Bronsted base hydroxides

A note on good practice The oxides and hydroxides of the alkali and alkaline earth metals are not Bronsted bases the oxide and hydroxide ions they contain are the bases (the cations are spectator ions). However, for convenience, chemists often refer to the compounds themselves as bases. 

This reaction shows that the methide ion is a very strong Bmnsted base. The species C22 is the acetylide ion, and the carbides that contain it are called acetylides. The acetylide ion is also a strong Bronsted base, and acctylides react with water to produce ethyne (acetylene) and the corresponding hydroxide. Calcium carbide, CaC2, is the most common saline carbide. 

Because of its high negative charge density, the oxide ion is a very strong Bronsted base. Therefore, when an ionic oxide is placed in water, there is proton transfer to produce hydroxide ions. 

The use of amines allows much higher nucleophile concentrations than those achievable with Bronsted bases. We have used solutions as concentrated as 6 M Me N. This vast difference in available nucleophile concentration partially explains the huge increase in rate afforded by NMe3 over the rate with Bronsted bases. Very large concentrations of hydroxide may promote the base attack step but can decrease the rate of the WGSR due to inhibition of the protonation of the metal hydride species. 

An example of a Bronsted base is an oxide ion, such as the oxide ion in calcium oxide, CaO. When CaO dissolves in water, the strong negative charge of the small O2- ion pulls a proton out of a neighboring HzO molecule (Fig. 10.4). By accepting a proton, the oxide ion has become protonated. The oxide ion (a Lewis base, Section 2.13) forms a coordinate covalent bond to the proton (a Lewis acid) by providing both the electrons in the bond and becomes a hydroxide ion ... 

The Michael reaction is the conjugate addition of a soft enolate, commonly derived from a P-dicarbonyl compound 24, to an acceptor-activated alkene such as enone 41a, resulting in a 1,5-dioxo constituted product 42 (Scheme 8.14) [52]. Traditionally, these reactions are catalyzed by Bronsted bases such as tertiary amines and alkali metal alkoxides and hydroxides. However, the strongly basic conditions are often a limiting factor since they can cause undesirable side- and subsequent reactions, such as aldol cyclizations and retro-Claisen-type decompositions. To address this issue, acid- [53] and metal-catalyzed [54] Michael reactions have been developed in order to carry out the reactions under milder conditions. 

A Bronsted base can accept a proton from water to give hydroxide ion (OH ). 

Bronsted bases, ions or molecules that will take on protons, are generally negative ions or neutral molecules there are a few of such bases that are positively charged, the most important being hydrated cations that have lost protons, for example, Zn(H20)3(0H)+. Basic half-reactions of the hydroxide ion, the sulfate ion, the oxide ion, water, the hydrogen carbonate ion, and the zinc-containing cation, mentioned above, are listed ... 

The preceding section describes the primary reaction chemistry of superoxide ion (02 -) to be that of (a) a Bronsted base (proton transfer from substrate), (b) a nucleophile (via displacement or addition), (c) a one-electron reductant, and (d) a dehydrogenase of secondary amine groups. The chemistry is characteristic of all oxyanions (H0 (R0 ), HOO (ROO ), and 02 -)> but the relative reactivity for each is determined by its pXa and one-electron oxidation potential, which are strongly affected by the anionic solvation energy of the solvent matrix. The present focus is on the reactivity of hydroxide ion (HO ), but the principles apply to all oxyanions and permit assessments of their relative reactivity. 

Some substances can act as a Bronsted-Lowry acid in one reaction and a Bronsted-Lowry base in another. Consider the following net ionic equations for the reaction of dihydrogen phosphate ion with either the acid hydrochloric acid or the strong base hydroxide. 

The mechanism operating in enantioselective Michael reactions under PTC conditions involves the formation of a chiral ion-pair between the nucleophile and the catalyst as the key phenomenon operating in the stereocontrolled formation of the new stereogenic center. The reaction typically incorporates an acidic pro-nucleophile (in almost all cases an enolizable carbonyl compound), the Michael acceptor, the catalyst and a Bronsted base, which is typically an inorganic salt such as a hydroxide or a carbonate (Figure 5.1). It starts with the... 

