Substitution at Alkenyl and Aryl

Problems 2.7. Draw mechanisms for the following two aromatic substitution reactions. Note The second problem will be much easier to solve if you draw the by-product. 

Nucleophilic substitution can also occur at aryl rings not substituted with electron-withdrawing groups, although it is not seen as often. Two mechanisms are possible SrnI and elimination-addition. The SrnI mechanism is a radical chain mechanism. (The R in SrnI stands for radical.) As such, it has three parts initiation, propagation, and termination. Consider the following reaction  

One can draw several resonance structures for the radical anion. This text uses an unusual resonance structure for the radical anion [R -X] in which there is a three-electron, two-center bond between R and X. This resonance structure suggests that the odd electron is localized in the R—X cr orbital, making half a bond between R and X. The toilet bowl resonance stmcture is probably a better description of the ground state, reflecting the fact 

A curved arrow is not used to show an electron transfer. 

The propagation part of an SrnI reaction has three steps. In the first step, dissociates into X and -R. The departure of X is facilitated by the electron in the antibonding tr orbital. 

Aromatic compounds that are substituted with electron-withdrawing groups (usually nitro groups) undergo nucleophilic aromatic substitution reactions by this mechanism. Because addition of the nucleophile to the ring disrupts the aromaticity, two electron-withdrawing groups are usually required to make the aro- 

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