Brpnsted-Lowry bases

Table 6.3 Typical properties of Lowry-Brpnsted bases Base property Example from everyday life...

A mechanism for this reaction has been proposed and is summarized in Sch. 10. The catalyst 64 is thought to be bifunctional with the aluminum center operating as a Lewis acid and the lithium naphthoxide operating as a Lowry-Brpnsted base. It was envisaged that the aldehyde coordinates with the aluminum to give the complex 69 and deprotonation of the dimethyl phosphite then gives the aggregate 70 in which the phosphite anion is positioned for P-alkylation of the aldehyde that will occur selectively from the si face when the catalyst is prepared from (f )-BINOL. 

All aqueous solutions naturally contain hydroxide ions in consequence of the auto-protolytic reaction in Equation (6.2). As we have seen, there will be equal numbers of solvated protons and solvated hydroxide ions unless we add an acid or base to it. A solution containing more solvated protons than hydroxide ions is said to be an acid within the Lowry-Brpnsted theory, and a solution comprising more hydroxide ions than solvated protons is said to be a base. 

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. 

Method 2 Brpnsted-LoWri tackles bases u/ithout a ht dromde ion... 

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. 

In 1923, Johannes N. Brpnsted (1879-1947) and Thomas M. Lowry (1874-1936) independently defined acids and bases in a different way from the Arrhenins definitions. The resulting theory is sometimes called the Brpnsted-Lowry theory, bnt more often is referred to as just the Brpnsted theory. The Brpnsted theory extends the definitions of acid and base in a way that explains more than the Arrhenins definitions can explain. According to this theory, a Brpnsted acid is a proton donor, and a Brpnsted base is a proton acceptor. 

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. 

See pH. When dealing with chemical reactions in solvents other than water, it is sometimes convenient to define an acid as a substance that ionizes to give the positive ion of the solvent. The common definitions of acid have been extended as more detailed studies of chemical reactions have been made. The Lowry-Brpnsted definition of an acid as a substance that can give up a proton is more useful in connection with an understanding of bases (see base). Perhaps the most significant contribution to the theory of acids was the electron-pair concept introduced by G. N. Lewis around 1915. 

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

Knowledge Required (1) The interpretation of acid equilibrium constants, K. (2) Meaning of terms associated with weak and strong acids and bases. (3) Brpnsted-Lowry acid-base definition. 

Lewis defined a base as an electron-pair donor and an acid as an electron-pair acceptor. Modem inorganic chemistry extensively uses the Lewis definition, which encompasses the Br0nsted-Lowry definition, since accepts an electron pair from a Brpnsted base during protonation. The Lewis definition dramatically expands the acid list to include metal ions and main group compounds, and provides a framework for nonaqueous reactions. The Lewis definition includes reactions such as... 

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. 

How are the Arrhenius and Brpnsted-Lowry acid-base definitions different How are they similar Name two Brpnsted-Lowry bases that are not Arrhenius bases. Can you do the same for acids Explain. 

When an Fe + salt is dissolved in water, the solution becomes acidic due to formation of Fe(H20)50H + and H3O+. The overall process involves both Lewis and Brpnsted-Lowry acid-base reactions. Write the equations for the process. 

The bicarbonate ion has the ability to act as an acid in the presence of a base and as a base in the presence of an acid, so it is said to be amphiprotic. Illustrate this behavior with water by writing Brpnsted-Lowry acid-base reactions. Also illustrate this property by selecting a common strong acid and base to react with the bicarbonate ion. 

Clearly, the two reactions are analogous and demonstrate that the reaction between hydroxide ion and hydrogen bromide is simultaneously a Brpnsted-Lowry acid-base reaction and a Lewis acid/Lewis base reaction. Br0nsted-Lowry acid-base reactions constitute a subcategory of Lewis acid/Lewis 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 ... 

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