Oxygen bonds, carbon

Clearly such bonding would produce two different carbon-oxygen bond distances (p. 48) but in fact all bonds are found to be identical and intermediate in length between the expected C=0 and C—O bond distances. We conclude, therefore, that the true structure of the carbonate ion cannot be accurately represented by any one diagram of the type shown and a number of resonance structures are suggested (p. 50). 

Carbon dioxide has a linear structure. The simple double-bonded formula, however, does not fully explain the structure since the measured carbon-oxygen bond lengths are equal but intermediate between those expected for a double and a triple bond. A more accurate representation is, therefore, obtained by considering carbon dioxide as a resonance hybrid of the three structures given below ... 

All the carbon-oxygen bonds are found to be of equal length and intermediate between carbon-oxygen single and double bond length. 

Carbon-oxygen bonds are formed by the Ullmann reaction (- coupling of aryl halides with copper) which has been varied in alkaloid chemistry to produce diaryl ethers instead of biaryls. This is achieved by the use of CuO in basic media (T. Kametani, 1969 R.W. Dos-kotch, 1971). 

Oxidation of carbon corresponds to an increase in the number of bonds between carbon and oxygen or to a decrease in the number of carbon-hydrogen bonds Conversely reduction corresponds to an increase in the number of carbon-hydrogen bonds or to a decrease in the number of carbon-oxygen bonds From Table 2 4 it can be seen that each successive increase m oxidation state increases the number of bonds between carbon and oxygen and decreases the number of carbon-hydrogen bonds Methane has four C—H bonds and no C—O bonds car bon dioxide has four C—O bonds and no C—H bonds... 

Primary alcohols do not dehydrate as readily as secondary or tertiary alcohols and their dehydration does not involve a primary carbocation A proton is lost from the (3 carbon m the same step m which carbon-oxygen bond cleavage occurs The mechanism is E2... 

In the acid catalyzed dehydration of 2 methyl 1 propanol what carbocation would be formed if a hydride shift accompanied cleavage of the carbon-oxygen bond in the alkyloxonium lon" What ion would be formed as a result of a methyl shift" Which pathway do you think will predominate a hydnde shift or a methyl shift" ... 

Step 3 Carbon migrates from boron to oxygen displacing hydroxide ion Carbon migrates with the pair of electrons m the carbon-boron bond these become the electrons m the carbon-oxygen bond... 

Typical carbon-oxygen bond distances m ethers are similar to those of alcohols (—142 pm) and are shorter than carbon-carbon bond distances m alkanes (—153 pm) An ether oxygen affects the conformation of a molecule m much the same way that a CH2 unit does The most stable conformation of diethyl ether is the all staggered anti conformation Tetrahydropyran is most stable m the chair conformation—a fact that has an important bearing on the structures of many carbohydrates... 

Thus epoxides like cyclopropanes have significant angle strain They tend to undergo reactions that open the three membered nng by cleaving one of the carbon-oxygen bonds... 

Their polar carbon-oxygen bonds and the presence of unshared electron pairs at oxygen contribute to the ability of ethers to form Lewis acid Lewis base complexes with metal ions... 

A long standing method for the preparation of ethers is the Williamson ether synthesis Nucleophilic substitution of an alkyl halide by an alkoxide gives the carbon-oxygen bond of an ether... 

Just as the carbon-oxygen bond of alcohols is cleaved on reaction with hydrogen halides (Section 4 8) so too is an ether linkage broken... 

With mixed ethers of the type ROR the question of which carbon-oxygen bond... 

The experimental observations combine with the principles of nucleophilic substi tution to give the picture of epoxide ring opening shown m Figure 16 5 The nucleophile attacks the less crowded carbon from the side opposite the carbon-oxygen bond Bond... 

Step 2 Nucleophilic attack by water on carbon of the oxonium ion The carbon-oxygen bond of the ring is broken in this step and the ring opens... 

Section 16 3 The carbon-oxygen bond of ethers is polar and ethers can act as proton acceptors m hydrogen bonds with water and alcohols... 

The structural features of the carboxyl group are most apparent m formic acid Formic acid IS planar with one of its carbon-oxygen bonds shorter than the other and with bond angles at carbon close to 120°... 

Likewise the experimentally measured pattern of carbon-oxygen bond lengths m acetic acid is different from that of acetate ion Acetic acid has a short C=0 and a long C—O distance In ammonium acetate though both carbon-oxygen distances are equal... 

The answer to this question is critical because it tells us whether the carbon-oxygen bond of the alcohol or a carbon-oxygen of the carboxylic acid is broken during esterification... 

The carbon-oxygen bond broken m the process is therefore the one between oxygen and the acyl group The bond between oxygen and the ethyl group remains intact An 8 2 reaction at the ethyl group would have broken this bond... 

As we ve seen on a number of occasions bonds to sp hybridized carbon are shorter than those to sp hybridized carbon and the case of phenols is no exception The carbon-oxygen bond distance in phenol is slightly less than that in methanol... 

In resonance terms the shorter carbon-oxygen bond distance in phenol is attrib uted to the partial double bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring... 

The cleavage of dialkyl ethers by hydrogen halides was discussed in Section 16 8 where It was noted that the same pair of alkyl halides results irrespective of the order in which the carbon-oxygen bonds of the ether are broken... 

In presence of—C=C— or of aromatic ring In presence of—C=0 bond In presence of two carbon-oxygen bonds In presence of two carbon-carbon double bonds Aryl-C=0... 

The amyl alcohols undergo the typical reactions of alcohols which are characterized by cleavage at either the oxygen—hydrogen or carbon—oxygen bonds. 

A weakened carbon—oxygen bond results from the combined ( - and TT-bonding, allowing the metal-bonded carbon monoxide to react more readily... 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

Benzyl and alkyl tnalkylsilyl ethers undergo clean fluonnation to give good yields of benzyl and alkyl fluorides, respectively, when reacted with a combination of d quaternary ammonium fluoride and methanesulfonyl orp- toluenesulfonyl fluoride. The reactions are applicable strictly to a primary carbon-oxygen bond, secondary and tertiary alkyl silyl ethers remain intact or, under forcing conditions, aie dehydrated to olefins [29] (equation 22)... 

In contrast to phosphorus esters, sulfur esters are usually cleaved at the carbon-oxygen bond with carbon-fluorine bond formation Cleavage of esteri nf methanesulfonic acid, p-toluenesidfonic acid, and especially trifluoromethane-sulfonic acid (tnflic acid) by fluoride ion is the most widely used method for the conversion of hydroxy compounds to fluoro derivatives Potassium fluoride, triethylamine trihydrofluoride, and tetrabutylammonium fluoride are common sources of the fluoride ion For the cleavage of a variety of alkyl mesylates and tosylates with potassium fluoride, polyethylene glycol 400 is a solvent of choice, the yields are limited by solvolysis of the leaving group by the solvent, but this phenomenon is controlled by bulky substituents, either in the sulfonic acid part or in the alcohol part of the ester [42] (equation 29)... 

The carbon-oxygen bond is strong, like the normal carbon-fluorine bond, it is difficult to reduce However, some structural features facilitate its reductive cleavage Aryl esters of perfluoroalkanesulfomc acids can be cleaved in good yield by... 

Most of the material presented in this section are reactions of sulfur trioxide. This compound is ambivalent and frequently forms a carbon-sulfur bond (true sulfonation), but it can form a carbon-oxygen bond as well. Examples of both types of bonding are included... 

Additions Forming Carbon-Oxygen Bonds by L. G. Sprague... 

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