Halogen general rule

Twenty years later, by studying the chlorination of cis- and trans-di-t-butylethylenes, Fahey (1966) showed that halogen bridging is a general rule. It is expected that, owing to steric repulsions between the two branched groups, the cis-alkene would prefer to react via an open / -chlorocarbocation [14] where free rotation can occur rather than via a chloronium ion [15]. 

Cotton-polyester blends For blends sometimes finishes, which are successful on one type of fiber, may prove to have antagonistic effect on blends.3 The general rule is to apply finish suitable to the majority fiber present or apply halogen-based coating (see Section 24.5.1.2). Most nondurable finishes for cotton will function on cotton-rich blends with polyester, but not for the polyester rich blend unless some bromine is present.3 Durable treatments such as Proban and Pyrovatex type finishes can be applied on higher cotton content fabrics, e.g., 80 20 cotton/polyester. 

The halobenzenes are exceptions to the general rules. Halogens are deactivating groups, yet they are ortho, para-directors. We can explain this unusual combination of properties by considering that... 

As a general rule in enzymatic reductions, the 1,4-reduction of enones is preferred over the 1,2-reduction mode. However, when an electronegative substituent, such as halogen, is introduced that stabilizes the double bond, enzymatic reduction to allylic alcohols may be achieved. ... 

Examination of reactions that involve attack not only by halogen atoms but by other free radicals as well has shown that this is a general rule benzylic hydrogens are extremely easy to abstract and thus resemble allylic hydrogens. We can now expand the reactivity sequence of Sec. 6.22 ... 

---