Cyano, terminal substituents

The tetramethylene is a resonance hybrid of 1,4-diradical (, = y in Eq. (22)) and zwitterionic (, = +,—) limiting structures. The character of the tetramethylene is determined by the nature of the terminal substituents. A very strong donor substituent at one of the terminal carbons and a very strong acceptor substituent at the other leads to zwitterionic intermediates. Otherwise, for instance, phenyl or vinyl group at the donor terminal and diester, cyano-ester or anhydride at the acceptor terminal, will favor the diradical form. 

For many terminal substituents, and especially for cyano compounds, the magnitude of ye is much less than expected. This, and other effects, are probably due to antiparallel ordering of the polar molecules, which is reduced by a lateral fluoro substituent adjacent to the cyano group (Table 2.5). The extent of antiparallel ordering (for example, in a mixture) cannot be accurately predicted, and the dielectric anisotropies of mixtures are, as a result, also difficult to predict precisely. 

X indicates a small substituent, which may be an atom such as hydrogen (H) or chlorine (Cl) or it may he a group such as methyl (CH3), cyano (CN), carhoxyl (COOH), carbomethoxy (COOCH3), etc. The growing chain is terminated by collision with another chain or other radical source or by one of several other mechanisms. The number of monomer units in the polymer chain is the degree of polymerization, abbreviated DP. If the degree of polymerization is very low, the product is sometimes referred to as an oligomer. 

Substitution at both terminal diene-synthon positions is allowed only if the substituent is a primary atom or a triply bonded functional group (such as a cyano group). 

At least one terminal chain, such as an alkyl (R) or alkoxy chain (RO) is attached to the molecular core. The second group may be a similar group or a small substituent such as a halogen atom (F, Cl, Br, I) or a cyano (CN), isothio-cyanato (NCS), dimethylamino (N(CH3)2) or nitro (NO2) group or a fluorinated group such as trifluoromethyl (CF3), trifluoromethoxy (CF3O), difluoro-methoxy (CF2HO), etc. Compounds with two short terminal alkyl or alkoxy chains tend to exhibit a nematic phase. Mesomorphic compounds with short... 

The MO data for cyano-, formyl- and ethynylacetylene (Figure 6) fail to indicate the correct positional isomer. Instead of C, attack, C/a or Michael addition products are the rule for these terminal alkynes (equations 32-34). Of course, when nucleophilic attachment occurs at C j, the substituents are capable of stabilizing incipient... 

Characterization of surface-functionalized dendritic macromolecules was achieved by the same methods and techniques as described previously. However, further information could be obtained from the H and 13C NMR, and in the cyano case, from the infrared spectra. By observation of the unique resonances for the functionality itself or the phenyl ring to which it is attached, the number of functional groups at the chain ends could be quantified. For example, the H NMR spectrum of 13 shows an ABq (7.46 and 7.61 ppm) at lower field to the resonance for the terminal phenyl rings (7.30-7.40 ppm). This unique and distinct set of resonances is due to a p-cyano-substituted phenyl ring. By comparing integration values, it was confirmed that only one of the exterior 48 phenyl rings carries a cyano substituent. Similar results were obtained for the brominated derivative 16. 

E.Z -photoisomerization of the fluorinated l-phenyl-4-carboalkoxybutadienes 21 proceeds regioselectively about the phenyl-substituted double bond, consistent with the Dauben intermediate . Saltiel and coworkers have reported a particularly interesting example of solvent- and substituent-dependent regioselectivity in the E.Z-photoisomerization of , , -l,6-diaryl-l,3,5-hexatrienes (22) the quantum yields for isomerization about the central double bond in the parent molecule (22 X = H) are insensitive to solvent polarity, while those for isomerization about a terminal bond are enhanced seven-fold in acetonitrile compared with hydrocarbon solvents". Isomerization of the substituted derivatives proceeds regioselectively to yield the 1-Z and 3-Z isomers, and in both cases the quantum yield for terminal bond isomerization is enhanced dramatically in polar solvents. The results were explained in terms of competing torsional relaxation about the central and terminal bonds via biradicaloid and zwitterionic twisted intermediates, respectively. For the parent and cyano derivatives, the latter is polarized in the same sense proposed by Dauben and Ritscher. Interestingly, the polarization appears to be reversed in the case of the methoxy derivative. 

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