Para-difluorobenzene

Figure 15. Spectrum of para-difluorobenzene (a) high-resolution photoelectron spectrum [19] (b) ZEKE spectrum in the same energy range [21],...

From these data it can be pointed out that for a given size of the clusters the proton affinity of water is smaller than for the other solvents consequently, for fluorobenzene/methanol or para-difluorobenzene/water systems, a proton affinity of 205/215 kcal mol-1 seems to be the limit of the reaction process (it is reached for two molecules of methanol and three molecules of water). 

Carbon-boron clusters ( carboranes ) have been shown to react with Cr-coordinated halo-arenes. For example, reaction of 2 equiv. of LiC2BjoHio(CH3) with para-difluorobenzene-tricarbonylchromium complex in refluxing THF results in the displacement of both fluoride substituents from the arene ring to yield the para-phenylene compound, albeit in just 9 % yield owing to the effect of the steric bulk of the carborane [55]. 

At the equilibrium geometries of the neutrals, the symmetry assignments of these cationic doublet states, ordered by ascending vertical ionization potentials, are as follows. The obvious notations Bz+, E-Bz+, 1,2, 1,3, 1,4 and 1,2,3 correspond to the benzene, mono-fluorobenzene, ortho-, meta-, para-difluorobenzene isomers and 1,2,3-trifluorobenzene, respectively. (The numbers refer to the position of the... 

Figure 10.10 Plot of the dimer lifetime versus the total quantum number change Any in the predissociation reaction. The three points that are well below the line are due to (NOlj, (HF)2, and para-difluorobenzene-Ar (p-DFB). The latter does not fit because of Fermi resonance which mixes the zero order bright state of p-DFB. Taken with permission from Ewing (1987).

SURFACE TENSION OF PARA-DIFLUOROBENZENE, HEXAFLUOROBENZENE, AND SOME PARA-DIFLUOROBENZENE-PARA-XYLENE SOLUTIONS. 

Figure 3 In para-difluorobenzene, the vibrational structure of the cation was not fully resolved until the Introduction of ZEKE spectroscopy.

In contrast to the difluorobenzenes, the ortho isomer of the dihydroxy-substituted benzenes is the most stable followed by the meta and then the para isomer. The meta isomer is 0.9 kcal/mol above the ortho isomer and the para is 2.4 kcal/mol higher. The para/ortho ratio has been measured as 6 94 over HZSM-5 catalysts.2 Although the temperature at which this measurement was made was not originally reported, the ratio is approximately invariant over the range of 250-350°C. Taking 250°C as the temperature, we obtain AG (ortho-para) =2.9 kcal/mol. 

A variety of electronic effects control the relative energies of the disubstituted benzenes. For the difluorobenzenes, the meta and para isomers are of comparable energy the ortho isomer is clearly less stable. This is most likely due to the electrostatic repulsions of the two C-F bond dipoles which are aligned in the same direction. This is just like the difluoroethylenes where the 1,2-cis isomer is much less stable than the 1,1-isomer.poj. he dihydroxybenzenes. 

Although benzene itself absorbs at 7.3 ppm in its H NMR spectrum, the protons on substituted benzenes absorb either upfield or downfield from this value, depending on the substituent. Explain the observed values for the para disubstituted benzene derivatives X and Y. Then explain why p-difluorobenzene shows a single peak in its NMR spectrum at 7.00 ppm. 

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