C O single bonds

C—O single bonds (alcohols, epoxides, ethers, acetates, etc.). 

Ethers are difficult to identify by IR spectroscopy. Although they show an absorption due to C-O single-bond stretching in the range 1050 to 1150 cnr1, many other kinds of absorptions occur in the same range. Figure 18.3 shows the IR spectrum of diethyl ether and identifies the C-0 stretch. 

Tables 2.3 and 2.4 list a selection of typical dissociation energies. The values given in Table 2.4 are average dissociation energies for a number of different molecules. For instance, the strength quoted for a C—O single bond is the average strength of such bonds in a selection of organic molecules, such as methanol (CH3—OH), ethanol (CH,CH2—OH), and dimethyl ether (CH,—O—Cl l5). The values should therefore be regarded as typical rather than as accurate values for a particular molecule.

There are many other molecules in which some of the electrons are less localized than is implied by a single Lewis structure and can therefore be represented by two or more resonance structures. For example, the three bonds in the carbonate ion all have the same length of 131 pm, which is intermediate between that of the C—O single bond in methanol (143 pm) and that of the C=0 double bond in methanal (acetaldehyde) (121 pm). So the carbonate ion can be conveniently represented by the following three resonance structures ... 

Since this can formally be viewed as an addition of M-H to C=0, the bond order is reduced in this case to a C-O single bond. Reductive ehmination generates the hydrogenated product and an unsaturated metal complex that subsequently re-enters the catalytic cycle. Many subtleties of this mechanism have been delineated in studies of hydrogenations of C=C and C=0 bonds, and catalysts that follow this mechanism have been very successful. 

The structures of a- and /3-D-glucopyranose may be compared with that of methyl a-D-glucopyranoside.26 In the latter, the C-l-O-1 bond is shorter than a C-O single bond, but not as short as in a-D-gluco-pyranose. Part of this shortening may affect the C-O bonds of the ring carbon atoms, as the C-l-O-5 bond is slightly shorter than the C-5-0-5 bond. 

Fig. 8 General structure of semi-flexible oligobenzoate Hekates 34—36. (a) These stars may consist of three identical arms (34), two (35) or three (36) different arms. Thereby the length (n,m,l), the peripheral chains (R, R", R "), the linking groups X (OOC, COO, CONH) to the core and the substituents Y (H, I) can he adjusted, (b) Borderline conformers - star-shaped, f.-shaped and cone-shaped conformers - for a non-symmetric oligobenzoate scaffold (X = OOC). They can be created by rotation about the C-O single bond within the carboxy linking group to the core...

The carbonyl bond is shorter than a C-O single bond (1.22 A vs. 1.43 A) and is also stronger since two bonds are present as opposed to one (732 kj mol-1 vs. 385 kj mol-1). The carbonyl group is more reactive than a C-O single bond because of the relatively weak n bond. 

Selenium dioxide is able to a-oxygenate ketones via their enol tautomers. As is demonstrated in Figure 12.10 by the reaction of selenium dioxide with cyclohexanone, the actual electrophilic substitution product C is unstable. The latter contains selenium in the oxidation state +2 that takes the opportunity to transform into selenium in the oxidation state 0, i.e., elemental selenium, by way of the fragmentation reaction indicated. Thereby, the a-C O single bond of the primary product C is transformed into the a-C=0 double bond of the final product B (which, however, is largely present as the tautomeric enol A). 

The putative cation that would be generated by C—N bond cleavage in D would contain an O atom with a very weak pi electron donating ability (+M effect). This is because the z BUjAl group, which binds to this O atom, acts as a Lewis acid. Consequently, the cation in question would be an acceptor-substituted carbenium ion (with a C—O single bond) rather than an oxocarbenium ion (with a C=0 double bond). 

The correlation signals of the INADEQUATE experiment directly build up the ring skeleton A of the compound. Here characteristic 13C shifts (8C = 123.1, 137.6 148.9, 109.1) establish the existence and position of two double bonds and of one tetrahedral C-O single bond (Sc = 70.5). DEPT spectra for the analysis of the CH multiplicities become unnecessary, because the INADEQUATE plot itself gives the number of CC bonds that radiate from each C atom. 

The region of C-O single bond vibrations at 1300— 1100 wavenumbers with a significant selectivity for different groups when combined with the simultaneously occurring C=0 vibrations. 

Calculate the amount of charge separation for a typical C —O single bond, with a bond length of 1.43 A and a dipole moment of 0.86 D. 

See how similar these two mechanisms are. In particular, they are the same at the carbonyl group itself. Electrons move into the C=0 n orbital the C=0 bond becomes a C-O single bond as a negative charge develops on the oxygen atom. It should come as no surprise that the order of reactivity for enolization is the same as the order of reactivity towards nucleophilic attack. 

Furan is not very aromatic and if there is the prospect of forming stable bonds such as C-O single bonds by addition, this may be preferred to substitution. A famous example is the reaction of furan with bromine in methanol. In nonhydroxylic solvents, polybromination occurs as expected, but in MeOH no bromine is added at all ... 

The C-O single bond of a carboxylic acid is shorter than the C—O single bond of an alcohol. This can be explained by looking at the hybridization of the respective carbon atoms. In the alcohol, the carbon is sp hybridized, whereas in the carboxylic acid the carbon is sp hybridized. As a result, the higher percent -character in the sp hybrid orbital shortens the C-O bond in the carboxylic acid. 

Experimental data support this resonance description of acetate. The acetate anion has two C-O bonds of equal length (1.27 A) and intermediate between the length of a C- O single bond (1.36 A) and C=0 (1.21 A). 

Kabalka and co-workers have shown that an attenuated form of BI3, BI3 NEt2Ph, will cleave a variety of compounds containing C-O single bonds at elevated temperatures [20]. Solutions of this reagent are prepared by reacting the commercially available amine-borane complex with I2 in benzene at 80 °C for several hours. This reagent cleaves ethers [21], esters [20], and geminal diacetates [21]. Esters [20] are cleaved to an activated acyl intermediate RCOX which can be used to prepare acids, other esters, and amides (Eq. 10). 

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