Brpnsted-Lowry acid-base reactions

Analyzing acid-base reactions according to the Brpnsted-Lowry picture provides yet another benefit Table 1 7 which lists acids according to their strength m descending... 

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. 

Section 1.17 The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Brpnsted-Lowry picture. A Lewis acid is an electron-pair acceptor. A Lewis base is an electron-pair donor. The Lewis approach incorporates the Brpnsted-Lowry approach as a subcategory in which the atom that accepts the electron pair in the Lewis acid is a proton. 

Some of the terms associated with acids and bases have evolved specific meanings in organic chemistry. When organic chemists use the term base, they usually mean a proton acceptor (a Brpnsted-Lowry base). Similarly, the term acid usually means a proton donor (a Brpnsted-Lowry acid). When the acid-base reaction involves formation of a bond to some other element (especially carbon), organic chemists refer to the electron donor as a nucleophile (Lewis base) and the electron acceptor as an electrophile (Lewis acid). 

In this case, no proton has been transferred, so this is not a Brpnsted-Lowry acid-base reaction. Instead, a bond has formed between the C = 0 carbon atom and the oxygen of the CH3—O group. Drawing the Lewis structures helps to show that the CH3—O group (the nucleophile in this reaction) donates the electrons to form the new bond to acetaldehyde (the electrophile). This result agrees with our intuition that a negatively charged ion is likely to be electron-rich and therefore an electron donor. 

The Br0nsted-Lowry theory expands the definition of acids and bases to allow us to explain n ch more ol solution chemistry. For example, the Brpnsted-Lowry theory allows us to explain why a solution af ammonium nitrate tests acidic and a solution of potassium acetate tests basic. Most of the substances that we cofcider acids in the Arrhenius theory are also acids in the Brpnsted-Lowry theory, and the same is true of bases. Injboth theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the weak acid, the ions, and the w ater. A similar statement can be made about weak bases ... 

Acidity is measured by an equilibrium constant. When a Brpnsted-Lowry acid H- A is dissolved in water, an acid-base reaction occurs, and an equilibrium constant can be written for the reaction. 

Furthermore, when the substitution product bears a positive charge and also contains a proton bonded to O or N, the initial substitution product readily loses a proton in a Brpnsted-Lowry acid-base reaction, forming a neutral product. 

A hydroxide nucleophile is needed to synthesize an alcohol, and salts such as NaOH and KOH are inexpensive and commercially available. An alkoxide salt is needed to make an ether. Simple alkoxides such as sodium methoxide (NaOCH3) can be purchased, but others are prepared from alcohols by a Brpnsted-Lowry acid-base reaction. For example, sodium ethoxide (NaOCH2CH3) is prepared by treating ethanol with NaH. 

The cyclopentadienyl anion is readily formed from cyclopentadiene by a Brpnsted-Lowry acid-base reaction. 

Proton transfer reaction (Section 2.2) A Brpnsted-Lowry acid-base reaction a reaction that results in the transfer of a proton from an acid to a base. 

Gascons HCl molecnles interact with gaseous NH3 molecules to form a white smoke made up of sohd NH4CI particles. Explain whether or not this is an acid-base reaction according to both the Arrhenins model and the Brpnsted-Lowry model. (19.1)... 

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

In the early 20 century, the chemists Johannes Brpnsted and Thomas Lowry realized the proton-transfer nature of acid-base reactions. They defined an acid as a molecule (or ion) that donates a proton, and a base as a molecule (or ion) that accepts a proton. Therefore, in the aqueous reaction between strong acid and... 

From the Brpnsted-Lowry perspective, the only requirement for an acid-base reaction is that one species donates a proton and another species accepts it an acid-base reaction is a proton-transfer process. Acid-base reactions can occur between gases, in nonaqueous solutions, and in heterogeneous mixtures, as well as in aqueous solutions. 

Table 18.2 shows some Brpnsted-Lowry acid-base reactions. Notice that... 

Thus, 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. 

Earlier we saw that certain hydrated metal ions act as Brpnsted-Lowry acids. In the Lewis sense, the hydration process itself is an acid-base reaction. The hydrated cation is the adduct, as lone electron pairs on the O atoms of water form... 

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

What is the key structural feature of all Brpnsted-Lowry bases How does this feature function in an acid-base reaction ... 

From a kinetic standpoint, acid-base reactions are among the fastest reactions known, especially in aqueous solvents. They are also used to generate reactive intermediates that can be part of other reaction types. Acids are attacked by Lewis or Brpnsted-Lowry bases. Acid-base reactions are integral to many reactions, but they are not always easy to describe by a specific transform since they may be an adjunct to the desired... 

According to Brpnsted and Lowry, an acid-base reaction is defined in terms of a proton transfer. By this definition, the reaction of HCl in water is the following ... 

Although benzene has six 7i-electrons, it cannot donate electrons as well as a simple alkene with a single n-bond. Benzene is less reactive than a simple alkene. The reaction of an alkene with HCl is an acid-base reaction, where the 7i-bond of the alkene is the Brpnsted-Lowry base. If benzene does not react with HCl, benzene must be a weaker base than the alkene. Benzene is a weaker base (unable to donate electrons as efficiently) because the six 7t-electrons of benzene are delocalized on six carbons, whereas the two 7t-electrons of an alkene are only distributed between two carbons. In other words, benzene is resonance stabilized, making it less reactive. The more delocalized the electrons are, the lower is the net electron density for any point between the carbon atoms. If a reaction has to occur at one carbon in any reaction, then the net electron density of one carbon in benzene is less than that of one carbon in an alkene. Benzene is less reactive than alkenes because more energy is required to disrupt the 7t-system (an endothermic process see Chapter 7, Section 7.5) and the electron delocalization makes less electron density available for donation. 

An alkene has a C=C unit with a 7t-bond that is not polarized. The 7t-bond extends in space above and below the plane of the molecule, as in Figure 6.5. The 7t-bond is an electron-rich species, and those electrons can be donated to a positive center, initiating a chemical reaction. In other words, an alkene is a weak Br0nsted-Lowry base. The 7t-bond is a weak Brpnsted-Lowry base and reaction occurs only with a strong acid. This acid-base reaction is illustrated by ethene reacting with a proton H+ to give 55A. Donation of the electrons from the 7i-bond (blue arrow, see 56) to the acid (H of HCl) leads to formation of a new o-C-H bond (see 56), and a positive carbon C+ (a carbocation see Chapter 10, Section 10.2) on the other carbon atom of the C=C unit. Note that the positive carbon atom is represented as a p-orbital that has no electrons. Note also that the double-headed arrow indicates transfer of two electrons from C=C to W to form a new o-covalent bond. 

A reversible chemical reaction is one where the compounds normally defined as products react to regenerate the compounds normally defined as starting materials, and the two reactions are competitive. All of the Brpnsted-Lowry acid-base reactions discussed in previous chapters are equilibrium reactions, defined by an equilibrium constant K. They are acid-base reactions, so is used, but the principle is fundamentally the same. The fundamental principles of equilibria and reversible reactions can be applied to other systems. 

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