In the first part of this experiment acetic anhydride was used to prepare acetanilide which could then be readily brominated to form a mono-brominated product, with the bromine positioned at either the ortho, meta or para position on the aromatic ring. Acetic anhydride is very reactive towards nucleophiles and this reactivity is the result of the difference in electronegativities of the carbon and oxygen atoms that are bonded in acetic anhydride. This difference in electronegativities causes one of the carbonyl groups in acetic anhydride to break its carbon-oxygen double bond with the oxygen atom taking the pair of electrons from the pi bond and results in a negative charge on oxygen and a positive charge on carbon. The positive charge on carbon is then stabilized by the donation of a lone pair of electrons from oxygen, which is attached to both of the carbonyl groups in acetic anhydride, and results in the formation of an O=C bond with the oxygen containing a positive charge. The formation of the oxygen with the positive charge causes the electrons in C-O bond to be pulled more closely to the oxygen and results in the carbon on the unaltered carbonyl group to be very electron deficient and this is the cause of the reactivity of acid anhydrides.
The first step involves the nucleophilic acyl substitution reaction of acetic anhydride with aniline to form acetanilide. The reaction is initiated by the donation of the lone pair of electrons on the nitrogen atom on aniline to the electron deficient carbonyl carbon atom in acetic anhydride, which is followed by the carbon oxygen double bond breaking in the electron deficient carbon with the oxygen taking the pair of electrons and resulting in a negative charge being present on oxygen and results in the formation of a tetrahedral intermediate. The next step involves the deprotonation of the positively charged nitrogen, which is then followed by reformation of the oxygen-carbon double bond and the loss of the...
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