Lateral fluoro substituents

When compared with compound 63, the lateral fluoro substituent of compound 77 causes a dramatic fall in the melting point and a similar reduction in the j is seen these... 

The lateral fluoro substituents in eompounds 77 and 78 are even more significant in then-effect on physical properties shown in Table 3.3, namely the dieleetric anisotropy (Ae). Here the Ae values are extrapolated from mixtures in I-euteetic (a commercially available nematic mixture based on homologues of compound 80). The Ae of the parent system is reasonably high (40.1) and the first lateral fluoro substituent (41.0) does little to enhance the positive value because the enhanced polarity is reflected in the parallel and... 

The parent material (79) has a high melting point and has a high smectic phase stability. The lateral fluoro substituent dismpts the lamellar packing and reduces the smectic phase stability by far more than the T j, which when combined with the reduced... 

In some stractural situations a lateral fluoro substituent may just reduce the T value by... 

The properties required of ferroelectric hosts therefore means that materials such as esters, Schiff s bases, and azo compounds are unsuitable because of their slow response times and poor stability. The best materials discovered so far, rely on removal of any functional or linking groups which might increase the viscosity or lower the stability. Figure 39 shows a variety of families of host materials [31,33,48-51 ]. These materials have a number of attributes in common. First, they are devoid of linking groups, thus the aromatic or heterocyclic rings in the core are directly linked. Second, only alkyl or alkoxy terminal chains are used in order to maintain as low a viscosity as possible. Third, some materials carry lateral fluoro substituents in order to increase... 

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. 

Esters are very common liquid crystal compounds, and lateral fluoro substitution has provided some interesting modifications to melting points, transition temperatures, mesophase morpholo and physical properties. Compounds such as 66 [26] allow for the gen ation of high positive dielectric anisotropy, but the fluoro substituent has caused a large reduction in the nematic phase stability when compared with compound 20, due to the reduction in antiparallel correlations. However, the fluoro substituent in compound 67 [26] is somewhat shielded by the zig-zag nature of the ester linkage hence the nematic-isotropic transition temperature is identical to that of compound 20. The lateral fluoro substituent in compound 68 [51] is not as shielded by the zig-zag structure and the nematic-isotropic transition temperature is much reduced, but not to the same extent as for compound 66. 

If the water content of the diazotization system is too high, the halogen atom in halogen-substituted mono- and dinitroanilines may be replaced by a hydroxy group in a bimolecular aromatic substitution. Analogous behavior was observed by Stacey s group30 in the diazotization of pentafluoroaniline, where the 4-fluoro substituent became hydrolyzed. Later, Sonoda and Kobayashi s group31 found that this side reaction does not take place if the diazotization is conducted in a dichloromethane-aqueous sulfuric acid two-phase system in the presence of tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate. 

Fig. 14 Monomers with laterally-attached mesogens featuring fluoro substituents...

Compound (VIII.98), a folic acid analogue with a y-fluoro substituent in the side-chain was described first by Alekseeva e/ al. [281] and several years later by Bergmann and Chun-Hsu [282]. The synthesis of (VIII.98), as mixture of threo and erythro isomers, was achieved via the Waller method from V-(4-ami-nobenzoyl)-y-fluoroglutamic acid, 2,4,5-triamino-6(lif)-pyrimidinone, and 2,3-dibromopropionaldehyde, but the yield was low (5.6%). y-Fluoroglutamic acid, as a mixture of D- and L-enantiomers, was prepared from diethyl 2-fluoromalonate by condensation with ethyl 2-acetamidoacrylate followed by hydrolysis and decarboxylation in refluxing 12 M HCl, or from ethyl 3-chloro-2-hydroxypropanoate by a sequence consisting of (i) 0-t-butylation with CH2 = C(CH3)2, (ii) condensation with diethyl 2-acetamidomalonate, (hi)... 

Lateral substituents may be attached to the core system or located in the terminal aliphatic chain. By and large, lateral substituents positioned on the core or in the terminal chain(s) disfavour smectic mesophase formation. Smectic mesophase formation is principally depressed more by the steric bulk of the lateral group than by its polarity. Thus for example, fluoro substituents at the core or in the chain(s) are less effective at depressing smectic mesophase formation than is a methyl substituent [14, 31], nevertheless fluoro substitution still lowers transition temperatures of the phase transition to the smectic state in comparison to the unsubstituted analogue. 

No simple rationalization for the observed values of the 19F resonances could be made for the peracetylated 2-deoxy-2-fluoro-D-gIuco-and -manno-pyranosyl fluorides, apart from the fact that the anomeric fluorine substituent always gives the lower field-resonance of the two198 this should, however, be compared with the explanation described287 (see later in this Section) for the observed differences in 19F... 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

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