Carbonyl group, reactions

The role of the base is apparently primarily that of a proton remover from the reactive methylene group thus if B represents the base, reaction (i) gives the carbanion, which then combines with the positive carbon of the carbonyl group (reaction ii) the product regains a proton from the piperidinium ion, and then by loss of water followed by mono-decarboxylation of the malonic acid residue gives the final acid. 

Carbonyl Group Reactions. Mandelonitrile [532-28-5] is formed by the addition of hydrogen cyanide to the carbonyl double bond. 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

The carbonyl group reactivities in thiophenes and benzenes are very similar, as shown by the similar rates of alkaline hydrolysis of esters and by the great similarity of the thiophenealdehydes to benzaldehyde in numerous carbonyl group reactions. This has been ascribed to the counteracting —I- -M effects of the thienyl group in this kind of reactions. ... 

The addition of a nucleophile to a polar C=0 bond is the key step in thre< of the four major carbonyl-group reactions. We saw in Chapter 19 that when. nucleophile adds to an aldehyde or ketone, the initially formed tetrahedra intermediate either can be protonated to yield an alcohol or can eliminate th< carbonyl oxygen, leading to a new C=Nu bond. When a nucleophile adds to carboxylic acid derivative, however, a different reaction course is followed. Tin initially formed tetrahedral intermediate eliminates one of the two substituent originally bonded to the carbonyl carbon, leading to a net nucleophilic acy substitution reaction (Figure 21.1. ... 

The following reaction, called the benzilic acid rearrangement, tajkes place by typical carbonyl-group reactions. Propose a mechanism fPh = phenyl). 

We ve now studied three of the four general kinds of carbonyl-group reactions and have seen two general kinds of behavior. In nucleophilic addition and nucleophilic acyl substitution reactions, a carbonyl compound behaves as an electrophile. In -substitution reactions, however, a carbonyl compound behaves as a nucleophile when it is converted into its enol or enolate ion. In the carbonyl condensation reaction that we ll study in this chapter, the carbonyl compound behaves both as an electrophile and as a nucleophile. 

Two of the four general carbonyl-group reactions—carbonyl condensations and basic conditions and involve enolate-ion intermediates. Because the experimental conditions for the two reactions... 

You are what you eat. Food molecules are metabolized by pathways that involve the four major carbonyl-group reactions. 

Biochemistry is carbonyl chemistiy. Almost all metabolic pathways used by living organisms involve one or more of the four fundamental carbonvl-group reactions we ve seen in Chapters 19 through 23. The digestion and metabolic breakdown of all the major classes of food molecules—fats, carbohydrates, and proteins—take place by nucleophilic addition reactions, nucleophilic acyl substitutions, a substitutions, and carbonyl condensations. Similarly, hormones and other crucial biological molecules are built up from smaller precursors by these same carbonyl-group reactions. 

Fructose, a /3-hydroxy ketone, is then cleaved into two three-carbon molecules—one ketone and one aldehyde—by a reverse aldol reaction. Still further carbonyl-group reactions then occur until pyruvate results. 

These few examples are only an introduction we li look at several of the major metabolic pathways in much more detail in Chapter 29. The bottom line is that you haven t seen the end of carbonyl-group chemistry. A solid grasp of carbonyl-group reactions is crucial to an understanding of biochemistry. 

The Maunich reaction of a ketone, an amine, and an aldehyde is one of the few three-component reactions in organic chemistry. Cyclohexanone, for example, reads with dimethylamine and acetaldehyde to yield an amino ketone. The reaction takes place in two steps, both of which are typical carbonyl-group reactions. 

MONO AND TRIDIRECTIONAL 12-MEMBERED RING ZEOLITES AS ACID CATALYSTS FOR CARBONYL GROUP REACTIONS... 

In the reduction of radicals by ET, simple carbanions are practically never formed, and one-electron reduction of a carbon-centered radicals is only effective if the electron can be accommodated by the substituent, e.g., a carbonyl group [reaction (24), whereby upon electron transfer the enolate is formed (Akhlaq et al. 1987)]. Thus, in their reduction reactions these radicals react like heteroatom-centered radicals despite the fact that major spin density is at carbon. 

A proton has to be transferred from one oxygen atom to the other we have shown ethanol doing this job, with one molecule being protonated and one deprotonated. There is no overall consumption of ethanol in the pro to nation/deprotonation steps, and the order in which these steps happen is not important. In fact, you could reasonably write them in one step as shown in the margin, without involving the alcohol, and we do this in the next hemiacetal-forming reaction below. As with all these carbonyl group reactions, what is really important is the addition step, not what happens to the protons. 

Tire nitrogen atom in the pyridine ring is planar and trigonal with the lone pair in the plane of the ring. This makes it an imine. Most of the imines you have met before (in Chapter 14, for example), have been unstable intermediates in carbonyl group reactions, but in pyridine we have a stable imine—stable because of its aromaticity. All imines are more weakly basic than saturated amines and pyridine is a weak base with a pK of 5.5. This means that the pyridinium ion as about as strong an acid as a carboxylic acid. 

Cyclitols. A novel synthesis of conduritol (6) from benzoquinone utilizes 1 for protection of one C=C bond and for differentiation of the carbonyl groups. Reaction of the quinone with 1 gives the Diels-Alder adduct 2, which is converted selectively into 4,... 

The follownng reaction, called the bemstic ceid rearnan e/rten/, takea place by typi< cal carbonyl-group reactions. Propose a mecKanlem (Ph " phenyl). 

Stereoselective formation of the C—Sn bond is usually observed in the addition of a trialkylstan-nyl anion to a C-C double bond or carbonyl group. Reactions are also described where a new C—Sn bond is stereoselectively formed via migration of the stannyl group or by stannylation of a chiral carbanion. 

Both aromatic aldehydes and ketones undergo normal carbonyl group reactions with nucleophilic reagents. 

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