Aldehydes relative reactivities

Reductions by NaBKt are characterized by low enthalpies of activation (8-13kcal/mol) and large negative entropies of activation (—28 to —40eu). Aldehydes are substantially more reactive than ketones, as can be seen by comparison of the rate data for benzaldehyde and acetophenone. This relative reactivity is characteristic of nearly all carbonyl addition reactions. The reduced reactivity of ketones is attributed primarily to steric effects. Not only does the additional substituent increase the steric restrictions to approach of the nucleophile, but it also causes larger steric interaction in the tetrahedral product as the hybridization changes from trigonal to tetrahedral. 

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 in an alkene is more stable than a monosubstituted double bond, a ketone carbonyl is more stable than an aldehyde carbonyl. We ll see later in this chapter that structural effects on the relative stability of carbonyl groups in aldehydes and ketones are an important factor in their relative reactivity. 

Dehalogenation.1 DMBI effects dehalogenation of a-halo carbonyl compounds in a variety of ethereal solvents with formation of DMBI+X in generally high yield. The order of relative reactivity is Br > Cl > F (halides) and primary > secondary > tertiary (for the a-substituted position). In combination with HO Ac (1 equiv.) the reagent also reduces acyl chlorides to aldehydes (70-90% yield). 

MCBA enhances the solubility of the cobalt salts in MeCN solution, thereby ensuring better efficiency to a needed redox decomposition of the hydroperoxide intermediate of the substrate en route to the products". By using the HPI/Co(II)/MCBA/02 system in MeCN solution at 25 °C, competitive oxidations of p-X-substituted benzyl alcohols were run pairwise (Scheme 8). From the amount of the aldehydes produced, the relative reactivity (kx/ h) could be reckoned, and the acquired data provided a p = —0.68 in a Hammett plot vs. <7+. ... 

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

Carbon-14 content is measured by specially designed gas proportional counters (7. Aerosol samples are first converted to CO2 by combustion in a macroscale version of the thermal evolution technique. A clam shell oven was used to heat the sample for sequential evolution of organic and elemental carbon under equivalent conditions. Due to the possibility of thermal gradients, conditions in the macroscale apparatus were adjusted to produce the same recoveries of total carbon (yg C per cm of filter area) as for the microscale apparatus. Carbon-14 data are reported as % contemporary carbon based on the 1978 1 C02 content in the atmosphere. Aldehyde data referred to in this paper were obtained by impinger sampling in dinitrophenylhydrazine/acetonitrile solution and analysis of the derivatives by HPLC with UV detection (12). Olefin measurements were made by a specially designed ozone-chemiluminescence apparatus (13) difficulties in calibration accuracy and background drift with temperature limit its use to inferences of relative reactive hydrocarbon levels. 

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. 

As a result, aldehydes and ketones react with nucleophiles. The relative reactivity of the carbonyl group is determined by the number of R groups bonded to it. As the number of R groups around the carbonyl carbon increases, the reactivity of the carbonyl compound decreases, resulting in the following order of reactivity ... 

Both a- and y-oxygenated allylic stannanes add to aldehydes under thermal or Lewis-acid-promoted conditions. These reagents are less reactive and more acid-labile than their non-oxygenated counterparts. Consequently, the best results are obtained with relatively reactive aldehydes. Strong Lewis acids cannot be used because they tend to cause decomposition of the stannanes. Initial studies employed thermal conditions to effect the additions. Thus, the frans-a-OMOM crotylstannane, prepared from crotonaldehyde by addition of BuaSnLi and etherification of the alcohol adduct, afforded the anti-(Z) adduct upon treatment with benzaldehyde under reflux in toluene (Eq. 28) [46]. 

The relative reactivity of aldehydes and acetals toward a typical ketene silyl acetal in the presence of TiCU has been revealed by use of competition reactions (Eqs (22) [98, 99], (23) [98], and (24) [98]). Although yields are not necessarily high, perhaps because these experiments were conducted solely to compare the reactivity of the compounds, the results shown in these equations are quite informative (i) acetals are more reactive than the corresponding aldehyde, (ii) aliphatic aldehydes are more reactive than aromatic aldehydes, and (iii) electron-rich aromatic aldehydes are somewhat more reactive than the electron-deficient variety. 

Effect of Other Hydrocarbons. The complex role of C3H6 in this system has been analyzed in terms of its reactions with O, O3, and OH, and a similar analysis is applied to determining the role of other hydrocarbons in the smog chemistry. Relative rate constants for the reactions of a number of hydrocarbons with O, O3, and OH hydrocarbons are presented in Table I, together with the relative reactivities of these hydrocarbons based on NO-NO2 conversion rates observed in smog chambers. The relative OH rate constants correlate remarkably well with the reactivities for all the types of hydrocarbons listed in the table. By contrast, the O-atom and O3 rates correlate with the reactivities only for the olefinic compounds. For the aromatics, aldehydes, and alkanes in the table, the relative O-atom and O3 rate constants are negligibly small compared with the relative reactivities. The relative OH rate... 

Hydrides based on aluminum have proved most versatile and selective for the reduction of esters (RCO2R ) to aldehydes (RCHO), but it should be noted that the nature of R and R (aliphatic, aromatic, branched, electron-withdrawing or electron-donating, steric size) is important. Most of the reactions described here have been carried out on simple methyl or ethyl esters. It is frequently found that aliphatic esters give much better yields than do aromatic esters. Reductions are carried out at very low temperatures such that esters are still reactive to the hydride but any aldehyde produced is not. Alternatively, the intermediate formed by addition of metal hydride to the ester (15 Scheme 5) may be sufficiently stable that no significant amount of aldehyde is formed. Thus, if the rate constant 1 > k2 then little aldehyde will be released, and no aldehyde formed even if 3 > /(. It also seems likely that the difference in the Lewis basicity of esters and aldehydes affects the complex of rate constants shown as 1, ki and h, and hence affects the relative reactivity of these two functional groups towards the different hydride reagents. 

An interesting observation from organocuprate chemistry is that the initial step in 1,4-addition to enones may be electron transfer. Thus the relative reactivity of enones toward conjugate addition parallels their ease of reduction. One problem with any reaction between a ketone or aldehyde and a metal alkyl is deprotonation, when a hydrogens are present, to yield an enolate. Given the considerable basicity of metal alkyls, this side reaction should be anticipated. 

Cyclic oxocarbenium cations approximate the a- and jS-chelation models for allylations of substituted aldehydes. In this analogy, the nature and the degree of substitution of the carbon bound to the oxonium site replaces the role of coordinated bidentate Lewis acids. Aside from the obvious concerns about the relative reactivity of 126 (Scheme 5.2.27), as compared with metal cation activation of precomplexed aldehyde 127, the C—O bond of 126 is much shorter (1.514 A) than distances associated with the... 

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