Cannizzaro mechanism

Figure 12.6 presents the process flow diagram of the catalytic production of xylitol. Three main sections are (i) pretreatment, (ii) reaction-condensation, and (iii) crystallization. The pretreatment section deals with the pH adjustment and the removal of the impurities contained in the feed stream, soluble and insoluble solids, and remnants of the hydrolysis of biomass. But the main goal is the removal of the alkalis, whose presence catalyzes the conversion of xylose to xylonic acid (Cannizzaro mechanism). The first step of the downstream processes (unit PI) involves a series of treatment by ionic resins, activated carbon column, and chromatographic separations. Then, the pH is adjusted (unit Rl) in a range of 5-6 through the neutralization of acids (mainly acetic acid) by 1M solution of calcium hydroxide (Ca(OH)2) ... 

The Mecrwein-Ponndoi f-Verlev reaction involves reduction of a ketone by treatment with an excess of aluminum triisopropoxide. The mechanism of the process is closely related to the Cannizzaro reaction in that a hydride ion acts as a leaving group. Propose a mechanism. 

Camphor, molecular model of, 129 specific rotation of, 296 Cannizzaro, Stanislao, 724 Cannizzaro reaction. 724 mechanism of, 724 Caprolactam, nylon 6 from, 1213 Capsaicin, structure of. 78 -curbahlehyde, aldehyde name ending, 696... 

The mechanism of the Cannizzaro reaction involves a hydride shift (an... 

Under similar conditions, reactions between pyrrolidine derivatives 632 and MTAD proceed much more slowly and less cleanly with formation of a polymeric material. When the reaction is stopped before 50% conversion is reached, starting compound 632 is isolated as the main component (c. 40%) and compound 637 as a minor product (10-14%). Mechanistically, the most difficult problem lies in the fact that a reduction step has to be involved and no particular reduction agent is present. A proposed mechanism is shown in Scheme 103. The pathway includes a Cannizzaro-type hydride transfer between dipole 633 and product 634 (keto tautomer), resulting in the formation of the iminium derivative 635, which might be responsible for the polymeric material, and hydroxy derivative 636, the direct precursor of the final products 637. The low experimental yield of 637 could be explained by this mechanism <2003EJ01438>. 

D-Erythrose undergoes self-aldolization in alkali solution, to form d- / co-L- /3 C6 TO-3-octulopyranose by combination of the 1,2-enediol and aldehyde forms. In weak alkali at 105°, syrupy D-erythrose yields d- /ycero-tetrulose, jS-D-a/tro-L-g/ycero-l-octulofuranose, and a-Ti-gluco-i -g/ycero-3-octulopyranose. At 300° in alkali, the major products from syrupy D-erythrose were 1-5% of butanedione (biacetyl) with smaller proportions of pyrocatechol, 33, 2,5-dimethyl-2,5-cyclohexadiene-l,4-dione (2,5-dimethylbenzoquinone), and 2,5-dimethyl-1,4-benzenediol (2,5-dimethylhydroquinone). It was assumed that D-erythrose is reduced to erythritol by a Cannizzaro type of reaction, followed by dehydration of erythritol to form biacetyl. However, very low proportions (<1%) of biacetyl are formed from erythritol compared with D-erythrose itself. Apparently, some other mechanism predominates in the formation of biacetyl. 

Unlike the starting material, the product of this reduction contains an asymmetric carbon atom and it was found to be optically active. Since in the case of ketones a Cannizzaro reaction cannot take place, this mechanism for alcohol formation is out of the question. Consideration of the reduction of ketones thus clearly shows that an actual biohydrogenation is involved. 

Problem 15.20 Devise a mechanism for the Cannizzaro reaction from the reactions... 

The mechanism747 of the Cannizzaro reaction74 involves a hydride shift (an example of mechanism type 2, p. 1160). First OH adds to the C=0 to give 50, which may lose a proton in the basic solution to give the diion 51. 

The mechanism of this reaction, usually called the Cannizzaro reaction,4 combines many features of other processes studied in this chapter. The first step is reversible addition of hydroxide ion to the carbonyl group ... 

Compare your mechanism with that generally accepted for the Cannizzaro reaction. 

