Benzene atom polarization

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene we con sider the electronic properties of a trifluoromethyl group Because of their high elec tronegativity the three fluorine atoms polarize the electron distribution m their ct bonds to carbon so that carbon bears a partial positive charge... 

A general account of dipole-moment measurements and the use of these in conformational analysis has been given.3 At the University of East Anglia, for a study of the conformational equilibria of piperidines,119 121 electronic polarizations were estimated from tabulated bond polarizations122 (neglecting contributions of atomic polarization to the total polarization) or determined from refractive-index measurements. Most measurements were carried out in benzene or cyclohexane. 

However, if we look at the LUMO, we find that it has the form shown in Fig. 3-19, namely that of ip% of benzene, but polarized by the nitrogen atom. This polarization has reduced the coefficient at C-3, and the coefficient at C-4 is larger than that at C-2 for example, a simple Hiickel calculation7 for pyridine itself gives values of 0-454 and -0-383 respectively, and an energy of 0-56/1 (compare benzene with 1 / for this orbital). Thus, soft nucleophiles should... 

Physical Measurements. Molecular weights were determined in "x 0.07 M benzene solutions at 37°C by vapor pressure osmometry. Dipole moments were measured in dilute benzene solution at 25. 00 + 0.05°C using a previously described method (12). Values of the atomic polarization, P2 for the admh and acac ligands were estimated from the total polarization, TP2, of trans-Pd(admh)2 (13) and Co-(acac)g. Assuming that Co(acac)3 and trans- Pd(admh) have Dg and symmetry, respectively, the orientation molar polarizations,... 

Homoadamantanes.—The benzene-photosensitized polar addition of acetic acid or pivalic acid to 4-homoadamantene (763) gave 2,4-dehydrohomoadamantane (764) and either the esters (765) and (767) or the esters (766) and (768). The products are obtained from the 4-homoadamantyl cation, a proton source being essential to the formation of (764) in AcOD the resulting (764) and (765) both contained onedeuterium atom in the expected positions. Diazotization of 4-homoadamantylamine in AcOH yielded (763), (764), and (767) which is in contrast to the products of buffered acetolysis of 4-homoadamantyl tosylate, namely (763) and (767). A vibrationally excited form of the 4-homoadamantyl cation in the photosensitized addition and in the diazotization reaction accounts for the observations except that it is not clear why compounds (765) or (766) are formed only in the photochemical reactions. 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

AEyl chloride reacts with sodamide in Hquid ammonia to produce benzene when sodamide is in excess, hexadiene dimer is the principal product, with some trimer and tetramer (C24, six double bonds). AEylation at carbon atoms alpha to polar groups is used in the preparation of a-aEyl-substituted ketones and nittiles. Preparation of P-diketone derivatives, methionic acid derivatives, and malonic ester, cyanoacetic ester, and P-keto-ester derivatives, etc, involving substitution on an alpha carbon between two polar carbonyl groups, is particularly facEe. 

These effects can be attributed mainly to the inductive nature of the chlorine atoms, which reduces the electron density at position 4 and increases polarization of the 3,4-double bond. The dual reactivity of the chloropteridines has been further confirmed by the preparation of new adducts and substitution products. The addition reaction competes successfully, in a preparative sense, with the substitution reaction, if the latter is slowed down by a low temperature and a non-polar solvent. Compounds (12) and (13) react with dry ammonia in benzene at 5 °C to yield the 3,4-adducts (IS), which were shown by IR spectroscopy to contain little or none of the corresponding substitution product. The adducts decompose slowly in air and almost instantaneously in water or ethanol to give the original chloropteridine and ammonia. Certain other amines behave similarly, forming adducts which can be stored for a few days at -20 °C. Treatment of (12) and (13) in acetone with hydrogen sulfide or toluene-a-thiol gives adducts of the same type. 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

The pyrrolidine enamines of /l -3.]cetosteroids (111), on alkylation with methyl iodide, gave mainly the N-alkylated product (5,55) in nonpolar solvents such as benzene. The reaction in more polar solvents gave the 4-methylated product (5.S). The reaction of (111) with perchloryl fluoride involves attack at the C-4 atom to give, after acid hydrolysis, either 4-fluoro-zJ -3-ketone (119) or 4,4-difluoro-zJ -3-ketone 120), depending on the reaction conditions (59). 

In non-polar solvents, the reaction with piperidine is best represented by a two-term kinetic form indicating a mixed 2nd- and 3rd-order reaction. Also, base catalysis by tri-ri-butylamine was observed. This kinetic pattern is strongly reminiscent of the results obtained with nitro-activated benzenes.Another interesting result is that stepwise replacement of chlorine atoms by amino groups results in marked... 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

Obviously, this shift implies the self-association of DMSO. Further frequency shifts to even lower wave numbers (1050-1000 cm " ) are observed in both aprotic polar and protic solvents. In aprotic solvents such as acetonitrile and nitromethane, the association probably takes place between the S—O bond of DMSO and the —C=N or the —NOz group in the molecules by dipole-dipole interaction as shown in Scheme 331,32. Moreover, the stretching frequency for the S—O bond shifts to 1051 cm 1 in CHC13 and to 1010-1000 cm -1 in the presence of phenol in benzene or in aqueous solution33. These large frequency shifts are explained by the formation of hydrogen bonds between the oxygen atom in the S—O bond and the proton in the solvents. Thus, it has been... 

Predict whether each of the following molecules is likely to be polar or nonpolar (a) C HjN (pyridine, a molecule like benzene except that one —CH— group is replaced by a nitrogen atom) (b) C2H6 (ethane) (c) CHC1, (trichloromethane, also known as chloroform, a common organic solvent and once used as an anesthetic). 

There are three different dichlorobenzenes, CgH4Cl2, which differ in the relative positions of the chlorine atoms on the benzene ring, (a) Which of the three forms are polar ... 

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