Bonding in aldehydes

The carbon-carbon double bond in vinyl monomers and the carbon-oxygen double bond in aldehydes and ketones are the two main types of linkages that undergo chain... 

Progress in the polymerization of the carbonyl linkage did not result until there was an understanding of the effect of ceiling temperature (Tc) on polymerization (Sec. 3-9c). With the major exception of formaldehyde and one or two other aldehydes, carbonyl monomers have low ceiling temperatures (Table 5-13). Most carbonyl monomers have ceiling temperatures at or appreciably below room temperature. The low Tc values for carbonyl polymerizations are due primarily to the AH factor. The entropy of polymerization of the carbonyl double bond in aldehydes is approximately the same as that for the alkene double bond. The enthalpy of polymerization for the carbonyl double bond, however, is appreciably lower. Thus AH for acetaldehyde polymerization is only about 29 kJ mol-1 compared to the usual 80-90 kJ mol-1 for polymerization of the carbon-carbon double bond (Table 3-14) [Hashimoto et al., 1076, 1978],... 

There are several reviews on alkane oxidation by Ru complexes (principally by RuO ) including [1-6]. Most of the oxidations catalysed are of C-H bonds C-C cleavage is considered in 4.2. We begin with what in principle is the simplest oxidation, that of the C-H bond in aldehydes and related substrates. 

A linear relation was found28 to exist between the logarithm of the equilibrium constant for the addition of water and other nucleophiles to quinazoline and the y-value of Sander and Jencks. The latter is an equilibrium measure of the avidity of a nucleophile to add across the carbonyl bond in aldehydes.29... 

The carbon-oxygen double bond in aldehydes and ketones is similar and can be described in either of these two ways. If we adopt the iocalised-orbital description, formaldehyde will have two directed lone pairs in place of two of the C-H bonds in ethylene. In this case the axes of these hybrid orbitals will be in the molecular plane (unlike the oxygen lone pairs in water). Either the components of the double bond or the lone pairs can be transformed back into symmetry forms. The alternative description of the lone pairs would he one er-type along the 0-0 direction and one jr-type with axis perpendicular to the 0-0 bond hut in the molecular plane. It is the latter orbital which has the highest energy, so that an electron is removed from it in. ionisation or excitation to the lowest excited state. 

Watanabe et al. [78] reported that the addition of C-H bonds in aldehydes to olefins took place efficiently with the aid of Ru3(CO)12 under a CO atmosphere at 200 °C (Eq. 50). In the case of the reaction with 1-hexene, a mixture of linear and branched ketones was obtained in 35% and 12% yields, respectively. To accomplish this reaction in a catalytic manner, the presence of carbon monoxide appears to be essential for suppressing the decarbonylation of aldehydes and for stabilizing the active catalyst species on the basis of the following observations ... 

The acidifying effect of the remaining acceptor substituents of Table 10.1 decreases in the order —C(=0)—H > —C(=0)—alkyl > —C(=0)—O-alkyl, and the amide group —C(=0)—NR2 is even less effective. This ordering essentially reflects substituent effects on the stability of the C=0 double bond in the respective C,H-acidic compound. The resonance stabilization of these C=0 double bonds drastically increases in the order R—C(=0)—H < R—C(=0)—alkyl < R—C(=0)—0—alkyl < R—C(=0)— NR2 (cf. Table 6.1 see Section 7.2.1 for a comparison between the C=0 double bonds in aldehydes and ketones).This resonance stabilization is lost completely once the a-H-atom has been removed by way of deprotonation and the respective enolate has formed. 

The most important functional groups that participate in chain-growth polymerizations are the carbon-carbon double bond in alkenes and the carbon-oxygen double bond in aldehydes and ketones. In such polymerizations the active species A adds to one atom of the double bond and produces a new active species on the other atom ... 

Addition of C-H Bonds in Aldehydes to C-C Multiple Bonds and Related Reactions... 

The strength of the labile C—H bond in aldehydes is 86.5 kcal. mole" (cf. secondary C—H = 94.5 kcal. mole" ). Since Z)[ROO—H] = 90 kcal. mole" reaction (32) will be exothermic by about... 

