Question

3.) (12 points) For the following electrophilic aromatic substitiution reaction: 02N. CN CN CN HNO3 H2SO4 O2N в a) Which product is the major one? b) Draw a complete mechanism for the formation of the major product, labelling the rate-determining step. c) Referring back to the mechanism you drew in part b, explain your choice of part a

Answer 1

Rate of a electrophilic aromatic substitution reaction reaction depends on the groups which are already bonded with the aromatic ring. In electrophilic substitiution reaction the electron density on the benzene ring attack the electrophile and bond forms i.e higher the electron density on the ring, higher the rate. Now presence of electron donating group( -OH, -NH₂, Alkyl group etc.)

increases the electron density on the ring i.e such group activate the aromatic ring towards electrophilic substitution. Again electron withdrawing group (-NO₂, -Cl, -Br, -COR etc. ) deactivates the ring.

In this case the reactant is benzonitrile. It contains a -CN group in the benzene ring. It is electron withdrawing group. The resonating structure of benzonitrile is

Resonating structure of benzonitrile

From the resonating structure we see that -CN group withdraw electron mainly from the ortho and para position. i.e meta position has comparatively higher electron density than ortho and para position. Therefore in electrophilic substitution meta position will attack the electrophile and gives meta product as major product.

Nitration of benzonitrile shown below

C CN ANO3 NO2 NO2 Meehanism ONO2 H20 H-oNO :No7 (Slanr nee) Pata dudenmining Step Thin is themchanism rfeteshon o met-ptuct Sinlorly pa podvet olso phne ut it miemot P Rear iisusd abore