, a Lewis acid. 

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Electron-Pair Donation and the Lewis Acid-Base Definition... 

A Lewis acid is any species capable of accepting a pair of electrons, and a Lewis base is a species with a pair of electrons available for donation. The terms acceptor and donor are also commonly used. Lewis acids include H+ and metal cations, molecules such as BF3 with incomplete octets, and ones such as SiF4 where octet expansion is possible (see Topic Cl). Any species with nonbonding electrons is potentially a Lewis base, including molecules such as NH3 and anions such as F. The Lewis acid-base definition should not be confused with the Bronsted one (see Topic E2) Bronsted bases are also Lewis bases, and H+ is a Lewis acid, but Bronsted acids such as HC1 are not Lewis acids. 

The Lewis acid-base definition focuses on the donation or acceptance of an electron pair to form a new covalent bond in an adduct, the product of an acid-base reaction. Lewis bases donate the electron pair, and Lewis acids accept it. Thus, many species that do not contain El are Lewis acids. Molecules with polar double bonds act as Lewis acids, as do those with electron-deficient atoms. Metal ions act as Lewis acids when they dissolve in water, which acts as a Lewis base, to form an adduct, a hydrated cation. Many metal ions function as Lewis acids in biomolecules. 

Perhaps the greatest area in which the Lewis acid-base approach is most useful is that of coordination chemistry. In the formation of coordination compounds, Lewis acids such as Cr3+, Co3+, Pt2+, or Ag+ bind to a certain number (usually 2, 4, or 6) of groups as a result of electron pair donation and acceptance. Typical electron pair donors include H20, NH3, F , CN , and many other molecules and ions. The products, known as coordination compounds or coordination complexes, have definite structures that are predictable in terms of principles of bonding. Because of the importance of this area of inorganic chemistry, Chapters 16 through 22 in this book are devoted to coordination chemistry. 

In 1923 G. N. Lewis" proposed a definition of acid-base behavior in terms of electron-pair donation and acceptance. The Lewis definition is perhaps the most widely used of all because of its simplicity and wide applicability, espeaally in the field of organic reactions. Lewis defined a base as cn electron-pair donor and an acid as an electron-pair acceptor. In addition to all of llie reactions discussed above, the Lewis definition includes reactions in which no ions are formed and no hydrogen ions or other ions are transferred 2... 

Every base that we have discussed thus far—whether OH , H2O, an amine, or an anion—is an electron-pair donor. Everything that is a base in the Bronsted-Lowry sense (a proton acceptor) is also a base in the Lewis sense (an electron-pair donor). In the Lewis theory, however, a base can donate its electron pair to something other than H. The Lewis definition therefore greatly increases the number of species that can be considered acids in other words, is a Lewis acid but not the only one. For example, the reaction between NH3 and BF3 occurs because BF3 has a vacant orbital in its valence shell, ooo (Section 8.7) It therefore acts as an electron-pair acceptor (a Lewis acid) toward NH3, which donates the electron pair ... 

While the Bronsted acid/base terms specifically refer to proton donors and acceptors, respectively, the Lewis approach (named after G. N. Lewis, who introduced the idea in 1923) greatly broadens the definitions of what is an acid and what is a base. Recall that a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor. All common organic reactions that do not involve radicals or concerted pericyclic processes can in some manner be discussed as Lewis acid-base reactions. Similarly, all these reactions can be considered to be occurring between electrophiles and nucleophiles. Recall that an electrophile is any species seeking electrons and a nucleophile is any species seeking a nucleus (or positive charge) toward which it can donate its electrons. In this context, a Lewis base is synonymous with a nucleophile, and a Lewis acid is synonymous with an electrophile it just de-... 

The Lewis definition implies the presence of high electron density centres in Lewis bases, and low electron density centres in Lewis acids. In a reaction between a Lewis acid and a Lewis base the electron pair donated by the base is used to form a new sigma bond to the electron-deficient centre in the acid. The identification of Lewis bases follows basically the same guidelines as the identification of Br0nsted-Lowry bases. They frequently contain atoms that have non-bonding electrons, or lone pairs. In contrast Lewis acids frequently contain atoms with an incomplete octet, a full positive charge, or a partial positive charge. 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

The hydrogen ion accepts the lone pair of electrons from the ammonia to form the ammonium ion. The hydrogen ion, because it accepts a pair of electrons, is the Lewis acid. The ammonia, because it donates a pair of electrons, is the Lewis base. This reaction is also a Brpnsted-Lowry acid-base reaction. This illustrates that a substance may be an acid or a base by more than one definition. All Brpnsted-Lowry acids are Lewis acids, and all Brpnsted-Lowry bases are Lewis bases. However, the reverse is not necessarily true. 

Thus, by definition, electrophiles are electron-pair acceptors and nucleophiles are electron-pair donors. These definitions correspond closely to definitions used in the generalized theory of acids and bases proposed by G. N. Lewis (1923). According to Lewis, an acid is any substance that can accept an electron pair, and a base is any substance that can donate an electron pair to form a covalent bond. Therefore acids must be electrophiles and bases must be nucleophiles. For example, the methyl cation may be regarded as a Lewis acid, or an electrophile, because it accepts electrons from reagents such as chloride ion or methanol. In turn, because chloride ion and methanol donate electrons to the methyl cation they are classified as Lewis bases, or nucleophiles ... 

The Lewis definition of acids and bases is broader and more encompassing than the Bronsted-Lowry definition because it s not limited to substances that donate or accept just protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an electron pair. The donated electron pair is shared between tiie acid and the base in a covalent bond. 

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