Acetals formation mechanism

In the first mechanism (acetal formation), another molecule of ROH attacked, because there was no other way to remove the positive charge ... 

None of these mechanisms are complete. All three of them are missing the last steps. But compare the steps that are shown above. Notice that, once again, these mechanisms are identical up until the very end of each mechanism. And it is right at the end where we see differences in the final products. In the first mechanism (ROH as the nucleophile), we saw that another molecule of ROH attacks. In the second mechanism, we saw that deprotonation led to formation of an imine. But in the third reaction, we cannot just lose a proton the way we did in the mechanism of imine formation. So, we might be tempted to do what we did in the first mechanism (acetal formation). We might be tempted to say that another molecule of amine should attack. But there is something else that happens instead ... 

If you re not sure of the mechanism of acetal formation or just want to know a bit more about acetals, read frames 1-21 and 62-64 of the Carbonyl Programme. 

The mechanism for formation of benzaldehyde diethyl acetal which proceeds m two stages is presented m Figure 17 9 The first stage (steps 1-3) involves formation of a hemiacetal m the second stage (steps 4-7) the hemiacetal is converted to the acetal Nucleophilic addition to the carbonyl group characterizes the first stage carbocation chemistry the second The key carbocation intermediate is stabilized by electron release from oxygen... 

FIGURE 17 9 The mechanism of acetal formation from benzaldehyde and ethanol... 

Problem 17.8 asked you to write details of the mechanism describing formation of benzaldehyde diethyl acetal from benzaldehyde and ethanol. Write a stepwise mechanism for the acid hydrolysis of this acetal. 

Hydrolysis of the amide 720 gave the acid 721. Boiling 721 in acetic acid for a prolonged period gave the dihydrofuro[3,4-6]quinoline 722 whose possible mechanism of formation is shown in Scheme 125 (85JCS(P1)1897). 

Mechanism of acid-catalyzed acetal formation by reaction of an aldehyde or ketone with an alcohol. 

In practice, it s convenient to use 1 equivalent of a diol such as ethylene glycol as the alcohol and to form a cyclic acetal. The mechanism of cyclic acetal formation using ethylene glycol is exactly the same as that using 2 equivalents of methanol or other monoalcohol. The only difference is that both —OH groups are in the same molecule. 

McLafferty, Fred Warren, 732 McLafferty rearrangement. 416, 732 Mechanism (reaction), 139 acetal formation, 717-718 acetylide alkylation, 272 acid chloride formal ion with SOCl2, 795... 

Scheme 7 Proposed mechanism to iron(III)-catalyzed indenyl acetate formation...

Kanasawud and Crouzet have studied the mechanism for formation of volatile compounds by thermal degradation of p-carotene and lycopene in aqueous medium (Kanasawud and Crouzet 1990a,b). Such a model system is considered by the authors to be representative of the conditions found during the treatment of vegetable products. In the case of lycopene, two of the compounds identified, 2-methyl-2-hepten-6-one and citral, have already been found in the volatile fraction of tomato and tomato products. New compounds have been identified 5-hexen-2-one, hexane-2,5-dione, and 6-methyl-3,5-heptadien-2-one, possibly formed from transient pseudoionone and geranyl acetate. According to the kinetics of their formation, the authors concluded that most of these products are formed mainly from all-(E) -lycopene and not (Z)-isomers of lycopene, which are also found as minor products in the reaction mixture. 

In the preparation of D-glucosone by the direct oxidation of D-glucose, D-fructose, or D-mannose by such reagents as that of Fenton,37 cupric acetate,16- 46- 46 selenious acid,16-61 etc., the degree of oxidation must be carefully controlled if the osone, which is the first product, is to be the main product of the reaction. The nature and mechanism of formation of the products of further oxidation of D-glucosone are discussed on p. 68. 

