Base, Bronsted—Lowry organic

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Molecules. As pointed out in Chapter 4, there are many molecular weak bases, including the organic compounds known as amines. The simplest weak base is ammonia, whose reversible Bronsted-Lowry reaction with water is represented by the equation... 

The Bronsted-Lowry theory of acids and bases referred to in Section 10.7 can be applied equally well to reactions occurring during acid-base titrations in non-aqueous solvents. This is because their approach considers an acid as any substance which will tend to donate a proton, and a base as a substance which will accept a proton. Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end points when titrations are carried out in non-aqueous media. An additional advantage is that many substances which are insoluble in water are sufficiently soluble in organic solvents to permit their titration in these non-aqueous media. 

An example that refers to the third method additives can be employed is described below. Markedly enhanced enantioselectivity was reported for P. cepacia lipase and subtilisin Carlsberg with chiral substrates converted to salts by treatment with numerous Bronsted-Lowry adds or bases [63]. This effect was observed in various organic solvents but not in water, where the salts apparently dissociate to regenerate... 

A vast generalization beyond the Bronsted—Lowry acids and bases concepts is the concept of a Lewis29 base (an electron pair donor) and a Lewis acid (an electron pair acceptor). This concept has been used extensively in all branches of chemistry. In physical organic chemistry, quantities of the type pA = —logio[A] have used extensively to study reactivities—for example, in the Hammett equation. 

Hydroxide ("OH) can react as a Bronsted-Lowry base (and remove a proton), or a Lewis base (and attack a carbon atom), (a) What organic product is formed when OH reacts with the carbocation (CH3)3C as a Bronsted-Lowry base (b) What organic product is formed when OH reacts with (CH3)30 as a Lewis base ... 

Chapters 10 and 11 describe the special properties of liquid water. Because of its substantial dipole moment, water is especially effective as a solvent, stabilizing both polar and ionic solutes. Water is not only the solvent, but also participates in acid-base reactions as a reactant. Water plays an integral role in virtually all biochemical reactions essential to the survival of living organisms these reactions involve acids, bases, and ionic species. In view of the wide-ranging importance of these reactions, we devote the remainder of this chapter to acid-base behavior and related ionic reactions in aqueous solution. The Bronsted-Lowry definition of acids and bases is especially well suited to describe these reactions. 

A further important concept related to electronegativity and polarity is that of aciiiityand basicity. We ll see, in fact, thiit much of the chemistry of organic molecules can be explained by their acid-base behavior. You may recall from a course in general chemistry that there are two frequently used definitions of acidity the Br0usted-Lowry definitiou and the Lewis definition. We ll look at the Bronsted-Lowry definition in this and the next three sections and then discuss the Lewis definition in Section 2.11. 

Many mechanisms in organic chemistry start with an acid/base reaction. This may be just a simple Bronsted-Lowry protonation of a hydroxyl group, which results in the activation of a C-OH bond or it may be a Lewis acid/base reaction as, for example, when aluminium trichloride complexes with a halogenoalkane in the first step of the Friedel-Crafts reaction. In each case, the initial intermediate usually reacts further and leads to the desired product. In inorganic chemistry, the acid/base reaction may be all that is of interest, e.g. the treatment of a carbonate with an acid to liberate carbon dioxide. However, it is unusual in organic chemistry for the acid/base reaction to be an end in itself. It is for this reason that acid/base characteristics are normally considered as a property of the molecule, similar to the nucleophilic and electrophilic properties to which they are closely related, rather than as a fundamental reaction type as is the case in inorganic chemistry. 

If ammonia had been allowed to react with hydrogen chloride in the aqueous phase, then it would have fallen within the Arrhenius definition of an acid/base reaction. Notice that it is the water molecule that is central to the Arrhenius definition, and this is one of the main reasons why it is of little use in conventional organic chemistry. Similarly, because the Bronsted-Lowry definition does not depend on water, but only upon the presence of two compounds of complementary properties, it is this definition that is of more use to organic chemists. 

Many organic and biological reactions are acid-base reactions that do not lit within the Arrhenius or Bronsted-Lowry theories. Experienced chemists find the Lewis theory to be very usefol because so many other chemical reactions are covered by it. The less experienced sometimes find the theory less useful, but as their knowledge expands so does its utility. 

Many organic compounds can act as weak Bronsted-Lowry acids or bases. Their reactions involve the transfer of H+ ions, or protons (Section 10-4). Like similar reactions of inorganic compounds, these acid-base reactions of organic acids and bases are usually fast and reversible. Consequently, we can discuss the acidic or basic properties of organic compounds in terms of equilibrium constants (Section 18-4). 

Arrhenius concept Bronsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant Section 14.2 strong acid weak acid diprotic acid oxyacids organic acids carboxyl group monoprotic acids amphoteric substance autoionization... 

Two classes of acid-base reactions are fundamental in organic chemistry Bronsted—Lowry and Lewis acid-base reactions. We start our discussion with Bronsted—Lowry acid—base reactions. 

The Lewis theory, by virtue of its broader definition of acids, allows acid—base theory to include all of the Bronsted-Lowry reactions and, as we shall see, a great many others. Most of the reactions we shall study in organic chemistry involve Lewis acid—base interactions, and a sound understanding of Lewis acid—base chemistry will help greatly. 

Many organic reactions involve the transfer of a proton by an acid-base reaction. An important consideration, therefore, is the relative strengths of compounds that could potentially act as Bronsted-Lowry acids or bases in a reaction. 

You have reviewed the Bronsted-Lowry definition of acids and bases and the meanings of pH and pTQ. You have learned to identify the most acidic hydrogen atoms in a molecule based on a comparison of pIQ values. You will see in many cases that Brensted—Lowry acid-base reactions either initiate or complete an organic reaction, or prepare an organic molecule for further reaction. The Lewis definition of acids and bases may have been new to you. However, you will see over and over again that Lewis acid—base reactions which involve either the donation of an electron pair to form a new covalent bond or the departure of an electron pair to break a covalent bond are central steps in many organic reactions. The vast majority of organic reactions you will study are either Bronsted-Lowry or Lewis acid—base reactions. 

Apart from this modification, the Arrhenius definitions of acid and base are still valid and useful today, as long as we are talking about aqueous solutions. However, the Arrhenius concept of acids and bases is so intimately tied to reactions that take place in water that it has no good way to deal with acid-base reactions in nonaqueous solutions. For this reason, we concentrate in this chapter on the Bronsted-Lowry definitions of acids and bases, which are more useful to us in our discussion of reactions of organic compounds. 

Detailed work by Davis and Hetzer (1951-1958) on acid-base systems in nonaqueous liquids such as benzene constitutes an attempt to clarify the position relative to the Bronsted-Lowry and the Lewis concepts of acidity. Even in this work, however, liquids such as benzene were described as inert because they do not give rise to significant quantities of free hydrogen ions. It was asserted that in such solvents the product of the interaction of a base with a Bronsted acid and the product of the interaction of that base with a Lewis acid are essentially of the same type, a highly polar addition compound. For organic bases in liquids such as benzene, these authors urged the adoption of association rather than dissociation constants A+B C. They referred to spectroscopic evidence showing that the anion is not completely detached from the proton of the cation. In the system... 

Two acid-base definitions are used in organic chemistry today the Bronsted-Lowry definition [1] and the Lewis definition. 

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