As far back as 1894 Victor Meyer (1) reported research on the effect of an ortho substituent influencing the reactivity of an active group. He observed among other things that methyl, chloride, bromide, or iodide substituents produced a steric hindrance effect roughly proportional to the molecular weight of the substituent. He also observed that the nitro group had an effect out of proportion to its weight, which indicated that its chemical nature also had a bearing. Cases have been reported in which the nitro group had an accelerating effect rather than a hindering one (2).
Proceedings of the Iowa Academy of Science
©1938 Iowa Academy of Science, Inc.
Nitz, Otto and Raiford, L. Chas.
"The Effects of Substituents on the Reactions of Certain Vanillin Derivatives,"
Proceedings of the Iowa Academy of Science, 45(1), 127-132.
Available at: https://scholarworks.uni.edu/pias/vol45/iss1/24