Chemistry Reference
In-Depth Information
collapse of the anionic intermediate with loss of N-methyl-N-methoxyamine is pre-
vented. In Problem 6(f ), collapse of the anionic intermediate led to regeneration of
a carbonyl capable of reacting with a second methyllithium. In this example, this
does not happen, and quenching with hydrochloric acid allows exclusive formation
of the ketone shown. This process is illustrated below using arrow pushing.
Just as double bonds possess nucleophilic characteristics, so do aromatic rings. By
analyzing the charge distribution around an aromatic ring, sites of partial positive
charge and sites of partial negative charge can be identified. The sites of partial
positive charge are electrophilic in nature, and the sites of partial negative
charge are nucleophilic in nature. The partial charge distribution for methoxyben-
zene was the subject of Problem 2(h) from Chapter 1 and is shown below.
Having identified the nucleophilic sites, this mechanism now becomes an
addition-elimination reaction between methoxybenzene and acetyl chloride
where methoxybenzene is being added and chloride is being eliminated. As
shown below, using arrow pushing, electron movement starts with the methoxy
oxygen and moves through the aromatic ring. The addition-elimination steps occur
as shown in Problem 6(a). Finally, due to the conjugated and charged system, the
proton present on the reactive carbon atom of the phenyl ring becomes acidic. Loss