Peroxynitrite reacts with heme proteins such as prostacycline synthase (PGI2), microperoxidase, and the heme thiolate protein P450 to form a ferryl nitrogen dioxide complex as an intermediate [120]. Peroxynitrite also reacts with acetaldehyde with the rate constant of 680 1 mol 1 s" 1 forming a hypothetical adduct, which is decomposed into acetate, formate, and methyl radicals [121]. The oxidation of NADH and NADPH by peroxynitrite most certainly occurs by free radical mechanism [122,123], Kirsch and de Groot [122] concluded that peroxynitrite oxidized NADH by a one-electron transfer mechanism to form NAD and superoxide ... 

Fig. 21 Proposed mechanisms of formation of diacetates in the presence of cop-per(II)acetate as proposed by Nozaki [78]... 

The mechanism of formation of the sulfoxide (21a) from dibenzothiophene with chlorine in acetic acid has been studied. Sodium acetate showed a strong accelerating effect and the results suggested the formation of a dibenzothiophene-chlorine adduct which then decomposed giving the sulfoxide. 

Although the predominant product is the corresponding acetate (one could formulate either SnI or Sn2 mechanisms for formation of this product), about 30% of the alternative acetate is formed. This can be... 

Iron(ii) complexes of ethylenedithiodiacetic acid, diethylenetrithioacetic acid, and ethylenetetrathiotetra-acetic acid have all been reported. " " Isonitrile and other complexes. Refluxing [Fe(CNMe) ](HSOj2 "hh excess methylamine for 12 h in methanol gives the cation [Fe(CNMe)gNH2Me] which can be precipitated as its PF salt. " The structure of this cation has been determined by X-ray methods and is shown in (52). The location of the protons was confirmed by n.m.r. and the suggested mechanism of formation is as shown. 

Problem 15.35 Give mechanisms for (a) acid-catalyzed acetal formation. 

In a similar fashion to the formation of hydrate with water, aldehyde and ketone react with alcohol to form acetal and ketal, respectively. In the formation of an acetal, two molecules of alcohol add to the aldehyde, and one mole of water is eliminated. An alcohol, like water, is a poor nucleophile. Therefore, the acetal formation only occurs in the presence of anhydrous acid catalyst. Acetal or ketal formation is a reversible reaction, and the formation follows the same mechanism. The equilibrium lies towards the formation of acetal when an excess of alcohol is used. In hot aqueous acidic solution, acetals or ketals are hydrolysed back to the carbonyl compounds and alcohols. 

This mechanism (which is an SnIcA or A1 mechanism) is the reverse of that for acetal formation by reaction of an aldehyde and an alcohol (6-6). Among the facts supporting the... 

The mechanism of vinyl acetate formation is closely related to that of the Wacker oxidation (Scheme 9.11) that is, acetoxypalladation-palladium hydride elimination takes place.498,503 The coordinated alkene is attacked by the external nucleophile acetate ion, or the attack may occur within the coordination sphere. p-Hydride elimination followed by dissociation of the coordinated molecule yields directly the vinyl acetate end product. 

Exercise 15-17 The slow step in an alternative mechanism for acetal formation may be as follows ... 

Addition of hydrogen sulfide and thiols is qualitatively similar to reaction with alcohols in that there are two stages, formation of hemithioacetal (or hemithio-ketal) followed by acid-catalyzed elimination of the hydroxy group and substitution of a second —SR (Equations 8.47 and 8.48). The transformation has been studied less extensively than hydration and acetal formation, and relatively little information on mechanism is available. The initial addition appears to be specific base-catalyzed, an observation that implies that RS is the species that adds. The situation is thus similar to cyanide addition. General acid catalysis has, however, been found at pH 1 to 2 for addition of weakly acidic alkyl thiols, and the reaction rate as a function of pH has a minimum and rises both on the... 

The mechanism of acetal hydrolysis is the reverse of acetal formation. The first four steps convert the acetal to the hemiacetal. 

Write the steps in the mechanism of acetal formation and hydrolysis. Draw the structures of resonance contributors to intermediates in the mechanism. 

Scheme 4. A possible mechanism for formation of acetic acid from methane and CO.

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