= Solution
The <methoxy groups> activate the <aromatic ring> by electron donation through <chemical resonance> and direct electrophilic substitution ortho and para. Their para positions are already occupied by one another, so the first alkylation occurs at an available ortho position; the four initially vacant positions are equivalent by <symmetry>.
After placing the first <tert-amyl group> at position 2, \b[position 5 is favored for the second substitution]. It is para to that bulky group and ortho to the other <methoxy group>. Position 3 is crowded between methoxy and tert-amyl substituents, while position 6 is near the first <tert-amyl group> across the intervening methoxy-bearing <carbon>. The 2,5 arrangement distributes the bulky groups on opposite parts of the ring and preserves the equivalence seen in the spectrum.
Alkyl groups do not electronically deactivate the ring, so the stopping point needs a steric explanation. After two such substitutions the remaining positions are much more congested, each adjacent to a bulky <tert-amyl group> and a <methoxy group>. \b[Two large substituents are accommodated much more readily than a third], despite excess <alcohol>. The formula and <NMR> identify this sterically favored disubstitution; they do not imply that a <Friedel-Crafts reaction> always stops after two alkylations.
Back to article page