Exercise 17-43 Write a mechanism analogous to that for the Cannizzaro reaction for the benzilic acid transformation. What product would you expect to be formed from diphenylethanedione with potassium terf-butoxide in ferf-butyl alcohol Would you expect a benzilic acid-type rearrangement to occur with 2,3-butanedione Give your reasoning. 

The mechanism of the reaction could be either a base-induced enoiization reaction (Section 17-1) or ionization of the OH proton followed by a Cannizzaro-type reaction (Section 16-4E). Write each mechanism in detail and devise experiments that could be used to distinguish between them. 

This dismutation or disproportionation reaction is known as the Cannizzaro reaction. The mechanism of the reaction involves the production of the anion (1) which may transfer a hydride ion to a carbonyl carbon atom in another aldehyde molecule. The reaction sequence is completed by a proton transfer to yield the carboxylate anion and the alcohol. 

A number of mechanisms have been suggested for the Cannizzaro reaction.13-14 16 One mechanism, based upon a proposal by Lock,16 who patterned his suggestion after those put forward by Grignard and Fluchaire 16 and by Fredenhagen and Bonhoeffer,17 is the following. 

This reaction appears to be of limited usefulness as a preparative method. The fact that the ester is sometimes isolated39-41 is of interest in connection with the mechanism of the Cannizzaro reaction (p. 97). 

Reactions in which hydride leaves are less common but can occur if other reactions are precluded and the hydride is transferred directly to an electrophile. One example occurs when an aldehyde without any hydrogens on its a-carbon is treated with NaOH or KOH. (If the aldehyde has hydrogens on its a-carbon, the aldol condensation is faster and occurs instead.) In this reaction, called the Cannizzaro reaction, two molecules of aldehyde react. One is oxidized to a carboxylate anion and the other is reduced to a primary alcohol. The mechanism for this reaction is shown in Figure 20.5. The reaction begins in the same manner as the reactions described in Chapter 18 a hydroxide ion nucleophile attacks the carbonyl carbon of the aldehyde to form an anion. The reaction now begins to resemble the reactions in Chapter 19. 

The mechanism we have drawn here is slightly different from that in Chapter 27 where wc showed the dianion as an intermediate. The two reactions are related by base catalysis as we shall see. Now for some of the evidence and some of the alternative mechanisms that have been proposed for the Cannizzaro reaction. Most of these have been eliminated, leaving just the ones you have already met. Finally, we will see that even these mechanisms do not explain everything absolutely. 

Which step would be rate-determining for this mechanism It could not be step 1 since, if this were the case, then the rate law would be first-order with respect to the aldehyde rather than the observed second-order relationship. Also, if the reaction is carried out in water labelled with oxygen-18, the oxygen in the benzaldehyde exchanges with the 180 from the solvent much faster than the Cannizzaro reaction takes place. This can only be because of a rapid equilibrium in step 1 and so step 1 cannot be rate-determining. 

How can mechanism B be ruled out One way is to change the attacking nucleophile. The Cannizzaro reaction works equally well if methoxide is used in a mixture of methanol and water. If mechanism B were correct, the reaction with methoxide would be as follows. 

We said earlier that we can never prove a mechanism—only disprove it. Unfortunately, just as the correct mechanism seems to be found, there are some observations that make us doubt this mechanism. In Chapter 39 you saw how a technique called electron spin resonance (ESR) detects radicals and gives some information about their structure. When the Cannizzaro reaction was carried out with benzaldehyde and a number of substituted benzaldehydes in an ESR spectrometer, a radical was detected. For each aldehyde used, the ESR spectrum proved to be identical to that formed when the aldehyde was reduced using sodi-... 

Our mechanism does not explain this result but small amounts of radicals are formed in many reactions in which the products are actually formed by simple ionic processes. Detection of a species in a reaction mixture does not prove that it is an intermediate. Only a few chemists believe that radicals are involved in the Cannizzaro reaction. Most believe the mechanism we have given. 

The rest of the chapter is devoted to discussions of the methods we have briefly surveyed for the Cannizzaro reaction with examples of the use of each method. We give examples of many different types of reaction but we cannot give every type. You may rest assured that all of the mechanisms we have so far discussed in this book have been verified (not, of course, proved) by these sorts of methods. 

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