Brpnsted-Lowry proton transfer

Reviewing the three acid-base reactions presented, we find that all fit into the general equation for a Brpnsted-Lowry proton-transfer reaction ... 

In 1923, the Brpnsted-Lowiy proton transfer theory was introduced byj. N. Brpnsted (1897-1947), a Danish chemist, and T. M. Lowry (1847-1936), an English chemist. This theory states that an acid is a proton donor and a base is a proton acceptor. 

A more general theory of acids and bases was devised independently by Johannes Br0n sted (Denmark) and Thomas M Lowry (England) m 1923 In the Brpnsted-Lowry approach an acid is a proton donor, and a base is a proton acceptor The reaction that occurs between an acid and a base is proton transfer... 

Brpnsted-Lowry acids are proton donors, and Br0nsted-Lowry bases are proton acceptors. An acid increases the concentration of H,0+ in aqueous solution, and a base increases the concentration of OH. An acid-base pair related through the gain or loss of a single proton is described as conjugate. When a proton is transferred... 

In the Brpnsted-Lowry view, an acid doesn t dissociate in water it transfers a proton to water. Water acts as a base. 

In 1923, Brpnsted and Lowry defined acids and bases on the basis of the transfer of protons. A Brpnsted-Lowry acid is any species that can donate a proton, and a Brpnsted-Lowry base is any species that can accept a proton. These definitions also include all the Arrhenius acids and bases because compounds that dissociate to give H30+ are proton donors, and compounds that dissociate to give OH are proton acceptors. (Hydroxide ion accepts a proton to form H20.)... 

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. 

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. 

The reaction shows HCl acting as an Arrhenius acid, forming a hydro-nium ion. Because the reaction also involves a proton transfer, hydrochloric acid is also a Brpnsted-Lowry acid. All Arrhenius acids are, by definition, also Brpnsted-Lowry acids. 

While the Arrhenius definitions focus on the H and OH ions, the Brpnsted-Lowry definitions focus on the behavior of protons—that is, the transfer of a proton from one substance to another. 

A Br ousted-Lowry acid is a substance that transfers a proton to another substance. Since a hydrogen ion is a proton, all Arrhenius acids are Brpnsted-Lowry acids. However, the slight difference in definitions allows us to consider additional substances as acids. We can also consider reactions that do not occur in aqueous solutions. 

Of course, protons cannot exist as free particles in molecular solvents they should react with the solvent molecules, and such an interaction in aqueous solutions results in the formation of H5Oj (H+-2H20 or [H20--H--0H2]+) particles. Similarly, hydroxide ions in aqueous solutions exist as H302 (OH2- -OH-). The stronger the acid Ai compared to the acid A2, the more complete is the shift of reaction (1.1.6) to the right. The Brpnsted-Lowry definition thus extends the term acid-base interaction to reactions accompanied with proton transfer from one base to another. For example,... 

Usually, definitions of acids and bases are necessary for the classification of different kinds of chemical reactions, i.e. for dividing them into acid-base and other ones. For example, the Brpnsted-Lowry definition divides reactions into acid-base, which are characterized by redistribution of protons, and other ones. The wider Lewis definition makes the division of reactions into acid-base and redox ones, meaning that in the former case there is redistribution of electron density on account of electron pairs, and the latter case concerns reactions with the transfer of single electrons. Since chemistry concerns just the redistribution of electrons of external shells, we can classify all chemical reactions as acid-base ones by the Usanovitch definition. 

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. 

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. 

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)... 

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 ... 

Water is the base in this reaction. Acid and base reactions in general chemistry are always done in water. When a Brpnsted-Lowry acid such as HCl is placed in water, a proton is transferred to a water molecule (water is the base) a conjugate acid is formed (the hydronium ion HgO+) as well as a conjugate base (the chloride ion). In neutral pure water (no acid is present), the hydrogen ion concentration is about 1.0 x 10 M (pH of 7). An increase in the concentration of hydrogen ions above 1.0 x 10 M gives an acidic solution, with a pH less than 7. If the pH is greater than 7, it is considered a basic solution. 

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. 

Although the Arrhenius concept of acids and bases is useful, it is somewhat limited. For example, it tends to single out the OH ion as the source of base character, when other ions or molecules can play a similar role. In 1923, Johannes N. Brpnsted and Thomas M. Lowry independently noted that many reactions involve nothing more than the transfer of a proton (H ) between reactants, and they reaUzed that they could use this idea to expand the definitions of acids and bases to describe a large class of chemical reactions. In this view, acid—base reactions are proton-transfer reactions. 

In 1923 the Danish chemist Johannes N. Brpnsted (1879-1947) and, independently, the British chemist Thomas M. Lowry (1874-1936) pointed out that acid-base reactions can be seen as proton-transfer reactions and that acids and bases can be defined in terms of this proton (H ) transfer. According to the Br0nsted-Lowry concept, an acid is the species donating a proton in a proton-transfer reaction. A base is the species accepting the proton in a proton-transfer reaction. ... 

In this reaction in aqueous solution, a proton, H, is transferred from the H30 ion to the NH3 molecule, giving H2O and NH4 (Figure 16.3). Here H30 is the proton donor, or acid, and NH3 is the proton acceptor, or base. Note that in the Brpnsted— Lowry concept, acids (and bases) can be ions as well as molecular substances. 

Acid (Arrhenius definition) a substance that produces hydrogen ions, H, (hydronium ion, H30 ) when it dissolves in water. (4.4 and 16.1) (Brpnsted-Lowry definition) the species (molecule or ion) that donates a proton to another species in a proton-transfer reaction. (4.4 and 16.2)... 

Compare and contrast proton-transfer reactions with Brpnsted-Lowry acid-base reactions. 

We noted that reaction between ammonia and water reaches equilibrium. The fact is that most acid-base reactions reach equilibrium. Accordingly, the general Brpnsted-Lowry acid-base proton-transfer reaction can be written with a double arrow to show that it is reversible. Look carefully at the reverse reaction, for which the arrow and the labels for the reactants are printed in red ... 

Given the formulas of a potential Brpnsted-Lowry acid and a Br0nsted-Lowry base, write the equation for the possible proton-transfer reaction between them. 

The Strength of a Brpnsted-Lowry acid depends on the extent to which a proton can be separated from it and transferred to a base. Removing the proton involves breaking a bond to the proton, and it involves making the conjugate base more electrically negative. 

A second, more widely apphcable definition of acids and bases, called the Brpnsted-Lowry definition, was introduced in 1923. This definition focuses on the transfer ofH ions in an acid-base reaction. Since an H ion is a proton—a hydrogen atom without its electron—this definition focuses on the idea of a proton donor and a proton acceptor ... 

Unlike the situation for Brpnsted acidity, manifest by complete or partial proton transfer from acid to base, Lewis acidity is a more general concept. It encompasses the Brpnsted-Lowry definition and thus there is no universal scale (in contrast to the Hammett acidity... 

J. J. Brpnsted and T. M. Lowry (1923) explained these discrepancies using a new theory of acids and bases according to which acids are all compounds able to transfer a proton, and bases are compounds able to accept the proton. Thus, each acid is related to the base differing by one proton and providing a conjugated pair. 

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