(Chief Bee)
09-10-03 15:55
No 458173
(Rated as: good read)
Electrophilic Aromatic Substitution - Activation, Deactivation and Regioselectivity (../rhodium /eas-ra
Why does some aromatic compounds undergo substitution (bromination, nitration, alkylation etc.) much faster than others? Why are some unreactive and some so active that they polymerize? Why does Veratraldehyde (3,4-Dimethoxybenzaldehyde) brominate in the 6-position, while Vanillin (3-Methoxy-4-Hydroxybenzaldehyde) brominates in the 5-position?
It is all due to which groups are present on the ring to begin with - some are activating, some are deactivating the ring towards substitution. Also, some groups steer the substitution towards the so-called ortho/para* positions (usually these are also activating), while other steer the substitution towards the meta* positions (these are also as good as always deactivating, as in lowering the reaction rate compared to plain benzene).
* The positions one hop away from a substituent are said to be located ortho to it, the ones two hops away are meta to it, and the position three hops away (right across the ring) is in the para position.
(I'm Yust a Typo)
09-10-03 19:45
No 458197
This page isn't complete with a nice link to a table with hammett sigma constants, such as
http://wulfenite.fandm.edu/Data%20/Table
or
http://www.bluffton.edu/~bergerd/classes