The Orientation of Substituents in Aromatic Molecules

The Orientation of Substituents in Aromatic Molecules.—When a substituent is introduced into an aromatic molecule it may enter into certain of the available positions more readily than into others. This phenomenon has been extensively studied, and empirical rules have been formulated that describe the experimental results fairly satisfactorily. 

During the last 15 years a qualitative theory has been developed22 that accounts satisfactorily for the phenomenon in its major features, and a quantitative treatment based on quantum mechanics has been 

30 A comparison of the two methods of quantitative discussion of aromatic molecules has been published by Q. W. Wheland, J. Chem. Phys. 2V 474 (1934) see also G. VV. Wheland, Resonance in Organic Chemistry, John Wiley and Sons, New York, 1955. 

Substitution by an electrophilic reagent is thus assumed to take place preferentially at that carbon atom on which the negative charge is the largest. This assumption is a reasonable one, in view of the electronseeking character of these reagents. 

There are two principal ways in which the charge distribution can be affected by the group R, for each of which it has been assumed, and has been verified by quantum-mechanical calculations,24 that the ortho and para carbon atoms are about equally affected, the meta carbon atoms being affected to a much smaller extent. 

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Wheland, G. W. A Quantum Mechanical Investigation of the Orientation of Substituents in Aromatic Molecules. J. Amer. chem. Soc. 64, 900 (1942). 139- — The Theory of Resonance. New York Wiley 1944. 

Wheland GW (1942) A quantum mechanical investigation of the orientation of substituents in aromatic molecules. J Am Chem Soc 64 900-908... 

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