Ortho, para, meta -Directing group

The entrance of a third or fourth substituent can be predicted by Beilstein s rule. If a substituent Z- enters into a compound C H XY, both X and Y exert an influence, but the group with the predominant influence directs Z- to the position it will occupy. Since all meta-directing groups are deactivating, it follows that ortho—para activating groups predominate when one of them is present on the benzene ring. 

Ortho effect In electrophilic substitution of aromatic rings that already have two substituents, one of which is a meta directing group and that positioned meta to an ortho/para directing group, then the new substituent tends to go to the ortho position. In mass spectrometry, the ortho effect is the loss of a neutral species, usually a stable molecule such as water, derived from the ortho-... 

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