Relative Reactivity of Aldehydes and Ketones

One further comparison Aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes. The 

CHAPTER 19 I Aldehydes and Ketones Nucleophilic Addition Reactions 

Problem 19.6 Which would you expect to be more reactive toward nucleophilic additions, p-methoxy-benzaldehyde or p-nitrobenzaldehyde Explain. 

Aldehydes and ketones undergo reaction with water to yield 1,1-diols, j geminal (gem) diols. The hydration reaction is reversible, and a gem di can eliminate water to regenerate a ketone or aldehyde. 

The exact position of the equilibrium between a gem diol and a keta or aldehyde depends on the structure of the carbonyl compound. Although the equilibrium generally favors the less crowded carbonyl compound f( steric reasons, the gem diol is favored for a few simple aldehydes. For exam-1 

Aldehydes are generally more reactive than ketones for both steric and electronic reasons. 

On steric grounds. The nucleophile can attack the aldehyde carbonyl carbon atom more readily as this has only one (rather than two) alkyl group(s) bonded to it. The transition-state resulting from addition to the aldehyde is less crowded and lower in energy. 

Attack hindered by Attack hindered only one alkyl group by two alkyl groups 

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Steric factors also play an important role in the reactivity of aldehydes and ketones. We may look at the relative ease with which the attacking nucleophile can approach the carbonyl carbon or consider how steric factors influence the stability of the transition state leading to the final product. 

During the mechanistic studies of estrogen biosynthesis, selective oxidation of androstene-3,17,19-trione (n) to the corresponding carboxylic acid was found to proceed by iron porphyrin complexes (Scheme 14A) [253]. On the basis of substituent effect on the benzaldehyde oxidation and kinetic isotope effect, direct hydrogen abstraction mechanism has been proposed [254]. The relative reactivity of aldehydes and alkenes is as follows cyclooctene, styrene > aldehyde, terminal alkene > a, 3-unsaturated ketone. 

Relative reactivities of aldehydes, ketones, esters, acid chlorides and anhydrides. 

Hypervalent silicon componnds have found wide utility in organic synthesis. In general, pentacoordinated anionic silicates are more reactive toward nucleophiles than are tetracoordinated silanes. For example, Mes2SiF2 is nmeactive toward water, while (the 18-crown-6 potassium salt of) Mes2SiF3 is completely hydrolyzed within minutes. Similarly, the pentacoordinate anion HSi(OEt)4 is an effective reducing agent for aldehydes, ketones, and esters at or below room temperature (Scheme 2) no snch reaction occurs with HSi(OEt)3. The difference in relative reactivities of hypervalent and nonhypervalent species is relevant to the intermediates proposed in Section 7.6. 

Despite the relevance of these findings and the implications that they may have [43q, r] these excellent figures were not confirmed using catalysts with an identical composition and structure, namely, MnAPO-5 and MnAPO-18 [43s, t]. It was reported that n-hexane oxidation turnover rates (per redox-active Mn center) by oxygen were similar on MnAPO-5 and MnAPO-18, because the reactant may rapidly diffuse and reach the active site, regardless of the pore size in the microporous structure. No regiospedficity was detected for w-hexane oxidation to alkanols, aldehydes and ketones (7-8% terminal selectivity), and the relative reactivity of primary and secondary C—H bonds in w-hexane was identical in both catalysts and similar to that predicted from relative C—H bond energies in n-hexane. The selectivity to terminal adds was very low. 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

As an example of enolate-ion reactivity, aldehydes and ketones undergo base-promoted o halogenation. Even relatively weak bases such as hydroxide ion are effective for halogenation because it s not necessary to convert the ketone completely into its enolate ion. As soon as a small amount of enolate is generated, it reacts immediately with the halogen, removing it from the reaction and driving the equilibrium for further enolate ion formation. 

Relatively few studies of the reactions of allylboron compounds and ketones have appeared. Ketones are less reactive than aldehydes, and as a result these reactions tend to be much slower and often less diastereoselectivc. The reaction of (Z)-4,4,5,5-tetramethyl-2-[3-(tctrahy-dro-2/A-pyran-2-yloxy)-2-propenyl]-1,.3,2-dioxaborolane and ethyl 2-oxopropanoate, for example, was conducted under 6 kbar pressure at 45 C for 80 hours to give a 9 1 mixture of syn-and antz-diastereomers of 1 in 85% yield49. 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

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