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Ester Definition: An ester is an organic compound where the hydrogen in the compound's carboxyl group is replaced with a hydrocarbon group. Examples: Ethyl acetate is an ester. The hydrogen on the carboxyl group of acetic acid is replaced with an ethyl group. Article. Chemistry.
Electrophilic aromatic substitution (SEAr) is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile.oth the regioselectivity and the speed of an electrophilic aromatic substitution are affected by the substituents already attached to the benzene ring. In terms of regioselectivity, some groups promote substitution at the ortho or para positions, while other groups increase substitution at the meta position.