Question

1) Offer a reasonable explanation of why nitration of 1,4-dichlorobenzene yields mononitro derivative while N,N'diacetyl- 1,4Phenylene- diamine forms dinnitro compound.

2) Explain why p- nitrophenol is a stronger acid than phenol itself. Would P-methoxyphenol be a stronger or weaker acid than phenol? Expain.

Answer 1

Answer-1)

Chlorine (halogens) shows 2 opposing effects viz. +R effect and –I effect of which +R being relatively effective activates the towards electrophilic attack but unequally to ortho and para position. There is more activation of para position than ortho position as it has less –I effect. In 1,4-Dichlorobenzene case para position is already occupied and only ortho position is available for electrophilic aromatic substitution. But being less activated shows a single nitration reaction though such a more 3 positions are available.

But, in case of N-acetyl derivative, because of deactivating nature of N-acetyl group the ring is deactivated towards electrophilic attack at ortho and para position. In case of 1,4 di N-acetyl derivative the symmetric pattern of deactivators makes the ring reactivity normal and phenyl ring thus undergo electrophilic attack at ortho (which are meta to other) and forms the nitro compound this compound then undergo further reaction to give nitration at anathor ortho position (C6) as shown in diagram. Anathor positions are sterically hindered.

Its difficult to have resonance of lone pair of N in ring as its engaged with C=O but once it gets the chance it will more better stabilize the intermediate carbocation than Cl can. Because its 3d-orbital of Cl which is involved in overlapping with 2p orbital of C and in case of N its 2p-orbital and 2P-2P orbital overlap is far better than 3d-2p overlap.

Hence the reactivity of 1,4 di N-acetyl derivative is more than 1,4 dichloro derivative.

Answer-2) Acid strength is governed by the tendency of compound to donate proton and this in turn depend on stability of conjugate base thus generated. The conjugate base if getting more stability, the protons remains available in solution for more time and hence we can say more acidic is the compound.

In case od p-Nitrophenol the conjugate base p-Nitrophenoxide is well stabilized then phenoxide because of more numbers of canonical forms and NO2 showhs strong –I effect also hence p-Nitrophenol is stronger acid than phenol.

p-Methoxy phenol is less acidic than phenol as oMe has less –I but fr more +R effect which gives very less stability to conjugate base.

Hence increasing acidity order is p-Methoxyphenol < Phenol < p-Nitrophenol