The ortho effect also occurs when a meta-directing group is positioned in a meta arrangement relative to an ortho–para-directing group, a new substituent introduced into the molecule tends to preferentially occupy the ortho position relative to the meta-directing group rather than the para position.  Currently, there is no definitive explanation for the ortho effect, but it is hypothesized that there may be intramolecular assistance from the meta-directing group influencing the positioning of the incoming substituent. For example, the electrophilic aromatic nitration of 1-methyl-3-nitrobenzene affords 4-methyl-1,2-dinitrobenzene  and 1-methyl-2,3-dinitrobenzene in 60.1% and 28.4% yields, respectively. In contrast, 2-methyl-1,4-dinitrobenzene (2c) is isolated in only 9.9% yield. As witnessed in the above example, when a π-acceptor substituent (πAS) is meta to a π-donor substituent (πDS), the electrophilic aromatic nitration occurs ortho to the πAS rather than para.
Similar results were also observed on the nitration of 3-methylbenzoic acid in which 5-methyl-2-nitrobenzoic acid and 3-methyl-2-nitrobenzoic acid were obtained as the major compounds, whereas 3-methyl-4-nitrobenzoic acid was reported as a minor compound. Also in nitration of  the nitration of 3-bromobenzoic acid  5-bromo-2-nitrobenzoic acid (83%yield)  was obtained as major product and 3-bromo-2-nitrobenzoic acid (13% yield) as minor. On an interesting note the potential isomer 3-bromo-4-nitrobenzoic acid was not detected.