Activating Strengths

The objective of this lab was to determine the activating strengths of the directing groups in aniline, phenol, acetanilide and anisole by using melting point to identify the products of their electrophilic aromatic bromination reactions with pyridinium tribromide in glacial acetic acid.
The amount of substitution in an aromatic compound in a electrophilic aromatic bromination reaction is dependent upon the reactivity of the aromatic compound itself, which is determined by its substituents’ inductive effects. In the case of this experiment, four different aromatic compounds were used, containing either -NH2, -OH, -OCH3, and -NHCOCH3 as substituents. Of these four, the most activating substituent is -NH2 in aniline because nitrogen is less electronegative than the oxygens in -OH and -OCH3 in phenol and

This is beneficial for reactivity because the nitrogen in -NH2 in aniline is able to delocalize the positive charge of a carbocation by donating its electrons to the carbon during the transition state when aniline is brominated. The next strongest substituent would be -OH in phenol, followed by -OCH3 in anisole. This is because the three hydrogens on the carbon make the carbon slightly electronegative, slightly pulling away the electrons surrounding the oxygen directly attached to the benzene ring. The least reactive substituent would be -NHCOCH3 in acetanilide because the highly electronegative oxygen pulls away electrons from the nitrogen directly attached to the benzene ring, making the nitrogen less willing to stabilize the carbocation in the transition state in an electrophilic aromatic substitution reaction. Since all of the substituents are orth, para-directos, bromine in a bromination reaction would be substituted at either the 2 carbon, 4 carbon, 2 and 4 carbon, 2 and 6 carbon, or 2, 4, and 6 carbon. The more reactive the compound, the more substitution