Azoxy link

The azo group may be oxidized further, usually with a peracid such as 3-chloroper-benzoic acid to give azoxy linked materials examples include compounds 14-16 which are shown below [51, 54, 55], but only compounds 14 and 16 show chiral nematic phases. 

Structurally similar polymers with azo links, Polymers 9 and 10, formed no meso-phases when the rigid aromatic unit contained terminal carbonyl groups, but these formed nematic mesophases when the ester link was reversed The transition temperatures of the azo and azoxy polymers with the same spacer were quite close to each other. Substitution of methyl groups on the phenylene rings of these polymers reduced melting temperatures without adversely affecting nematic mesophase stability... 

Another type of unsaturated mesogenic structure, which is based on a structure in between an azo- and vinylene-type, is that in which a Schiff base unit links two phenylene groups. This unit is similar in geometry to the three previous mesogenic units, the azo, azoxy and stilbene groups The only reported example of this structure, however, was Polymer 8 with a decamethylene spacer, and it was quite similar in properties to the polymers just discussed. It also possessed a smectic phase, and the mesogenic unit had terminal carboxy groups. 

Azo Compounds.— The term azo is derived from the French word for nitrogen, viz., azote. Compounds designated by the name azo or some modification of it, e.g.y azo benzene, oxy azo benzene, amino azo benzene, hydrazo benzene, azoxy benzene, etc. represent a class of compounds in which two nitrogen atoms, each of which is linked to a separate benzene ring, are directly linked to each other by a double or single bond. 

More recently, Bouwman carried out a detailed study on the carbonylation of nitrobenzene in methanol with palladium bidentate phosphane complexes as catalysts [29-31]. After a careful analysis of the reaction, mixtures revealed that besides the frequently reported reduction products of nitrobenzene [methyl phenyl carbamate (MFC), A, 7/-diphenylurea (DPU), aniline, azobenzene (Azo) and azoxyben-zene (Azoxy)], large quantities of oxidation products of methanol were co-produced (dimethyl carbonate (DMC), dimethyl oxalate (DMO), methyl formate (MF), H2O, and CO). They proposed the Pd-imido species P2Pd = NPh, which is the central key intermediate that can link together all the reduction products of nitrobenzene and all the oxidation products of methanol into one unified mechanistic scheme. 

Examples of cyclic units are 1,4-phenylene, 2,6-naphthalene and fr s-1,4-cyclohexylene rings. The linking group connects the cyclic units on each side to give the chain a linear character. The preferred confirmation is planar (linear). A number of different groups fulfil these requirements direct bond, ester, imino, azo, azoxy, and methylol groups (semiflexible). 

Unsymmetrical azo- or azoxy-compoimds on reduction with tin and hydrochloric add produce a mixture of two amines, while hydrazo-compounds may yield amine(s) formed by normal reduction of the hydrazo-link and a diamine produced by a benzidine rearrangement. Nitro-samines, on reduction with tin and acid, give the secondary amines from which they were derived.)... 

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