Acid-base reactions resonance

In Section 1 9 we introduced curved arrows as a tool to systematically generate resonance structures by moving electrons The mam use of curved arrows however is to show the bonding changes that take place in chemical reactions The acid-base reactions to be discussed in Sections 1 12-1 17 furnish numer ous examples of this and deserve some preliminary comment... 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

Methylpyridinium ions (181) react reversibly with hydroxide to form a small proportion of the pseudo-base (182). The term pseudo is used to designate bases that react with acids measurably slowly, not instantaneously as for normal acid-base reactions. Fused benzene rings reduce the loss of resonance energy when the hetero ring loses its aromaticity and hence pseudo-bases are formed somewhat more readily by 1-methylquinolinium, 2-methylisoquinolinium and 10-methylphenan-thridinium, and much more readily by 10-methylacridinium ions. Pseudo-bases carrying the hydroxy group in the a-position are usually formed preferentially, but acridinium ions react at the y-position. 

Because the enolate ion is the preferred resonance structure so a better mechanism for the acid base reaction shows the enolate ion being formed simultaneously as the acidic proton is lost (Following fig.). As the hydroxide ion forms its bond to the acidic proton, the C-H bond breaks, and the electrons in that bond form a rbond to the carbonyl carbon atom. Simultaneously, the carbonyl n bond breaks in such a way that both electrons move onto the oxygen. The electronegative oxygen is responsible for making the a proton acidic. 

Base-mediated ester hydrolyses have a high driving force. This is because of the acid/base reaction between the carboxylic acid formed in the reaction, and the base used as the reagent. The resonance stabilization of the carboxylate is approximately 30 kcal/mol, which means a gain of about 16 kcal/mol compared to the starting material, the carboxylic ester (resonance stabilization 14 kcal/mol according to Table 6.1). Accordingly, the hydrolysis equilibrium lies completely on the side of the carboxylate. 

In other words, the unshared electron pair of the base, acetate ion, is delocalized (spread over both oxygens) by resonance. This electron pair is stabilized and less available for bonding to the proton, which localizes this electron pair in the sigma bond and costs resonance energy. The most common effect of resonance on an acid-base reaction is to delocalize and stabilize the unshared electron pair of the conjugate base, resulting in a stronger acid. 

Resonance stabilization of the enolate anion makes it a weaker base than other carbanions, so it is easier to generate by acid-base reactions. 

Weller s review (1961) is not confined to acid-base reactions but deals with the kinetics of excited state reactions in general. Vander Donckt (1970) covers developments of the acid-base section of the Weller field but pays more attention to physical organic aspects, such as applications of resonance theory to the interpretation of pA-shifts upon excitation and the application of linear free energy relationships. The reviews by Schulman and Winefordner (1970) and by Winefordner et al. (1971a) are directed towards possible analytical applications. 

Write equations for the following acid-base reactions. Label the conjugate acids and bases, and show any resonance stabilization. Predict whether the equilibrium favors the reactants or products. If in doubt, you can consult Appendix 4 for acids not shown in Table 1-5. 

Treatment of indene with NaNH2 forms its conjugate base in a Bronsted-Lowry acid-base reaction. Draw aii reasonable resonance structures for indene s conjugate base, and explain why the pKa of indene is lower than the pK of most hydrocarbons. 

Because removal of a proton from a carbon bonded to phosphoms generates a resonance-stabilized carbanion (the ylide). this proton is somewhat more acidic than other protons on an alkyl group in the phosphonium salt. Very. strong bases are still needed, though, to favor the products of this acid—base reaction. Common bases used for this reaction are the organoUthium reagents such as butyllithium, CH3CH2CH2CH2Li, abbreviated as BuLi. 

Resonance forms for the sulfite ion have not been included. There has been discussion concerning whether the compound should be considered a sulfonated hyponitrite (16) or a nitrosated hydroxylamine sulfonate (9). The compound can be prepared by nitrosating a sulfonated hydroxylamine (9). There is little to be gained by this dispute, for the structure is best described as in the above diagrams or in molecular orbital terminology. The reaction of sulfite ion and nitric oxide is best considered as a Lewis acid-base reaction in which nitric oxide behaves as the acid and sulfite ion as the donor. A reaction sequence can be formulated with the following equations ... 

Base-catalyzed enol formation occurs by an acid-base reaction between catalyst and carbonyl compound. The carbonyl compound acts as a weak protic acid and donate. one of its a hydrogens to the base. The resultant anion—an enolate ion—is then reprotonated to yield a neutral compound. Since the enolate ion is a resonance hybrid of two forms, it can be proto-nated either on the a carbon to regenerate the keto tautomer or on oxygen to give the enol tautomer (Figure 22.2, p. 905). 

A few points should be noted. (1) Bases may be anionic or neutral, and acids may be neutral or cationic. (2) The acid-base reaction is an equilibrium. The equilibrium may lie far to one side or the other, but it is still an equilibrium. (3) There is both an acid and a base on both sides of the equilibrium. (4) This equilibrium is not to be confused with resonance. (5) Proton transfer reactions are usually very fast, especially when the proton is transferred from one heteroatom to another. 

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