Bonding considerations carbon compounds, comparison

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

There is considerable multiple character in the carbene carbon-X(Y) bonds. An internal comparison of carbon-X(Y) bond lengths in each of the structures tabulated reveals a substantial shortening of carbene car-bon-X(Y) relative to other carbon-X(Y) bonds in the molecule. Carbene carbon-nitrogen distances are found to be shorter in the mono-N-substituted carbenes than in the bis-N-substituted compounds. 

The first isolable alkenetitanium complex, the bis(pentamethylcyclopentadienyl)-titanium—ethylene complex 5, was prepared by Bercaw et al. by reduction of bis(penta-methylcyclopentadienyl)titanium dichloride in toluene with sodium amalgam under an atmosphere of ethylene (ca. 700 Torr) or from ( (n-C5Mc5)2Ti 2(fJ-N2)2 by treatment with ethylene [42], X-ray crystal structure analyses of 5 and of the ethylenebis(aryloxy)trimethyl-phosphanyltitanium complex 6 [53] revealed that the coordination of ethylene causes a substantial increase in the carbon—carbon double bond length from 1.337(2) A in free ethylene to 1.438(5) A and 1.425(3) A, respectively. Considerable bending of the hydrogen atoms out of the plane of the ethylene molecule is also observed. By comparison with structural data for other ethylene complexes and three-membered heterocyclic compounds, the structures of 5 and 6 would appear to be intermediate along the continuum between a Ti(11)-ethylene (4A) and a Ti(IV)-metallacyclopropane (4B) (Scheme 11.1) as... 

More intriguing, however, is the effect on 7-anti-positioned carbon atoms, which are shifted downfield considerably. The values given for 252 (202), 253 (371), 254 (372), 255 (374), 256 (374), 257 (375), and 258 (376) in Scheme 62 are calculated by comparison with the corresponding parent compounds (nor-bomane, noibomene, adamantane, diamantane, and benzohomoadamantene, respectively). A plausible explanation for this effect was given by Christl and co-workers (203,204,377), who invoked an interaction between an unoccupied Walsh orbital and the HOMOs of the C(2)-C(3) and C(3)-C(4) bonds (259). 

Part of the interest in fluorocarbon systems lies in a comparison of the chemistry, and particularly reaction mechanisms, of fluorocarbon derivatives with those of the corresponding hydrocarbon compounds. Indeed, such comparisons pose quite a strenuous test on our theories of organic chemistry. As will be seen, our understanding of the influence of carbon-fluorine bonds on reaction mechanisms has made considerable progress. Nevertheless, it must be emphasised that fluorocarbon derivatives present much more complicated systems than their corresponding hydrocarbon compounds because, in addition to effects arising from different electronegativities, the effect of the lone pairs of electrons of fluorine that are not involved in o-bonds must be taken into consideration. Furthermore, the relative importance of these effects seems to be very dependent on the centre to which the fluorine is attached. 

Examples of thermochemical considerations of cupric enolates include the study of the binding of Cu + with kojic acid (16), a cyclic a-ketoenol. Comparison was made between the divalent cations of U02 +, Cu +, Zn +, Ni +, Co +, Cd +, Ca + where these metals are listed in decreasing order of binding constants over 6 powers of 10. In this case carbon-bonded metal seems most unreasonable because it would ruin the chelation as well as any aromaticity in the pyrone ring. It is admittedly an assumption that pyrones are aromatic. There are no one-ring pyrones for which there are enthalpy of formation data for gas phase species, as opposed to the benzoannelated compounds coumarin (I7)i07a, I07b chromone (is) " " "and xanthone (19) . Plausible, but unstable, Cu(II) enolates eliminate copper and form the 1,4-dicarbonyl compounds as shown in equation 8. 

By contrast, consider the above reaction with R = HC=C. Depending on which enthalpy of formation value is chosen for chloroacetylene, we find an endothermicity of ca. 7 kJ mol" or an exothermicity of ca. 35 kJ mol". Either value clearly documents that the stabilization of chloroacetylene is considerably less than that of carbonyl chloride despite the considerably shorter C-Cl bond in the former. Bond lengths and bond strengths seemingly do not correlate for the above chlorinated species. We hesitate to suggest that it is inherently aproblem for chlorinated species as opposed to a too indiscriminate comparison involving compounds with such disparately hybridized carbons. 

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