Base Bronsted

Reduction of dioxygen by electron transfer yields superoxide ion (02 ), which has its negative charge and electronic spin density delocalized between the two oxygens. As such it has limited radical character [H-OO bond energy AGbf, 2 kcal]2 and is a weak Bronsted base in water  

The dynamics for the hydrolysis and disproportionation of Ch in aqueous solutions have been characterized by pulse radiolysis.5-7 For all conditions the rate-limiting step is second order in O2 - concentration, and the maximum rate occurs at a pH that is equivalent to the pKa for HOO- (it decreases monotonically with further decreases in the hydrogen ion concentration). 

The propensity of 02 - to remove protons from substrates accounts for its reactivity with acidic reductants and their overall oxidation. Thus, combination of 02 with protic substrates [a-tocopherol, hydroquinone, 3,5-di-fcrf-butylcatechol, L(-t-)-ascorbic acid] yields products that are consistent with an apparent one-electron oxidation of the substrate and the production of HOOH. However, the results of electrochemical studies provide clear evidence that these substrates are not oxidized in aprotic media by direct one-electron transfer to O2 -. The primary step involves abstraction of a proton from the substrate by O2 to give substrate anion and the disproportionation products of HOO- (HOOH and O2). In turn, the substrate anion is oxidized by O2 in a multistep process to yield oxidation products and HOOH. Thus, by continuously purging the O2 that results from the disproportionation of 02 - when it is combined with a-tocopherol (by vigorous Ar bubbling through the solution), quantitative yields of substrate anion are obtained without significant oxidation. 

Although superoxide ion is a powerful nucleophile in aprotic solvents, it does not exhibit such reactivity in water, presumably because of its strong solvation by that medium (AHhydration/ 100 kcal) and its rapid hydrolysis and disproportionation. The reactivity of Oa - with alkyl halides via nucleophilic substitution was first reported in 1970. 3,14 These and subsequent kinetic studies35-t8 confirm that the reaction is first order in substrate, that the rates follow the order primary secondary tertiary for alkyl halides and tosylates, and that the attack by Oa - results in inversion of configuration (Sn2). 

The stoichiometries and kinetics for the reaction of 02 - with halogenated hydrocarbons (alkanes, alkenes, and aromatics) are summarized in Table 7-1.18-24 [The normalized first-order rate constants, fcj / [S], were determined by the rotated ring-disk electrode method under pseudo-first-order conditions ([substrate] [O2-)].  

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We are told that a proton is transferred from HCI to NH3 Therefore HCI is the Bronsted acid and NH3 is the Bronsted base... 

For which of the following Bronsted bases is the reaction with... 

Step 3 in Figure 5 6 shows water as the base which ab stracts a proton from the car bocation Other Bronsted bases present in the reaction mixture that can function in the same way include tert butyl alcohol and hydrogen sulfate ion... 

Bronsted acid See acid Bronsted base See base... 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

Both the zwitterion and the HC03 ion are amphiprotic, capable of acting as either a Bron-sted acid or a Bronsted base. 

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). 

Notice that because HzO accepts the hydrogen ion to form H30+, water is acting as a Bronsted base in this reaction. 

J.2 Classify each compound as either a Bronsted acid or a Bronsted base (a) H3As04 (b) CH NH2, a derivative of ammonia (c) CH3COOH (d) LiOH (e) HC103. 

What Are the Key Ideas Bronsted acids are proton donors Bronsted bases are proton acceptors. The composition of a solution of an acid or base immediately adjusts to satisfy the values of the equilibrium constants for all the proton transfer reactions taking place. 

Another example of a Bronsted base is ammonia. When ammonia dissolves in water, it is protonated when the lone pair of electrons on the N atom accepts a proton from 112( ) ... 

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. 

A Bronsted acid is a proton donor and a Bronsted base is a proton acceptor. 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

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. 

Answer (a) Bronsted acids, HN()2, H2P04 Bronsted bases, HP042 and N02 (b) Lewis acid, H+ Lewis bases, HP042 and NOz ]... 

A basic oxide is an oxide that reacts with water to form a Bronsted base, as in the reaction... 

Because water is amphiprotic—because it is both a Bronsted acid and a Bronsted base—proton transfer between water molecules occurs even in pure water, with one molecule acting as a proton donor and a neighboring molecule acting as a base ... 

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. 

Ammonia is very soluble in water because the NH3 molecules can form hydrogen bonds to H20 molecules. Ammonia is a weak Bronsted base in water it is also a reasonably strong Lewis base, particularly toward d-block elements. For example, it reacts with Cu2+(aq) ions to give a deep-blue complex (Fig. 15.4) ... 

In the second step, the hydrogen ion is pulled out of the ring by the HS04 ion acting as a Bronsted base. As in the bromination reaction, the restoration of the delocalization of the 7t-electrons facilitates the removal of the hydrogen ion. 

Used alone, base normally means a Bronsted base, base buffer See buffer. 

International System (SI) in terms of which all other units are defined. Examples kilogram for mass meter for length second for time kelvin for temperature ampere for electric current, basic ion An ion that acts as a Bronsted base. 

Bronsted base A proton acceptor (a species to which hydrogen ions, HT, can bond). Examples OH Cl CH,C02 HCO. NHV... 

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