Lateral or side-on substitution

Another interesting example is 1,4-phenylene dibenzoate (3.10) and its vinyl substituted homologue 2-vinyl-1,4-phenylene dibenzoate (3.11) (Zhou et al, 1993)  

It is also interesting to mention that the compound (3.11) is the first published example that has lateral but no terminal substitutions and yet forms a liquid crystal phase. By and large, the studies on substitution effect have shown that while the terminal substitutions are favorable to the thermal stability and the formation of a liquid crystal phase, the lateral substitutions are not. The larger the lateral substitution, the more harm it can do to the thermal stability of the liquid crystalline phase. A typical and very convincing example for this idea is given by homologues of the well known para-substituted azoxyanisole (3.12)  

Para-azoxyanisole is the terminally substituted homologue. It melts at 118 °C and forms a nematic phase. The clearing temperature is 135 °C. However, its laterally substituted meta- and ortho-forms both are nonliquid crystals. The meta-form melts at 51 °C, the ortho-form melts 

In this section we have discussed several structural aspects of liquid crystal compounds. However, these are not the only aspects that have a significant impact on the formation and property of liquid crystals. For 

---