Other rhodium complex also catalyzed the addition of the C-H bond in aldehyde to olefins [115-117]. The use of paraformaldehyde results in the formation of aldehydes [115]. Marder et al. proposed the reaction mechanism of CpRh(eth-ylene)2-catalyzed addition of C-H bond in aldehyde to ethylene by the use of isotope-labeling experiments [117]. They suggested that insertion of ethylene to the Rh-H bond must take place rapidly and reversibly, and this equilibrium must be established significantly faster than either aldehyde reductive elimination or product formation (Scheme 4). 

HCI + CH2 CH2 CHaCHja An example of nucleophilic addition is the addition of hydrogen cyanide across the carbonyl bond in aldehydes to form cyanohydrins. Addition-elimination reactions are ones in which the addition is followed by... 

Effects that are invisible and unimportant for conformational equilibria can play key roles in reactivity. For example, the large n - 0, jj interaction in aldehydes evolves, upon the C-Fl bond scission, into a 2c,3e-bond in acyl radical. The latter effect manifests itself as the source of dramatic weakening of the aldehyde C-H bond dissociation energy ( 88 kcal/mol) - much smaller than the BDE for C-Fl bond in ethene ( 111 kcal/ mol). The difference is especially striking since both carbon atoms are sp hybridized and expected to have relatively strong C-H bonds. However, the C(0)-H bond in aldehydes is even weaker than a typical C-H bond in allcanes. This structural feature and resulting ease of acyl radical formation has important consequences for the stability and reactivity of aldehydes under radical conditions (Figure 6.38). 

Figure 6 Activation of C-H bonds in aldehydes by an Mdium complex within a tetrahedral metal-organic host in aqueous solution, (a) Crystal structure of the host, viewed down the C2-axis of S5munetry (b) simplified view, showing the structure of one of the six identical ligands that comprise the edges of the tetrahedral host (c) schematic representation of host and (d) proposed mechanism for C-H bond activation by an encapsulated Ir complex.

Two carbonyl groups in TM 4.8 are present in the 1,5-position one belongs to the terminal aldehyde and the other to cyclic ketone. Generally, the C-H acidity of the a-C-H bond in aldehydes is somewhat higher (pA a 17) than in ketones (pAa 20). The carbonyl C atom in aldehydes is a stronger electrophile than in ketones. Ketone enolates have more nucleophilic a-C atoms than aldehyde enolates. The former... 

Most [2+2] cycloaddition reactions of isocyanates across the C=0 bond in aldehydes, ketones andlV-substimted amides proceed with elimination of carbon dioxide to give imines 105. The initial cycloadducts cannot be isolated because elimination of carbon dioxide is instantaneous. The reaction of aryl isocyanates with carbonyl compounds requires higher temperatures, while sulfonyl isocyanates react at room temperature ". This reaction can... 

The sequence begins by the initial condensation of the amine with the carbonyl component (Section 17-9) to produce the corresponding imine (for NH3 and primary amines) or iminium ion (secondary amines). Similar to the carbon-oxygen double bond in aldehydes and ketones, the carbon-nitrogen double bond in these intermediates is then reduced by simultaneous catalytic hydrogenation or by added special hydride reagents. 

Carbon-Oxygen Double Bonds in Aldehydes and Ketones... 

FUNCTIONAL GROUPS THAT CONTAIN OXYGEN Carbon-Oxygen Single Bonds in Alcohols and Ethers Carbon-Oxygen Double Bonds in Aldehydes and Ketones Carbon-Oxygen Bonds in Carboxylic Acids and Esters... 

The aldehyde peak in C is sharp (because of no hydrogen bonding in aldehydes). [1]... 

For direct aerobic a, -dehydrogenation of aldehydes and ketones with a new Pd(TFA)2/4,5-diazafluorenone catalyst, the turnover-limiting step is a-C-H cleavage. Means of oxidative cleavage of C-C bonds in aldehydes by O2, cyclic ketones " by nicotinium dichromate, and a-hydroxy ketones by O2 have been discussed. 

Oxidation of individual compounds can also be completed under controlled conditions to produce molecules of commercial interest. You may have noticed that in alcohols there is a single C-0 bond, in aldehydes there is a donble C=0 bond, and in carboxylic acids there are three C-0 bonds (two from the C=0 donble bond and one from the C attached to the OH group). We can conclude that the oxidation of primary alcohols (where the -OH group is at the end of the hydrocarbon chain producing a straight chain and not a branch) gives aldehydes, which are then oxidized into carboxylic acids. 

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