Bronsted Acid-Base Chemistry

Preparation of the alkynide anion involves simple Bronsted-Lowry acid-base chemistry. 

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. 

The first person to recognize the essential nature of acids and bases was Svante Arrhenius. Based on his experiments with electrolytes, Arrhenius postulated that acids produce hydrogen ions in aqueous solution, and bases produce hydroxide ions. At the time of its discovery the Arrhenius concept of acids and bases was a major step forward in quantifying acid—base chemistry, but this concept is limited because it applies only to aqueous solutions and allows for only one kind of base—the hydroxide ion. A more general definition of acids and bases was suggested independently by the Danish chemist Johannes N. Bronsted (1879-1947) and the English chemist Thomas M. Lowry (1874-1936) in 1923. In terms of the Bronsted—Lowry definition, an acid is a proton (H+) donor, and a base is a proton acceptor. For example, when gaseous HCl dissolves in water, each HCl molecule donates a proton to a water molecule, and so HCl qualifies as a Bronsted-Lowry acid. The molecule that accepts the proton—water in this case—is a Bronsted-Lowry base. 

Like most polynuclear oxy complexes and the surfaces of oxide minerals, these dimers are amphoteric in water and can dissociate by pathways that involve either proton uptake at a hydroxyl bridge or proton release from a bound water molecule. Therefore the stability of a dimer relates closely to its Bronsted acid-base chemistry. The decomposition pathways for hydroxy-bridged dimers are well understood from studies of... 

For aqueous solutions, the Bronsted-Lowry theory adequately describes the behavior of acids and bases. We shall limit our discussion of acid-base chemistry to aqueous solutions and use the following definitions ... 

Although the Arrhenius concept of acids and bases was a major step forward in understanding acid-base chemistry, this concept is limited because it allows for only one kind of base—the hydroxide ion. A more general definition of acids and bases was suggested by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. 

In the case of organic inorganic materials interaction (e.g., polymer-metal oxide), Bolger and Michaels [33] suggested a model based on Bronsted acid-base chemistry to account for the strength of the interaction. They defined a parameter A for organic adds and bases ... 

Bronsted dealt only with acid-base chemistry in water. 

The acid-base chemistry of amino acids is more complicated than shown in Equations 16.48 and 16.49, however. Because the COOH group can act as an acid and the NH2 group can act as a base, amino acids undergo a self-contained Bronsted-Lowry acid-base reaction in which the proton of the carboxyl group is transferred to the basic nitrogen atom ... 

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. 

Preparation of the aikynide anion involves simple Bronsted-Lowry acid-base chemistry. As you have seen (Sections 7.9 and 7.11), the hydrogen of a terminal alkyne is weakly acidic = 25), and with a strong base such as sodium amide it can be removed. The reason for this acidity was explained in Section 3.8A. 

The ability of protons to migrate from cloud to cloud among the highly electronegative atoms is a major feature of acid-base chemistry. In water solution, a Bronsted acid, like HF, may donate protons to water molecules. A Lewis acid, like A1(H20), can be seen to act like the Bronsted acids. The actual Lewis acid Al " withdraws electron cloud from the attached water molecules and also repels the H", which then jumps to neighboring water molecules. Either action increases the concentration of H3O and decreases the concentration... 

In Section 1.7 (p. 41), we introduced acids and bases. Now we know quite a bit more about structure and can return to the important subject of acids and bases in greater depth. In particular, we know about carbocations and carbanions, which play an important role in acid—base chemistry in organic chemistry The Lewis definition of acids and bases is far more inclusive than the Bronsted definition, which focuses solely on proton donation (Breasted acid) and acceptance (Breasted base). The archetypal Brensted acid-base reaction is the reaction between KOH and HCl to transfer a proton from HCl to HO. This reaction is a competition between the hydroxide and the chloride for a proton. In this case, the stronger base hydroxide wins easily (Rg. 2.57). 

A third concept was proposed by Gilbert N. I wis which includes reactions with acid-base characteristics that do not involve a proton transfer. A Lewis acid is a species that accepts a pair of electrons from another species in other words, it is an electron pair acceptor. Bronsted acid-base reactions are proton transfer reactions while Lewis acid-base reactions are electron pair transfers. AU Bronsted acids are also Lewis acids, but not all Lewis acids are Bronsted acids. Contrast the following reactions which could be described in terms of acid-base chemistry ... 

Amines are organic bases whose acid—base chemistry is like that of ammonia. For example, methyl-amine behaves as a Bronsted base because the nonbonded electron pair of the nitrogen atom can accept a proton from an acid such as hydronium ion. When methylamine accepts a proton, the conjugate acid, methyl-ammonium ion, is produced. 

Bronsted acid/base chemistry Proton donor Proton acceptor... 

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