Acid-base reactions showing mechanism

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. 

EXERCISE 3.38 Show the mechanism for the following acid-base reaction ... 

This method cannot be used with tertiary alkyl halides, because the competing elimination reaction predominates. The elimination reaction occurs because the rearward approach that is needed for an S 2 mechanism is impossible due to steric hindrance. An S 1 mechanism is likewise unfavored, because as the 3° carbon attempts to become a carbocation, the hydrogens on the adjacent carbons become acidic. Under these conditions, the alkoxide ion begins to show less nucleophilic character and, correspondingly, more basic character. This basic character leads to an acid-base reaction, which results in the generation of an elimination product (an alkene). 

In their extensive work on the mechanochemical reactions involving hydroxide-oxide mixtures. Senna and co-workers (Liao Senna, 1992, 1993 Watanabe et al., 1995b, 1996, Avvakumov et al., 2001) showed that the mechanism in these mixtures is governed by an acid-base reaction between different hydroxyl groups on the solid surface. The driving force for these reactions is the acid-base potential, i.e., the difference in the acid-base properties between an acidic and basic surface -OH group, which is determined by the type of metal on which it is bound, and therefore, on the strength of the M-OH bond (M denotes the... 

Early workers on acid-base catalysis had no clear ideas about the mechanism of catalysis, which was regarded as some kind of influence . These early views envisaged reactions which could take place in the absence of a catalyst but were facilitated by its presence however, evidence gradually accumulated to show that most of the reactions subject to acid-base catalysis cannot take place at all in the complete absence of catalysts, apparently spontaneous reactions being usually due to catalysis by solvent molecules or by some adventitious acidic or basic impurity. This indicates that the catalyst takes part in some fundamental way in the reaction mechanism, and the present view is that acid-base catalysis always involves an acid-base reaction between catalyst and substrate. 

Conjugated dienes and conjugated carbonyl compounds were discussed as more or less separate entities in the previous chapter. This chapter will show that one may react with the other. 1,3-Dienes react with alkenes, particularly with the C=C unit of a conjugated carbonyl compound, to form cyclohexene derivatives. This is a [4-1-2] cycloaddition, commonly known as the Diels-Alder reaction. The mechanism for this reaction requires an examination of the molecular orbitals of the reactants. This is one of the most important reactions in organic chemistry. It is also important to note that this reaction is not an acid-base reaction it is a new type of reaction and a new type of mechanism. The product is often a difunctional molecule. 

Experimental details are provided for key reactions to introduce the reaction. The point is to show that the experiment shows a result and then we try to understand that result in the context of the structural featrues of the reactive species. Mechanisms are given as part of the discussion of key reactions. The mechanism is discussed in most cases first as a walk-through of the reaction to understand how the transformation occurred, and then structures for the walkthrough are provided that constitute the mechanism. Many of these mechanistic steps will involve acid-base reactions—both Brpnsted-Lowry and Lewis. 

The acid-base reaction between hydrogen ion and hydroxide exemplifies this type of mechanism When combining an anion with a cation, draw the curved arrow starting at an electron pair on the anion and ending at the cation. NEVER start the curved arrow at the cation The arrow shows how electrons move, not atoms. Electrons move, and atoms follow. 

Review) Use curved arrows to show the mechanisms of the following two acid-base reactions. 

Show the mechanism of Synthetic Transformation 14.2, and draw an energy diagram (assume the acid-base reaction is much faster than either step in the elimination). 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Problem 19.18 t When o-phthalaldc-hyde is treated with base, o-(hydroxymethvl)benzoic acid is formed. Show the mechanism of tilts reaction. 

Carboxylic acids having a second carbonyl group two atoms away lose C02 (clecarboxylatc) through an intermediate enolate ion when treated with base. Write the mechanism of this decarboxylation reaction using curved arrows to show the electron flow in each step. 

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