Base, Bronsted—Lowry Lewis

Backbone (protein), 1028 Backside displacement. reaction and.363-364 von Baeyer, Adolf, 113 Baeyer strain theory, 113-114 Bakelile, structure of, 1218 Banana, esters in, 808 Barton, Derek, H. R., 389 Basal metabolic rate, 1169 Basal metabolism. 1169-1170 Base, Bronsted-Lowry, 49 Lewis, 57, 59-60 organic, 56-57 strengths of, 50-52 Base pair (DNA), 1103-1105 electrostatic potential maps of. 

Skill 10.1 Analyzing acids and bases according to acid-base theories (i.e., Arrhenius, Bronsted-Lowry, Lewis)... 

Bronsted-Lowry acids and bases donate or accept protons. A Lewis acid or base accepts or donates a pair of electrons. Key Terms Bronsted-Lowry acid Bronsted-Lowry base Bronsted-Lowry acid-base reaction 1 lie polyprotic acid diprotic acid triprotic acid Ul IvO Lewis base Lewis acid-base reaction... 

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 the acid and the base in a covalent bond. 

The Lewis definition of a base as a compound with a pair of nonbonding electrons that it can use to bond to a Lewis acid is similar to the Bronsted-Lowry definition. Thus, H20, with its two pairs of nonbonding electrons on oxygen, acts as a Lewis base by donating an electron pair to an H+ in forming the hydronium ion, H30+. 

The Bronsted-Lowry theory focuses on the transfer of a proton from one species to another. However, the concepts of acids and bases have a much wider significance than the transfer of protons. Even more substances can be classified as acids or bases under the definitions developed by G. N. Lewis ... 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

The various acid-base definitions are summarized in the Venn diagram (Fig. 2.1). From this it can be seen that the Usanovich definition subsumes the Lewis definition, which in turn subsumes all other definitions (i.e. Arrhenius, Bronsted-Lowry, Germann-Cady-Elsey, Lux-Flood). 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

KolthoflF, I. M. (1944). The Lewis and Bronsted-Lowry definitions of acids and bases. Journal of Physical Chemistry, 48, 51-7. 

C) The Bronsted-Lowry or proton theory interprets the acid-base reaction as a mere proton exchange between the acid (proton donor) and the base (proton acceptor) however, the Lewis theory or electron theory interprets the reaction as a donation and acceptance of a lone pair of electrons, where the... 

Acid-base behavior according to the Lewis theory has many of the same aspects as does acid-base theory according to the Bronsted-Lowry theory. 

At the microscopic level, acids are defined as proton (H ) donors (Bronsted-Lowry theory) or electron-pair acceptors (Lewis theory). Bases are defined as proton (H+) acceptors (Bronsted-Lowry theory) or electron-pair donors (Lewis theory). Consider the gas-phase reaction between hydrogen chloride and ammonia ... 

Amines cire both Bronsted-Lowry bases (they accept hydrogen ions from acids) and Lewis bases (they furnish an electron pair to Lewis acids). As Bronsted-Lowry bases they have values. Aliphatic amines have values of approximately 1(H, and aromatic amines have values near 10 °. (These values compare to a value of =10" for ammonia.)... 

Lewis defined a base as an electron pair donor and an acid as an electron pair acceptor. Lewis electron pair donor was the same as Bronsted-Lowry s proton acceptor, and therefore, was an equivalent way of defining a base. Lewis acids were defined as a substance with an empty valence shell that could accommodate a pair of electrons. This definition broadened the Bronsted-Lowry definition of an acid. The three definitions of acids and bases are summarized in Table 13.3. 

Each of the three definitions expands our concept of acids and bases. Arrhenius basic definition is adequate for understanding many of the properties of acids and bases. It is important to recognize, though, that acids and bases are not fixed labels that can be applied to a substance. Bronsted-Lowry and Lewis showed that acid-base characteristics are dependent on the reactions that take place between substances. A... 

