Bronsted bases definition

It is evident from equation (1) that the acidic and basi properties are independent of the electric charge of the molecul of the acid or base. Whereas formerly only electrically neutra molecules could be regarded as acids and bases, Bronsted definition admits that molecules which bear an electric charg may also behave as acids and bases. 

In the first instance, acidity influences the acid-base equilibria of the reactants. The amine is a Bronsted base. Aniline, a typical substrate, has pKa = 4.6, which means that the protonation shown in Scheme 3-11 is almost complete under normal conditions of diazotization (pH < 1). The base is definitely a much better reagent than the anilinium ion for nitrosation because the latter is an electrophilic substitution. One expects — simply on the basis of the equilibrium shown in Scheme 3-11 — that the rate of diazotization should decrease linearly with increasing acid concentration or, at higher acidities, with the Hammett acidity function h0 (for acidity functions see Rochester, 1970 Cox and Yates, 1983). 

The products of proton transfer in aqueous solution may also react with water. For example, the CIST ion produced when HCN loses a proton to water can accept a proton from a water molecule and form HCN again. Therefore, according to the Bronsted definition, CN is a base it is called the conjugate base of the acid HCN. In general, a conjugate base of an acid is the species left when the acid donates a proton ... 

The Bronsted definitions of acids and bases are more general than the Arrhenius definitions they also apply to species in nonaqueous solvents and even to gas-phase reactions. For example, when pure acetic acid is added to liquid ammonia, proton transfer takes place and the following equilibrium is reached ... 

The Lewis definition of a base is broader than the Bronsted definition. That is, although every Bronsted base is a Lewis base, not every Lewis base is a Bronsted base. For instance, carbon monoxide is an important Lewis base in its reactions with metals, but it is not a Bronsted base because it does not accept protons. 

An important implication of the Bronsted definitions of acids and bases is that the same substance may be able to function as both an acid and a base. For example, we have seen that a water molecule accepts a proton from an acid molecule (such as HC1 or HCN) to form an H30+ ion. So water is a base. I lowever, a water molecule can donate a proton to a base (such as O2- or NH3) and become an OH ion. So water is also an acid. We describe water as amphiprotic, meaning that an H20 molecule can act both as a proton donor and as a proton acceptor. 

Figure 7.4 is the graphical expression of the Henderson-Hassel-BALCH equation. The graph may he used for estimation of buffer composition, if the pK of the buffering substance is known. For example, the base A of an acetate buffer is the anion CHsCOO", the acid HA is CH3 COOH, and in a Tris buffer, TrisH" is the acid HA and unprotonated Tris is the base A (according to the Bronsted definition, the acid is that substance which has bound the hydrogen cation) ... 

In the Bronsted definition, an acid donates a proton and a base accepts a proton. The strengths of acids and bases are measured by the extent to which they lose or gain protons, respectively. In these reactions acids are converted to their copjugate bases and bases to their conjugate acids. Acid-base reactions go in the direction of forming the weaker acid and the weaker base. 

A Lewis base transfers an electron pair to a Lewis acid. A Bronsted acid transfers a proton to a Bnansted base. These exist in conjugate pairs at equilibrium. In an Arrhenius base, the proton acceptor (electron pair donor) is OH-. All Arrhenius acids/bases are Bronsted acids/bases and all Bransted acids/bases are Lewis acids/bases. Each definition contains a subset of the one that comes after it. 

This is an amphoteric compound because it behaves as both an acid and a base in the Bronsted definition. As an acid, it can donate an H+ and becomes the conjugate base ... 

Many substances in solution have the capability of releasing or absorbing protons. According to the classic Bronsted definition, acids are those substances that release protons and bases (alkalis) are those that accept protons. Thus, acetic acid is an acid, but acetate is a base. Ammonium ion (NH4+) is an acid, whereas ammonia (NH3) is a base. When an acid loses its proton, its conjugate base is formed, whereas when a base gains protons, its conjugate acid is generated. 

Before continuing on to the last definition of acids and bases, it will be helpful to consider the definitions for strong and weak acids within the context of the Bronsted-Lowry model of acids and bases. The definitions are really an extension of the Arrhenius ideas. In the Arrhenius definitions, strong acids and bases were those that ionize completely. Most Bronsted-Lowry acids and bases do not completely ionize in solution, so the strengths are determined based on the degree of ionization in solution. For example, acetic acid, found in vinegar, is a weak acid that is only about 1 percent ionized in solution. That means that when acetic acid, HC2H302, is placed in water, the reaction looks like ... 

Acids and bases were defined and described by early chemists, including Boyle, Lavoisier, Davy, Berzelius, Liebig, and Arrhenius. At the present time, depending on objectives, one of two definitions of acids and bases is likely to be accepted. These two definitions, by Bronsted and Lowry and by Lewis, were proposed about the same time. According to the Bronsted definition acids are substances having a tendency to lose a proton, and bases are those having a tendency to accept a proton. Thus, for an acid HA the acid-base half-reaction is... 

Where Ka is the acid dissociation constant. (Note that the definition of Ka is based on the Bronsted definition.) Values of Ka can vary tremendously (IQis to 10-60). after all anything with at least one proton can be considered an acid under some circumstances with this definition. The common definition of a strong acid is an acid which dissociates completely in a 1 M solution. The common strong acids in aqueous solution, such as sulphuric, nitric and hydrochloric acids have Ka values (for the first dissociation in the case of sulfuric) of 10 to 10. Thus they all dissociate completely (first dissociation only for sulphuric) in aqueous solution, though they wiU have different strengths in some other solvents. Most common organic acids are weak in aqueous solution, having Ka values of lO-s to Note that... 

Other definitions of acids and bases are usefui, the most notabie being those of Lewis, aiso proposed in 1923. However, for this chapter, the Bronsted definition is entireiy adequate. 

Scheme 3.1 is not a realistic expression, however, since protons are too reactive to exist independently and are rapidly taken up by the solvent. The proton-accepting entity, hy the Lowry-Bronsted definition, is a base, and the product formed when the proton has heen accepted by the solvent is an acid. Thus a second acid-base equilibrium occurs when the solvent accepts the proton, and this may be represented by... 

THE BRONSTED definition OF ACIDITY AND BASICITY. PROPERTIES OF ACIDS AND BASES... 

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). 

According to the Bronsted definition an acid is a proton donor, and a base is a proton acceptor. It follows that acid-base reactions can also be called proton transfer reactions. Thus, the reaction between acid HA and base B is written as... 

Base - Historically, a substance that yields an OH ion when it dissociates in solution, resulting in a pH>7. In the Bronsted definition, a base is a substance capable of accepting a proton in any type of reaction. The more general definition, due to G.N. Lewis, classifies any chemical species capable of donating an electron pair as a base. 

Acidity in the Bronsted definition involves donation of H to a base, water in this case. The reaction of acetic acid with water is shown as an equilibrium reaction. The equilibrium is only partially to the right-hand side because carboxylic acids are weak acids. 

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