Using the neutral KOH as our starting point, its dissociation releases an amount of K equivalent to the OH. The [K1 is a measure of the [OH ] released and is therefore equivalent to the protons consumed in this dissociation. Of course, there are many bases which do not directly release hydroxide ions (Bronsted bases). In the case of NH3, for example, protons are consumed to form an equal number of NH4 ions. Hence, the PBE for aqueous NH3 is ... 

Strictly speaking, these metal hydroxides are not Bransted bases because they cannot accept a proton. However, the hydroxide ion (OH ) formed when they ionize is a Bronsted base because it can accept a proton ... 

Acid-base properties so far have been discussed in terms of the Bronsted theory. To behave as a Bronsted base, for example, a substance must be able to accept protons. By this definition both the hydroxide ion and ammonia are bases ... 

Concept of an alkali In the previous pages NaOH was regarded as an Arrhenius base because it ionized to produce OH ions. NaOH, however, is not a Bronsted base because, as a molecule, it has little ability to accept a proton NaOH can act as a base solely because upon ionization it gives rise to OH ions which are very good proton acceptors. NaOH and other metallic hydroxides like KOH, therefore act as bases by proxy. Such compounds, under the bronsted theory, are known as alkalies. 

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 Bronsted acid is any compound that can donate a proton. A Bronsted base is any compound that can accept a proton. Let s look first at a very simple process, the reaction of solid potassium hydroxide (KOH) with gaseous hydrochloric acid (HCl).This reaction is nothing but a competition of two Bronsted bases (Cl and HO ) for a proton, H. The stronger Bronsted base (HO ) wins the competition (Fig. 6.16). 

As we saw in Figure 6.15, a molecule may be both a Bronsted acid and a Bronsted base. Consider water, for example. Water can both donate a proton (act as a Bronsted acid) and accept a proton (act as a Bronsted base). Figure 6.18 shows two reactions, (a) the protonation of water by hydrogen chloride, in which water acts as a base, and (b) the protonation of hydroxide ion by water, in which water is acting as the acid. 

The Grocjp 1A and heavy Group 2A metal hydrogddes are classified as strong bases. It is the hycboKxfe ion itself, however, that accepts a proton and is therefore the Bronsted base. Soluble metal hydroxides are simply scxirces of the hydroHcfe bn. 

I. Rodriguez, S. Iborra, F. Rey, A. Corma, Heterogeneized Bronsted base catalysts for fine chemicals production grafted quaternary organic ammonium hydroxides as catalyst for the production of chromenes and coumarins, Appl. Catal. A Gen. 194-195 (2000) 241-252. 

The most common Bronsted base is the hydroxide ion. It can accept a proton from an acid to give water. Ammonia is also a base it can accept a proton from an acid to give the ammonium ion. A curved arrow in the following equation shows the movement of the pair of electrons in the reaction of ammonia with hydronium ion. 

In Section 16.7, we saw that the conjugate base of a weak acid acts as a weak Bronsted base in water. Consider a solution of the salt sodium fluoride (NaF). Because NaF is a strong electrolyte, it dissociates completely in water to give a solution of sodium cations (Na" ) and fluoride anions (F ). The fluoride ion, which is the conjugate base of hydrofluoric acid, reacts with water to produce hydrofluoric acid and hydroxide ion ... 

The nitride ion is a very strong Bronsted base and reacts with water to produce ammonia and hydroxide ions ... 

Barrier-layer cells 658 Baryta see Barium hydroxide Bases. Bronsted-Lowry theory of, 21 dissociation constants of, (T) 833 hard, 54 ionisation of, 21... 

A note on good practice Sodium hydroxide is commonly termed a base however, from the Bronsted point of view it simply provides the base OH. ... 

---