In the same year that Bronsted and Lowry proposed their definition of acids and bases, an American chemist named Gilbert Lewis proposed an alternative definition that not only encompassed Bronsted-Lowry theory but also accounted for acid-base reactions in which a hydrogen ion isn t exchanged. Lewis s definition relies on tracking lone pairs of electrons. Under his theory, a base is any substance that donates a pair of electrons to form a coordinate covalent bond with another substance, while an acid is a substance that accepts that electron pair in such a reaction. As we explain in Chapter 5, a coordinate covalent bond is a covalent bond in which both of the bonding electrons are donated by one of the atoms forming the bond. 

All Br0nsted-Lowry acids are Lewis acids, but in practice, the term Lewis acid is generally reserved for Lewis acids that don t also fit the Bronsted-Lowry definition. The best way to spot a Lewis acid-base pair is to draw a Lewis dot structure of the reacting substances, noting the presence of lone pairs of electrons. (We introduce Lewis structures in Chapter 5.) For example, consider the reaction between ammonia (NH3) and boron trifluoride (BFj) ... 

In practice, it s much simpler to use the Arrhenius or Bronsted-Lowry definition of acid and base, but you ll need to use the Lewis definition when hydrogen ions aren t being exchanged. You can pick and choose among the definitions when you re asked to identify the acid and base in a reaction. 

Which definition of acids and bases is more universal the Bronsted-Lowry definition or the Lewis definition ... 

Since all proton acceptors have an unshared pair of electrons, and since all electron-pair donors can accept a proton, the Lewis and the Bronsted-Lowry definitions of a base are simply different ways of looking at the same property. All Lewis bases are Bronsted-Lowry bases, and all Bronsted-Lowry bases are Lewis bases. The Lewis definition of an acid, however, is considerably more general than the Bronsted-Lowry definition. Lewis acids include not only H+ but also other cations and neutral molecules having vacant valence orbitals that can accept a share in a pair of electrons donated by a Lewis base. 

For most aqueous acid-base chemistry, the Lewis definitions are too general and lack the symmetry of the acid-conjugate base relationship. We will mostly use the Bronsted-Lowry definitions. 

Bronsted-Lowry and Lewis definitions of acids and bases are reviewed in Sec. 7.6. Alcohols are comparable in acidity to water, but phenols are much more acidic. This increased acidity is due to charge delocalization (resonance) in phenoxide ions. Electron-withdrawing groups, such as -F and -N02, increase acidity, through either an inductive or a resonance effect, or both. 

The Lewis theory of acids and bases is more general than Bronsted-Lowry theory, but Bronsted-Lowry s definition is used more frequently. The terms "acid" and "base" most often refer to Bronsted acids and bases, and the term "Lewis acid" is usually reserved for chemicals like BF3 that are not Bronsted acids. 

According to the Lewis definition, an acid is an electron pair acceptor and a base is an electron pair donor. All Bronsted-Lowry bases are also Lewis bases. However, Lewis acids include many species that are not proton acids instead of H+, they have some other electron-deficient species that acts as the electron pair acceptor. An example of a Lewis acid-base reaction is provided by the following equation. In this reaction the boron of BF3 is electron/deficient (it has only six electrons in its valence shell). The oxygen of the ether is a Lewis base and uses a pair of electrons to form a bond to the boron, thus completing boron s octet. 

To avoid confusion, when the term acid or base is used in this text, it refers to a proton acid or base—that is, a Bronsted-Lowry acid or base. The term Lewis acid or Lewis base will be used when the discussion specifically concerns this type of acid or base. 

Acid-base reactions concepts of Arrhenius, Bronsted-Lowry, and Lewis coordination complexes amphoterism... 

The Lewis concept deals with the behavior of electron pairs in chemical reactions. The same electron pairs we looked at when we discussed molecular geometry (see Chapter 7) can be involved in many reactions. Substances that can form a covalent bond by accepting an electron pair from another substance are known as Lewis acids. Substances that can form a covalent bond by donating an electron pair to another substance are known as Lewis bases. Be careful that you don t mix these up with the Bronsted-Lowry acids and bases. It is easy to do since the words donate and accept are used, except they are associated with the opposite species (Bronsted-Lowry acids donate protons, while Lewis acids accept electron pairs). 

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