Question

The -N(CH_3)_2 substituent is very activating towards EAS. Commenting on both the sigma-and pi- effects, explain why it is so activating. Describe the "directing" effect of the -N(CH_3)_2 substituent and justify it by drawing some resonance structures. As proof that it is very highly activated, we mentioned in class an EAS reaction which occurs without catalyst. Draw this reaction and the product for N,N-dimethylaniline. Despite being very highly activated toward EAS, most EAS reactions will fail for N,N-dimethylaniline. Describe why nitration will fail (first provide reaction conditions necessary for nitration). What will happen instead which will (largely) prevent nitration? Describe why Friedel-Crafts acylation will fail (first provide reaction conditions necessary for FC acylation with acetyl chloride, CH_3COCl). What will happen instead which will prevent acylation?

Answer 1

Solution:

Part (a)

-N(CH₃)₂ is very active in EAS (Electrophilic Aromatic Substitution), the resonance structure of cyclic compound favours for electrophilic substituition and -N(CH₃)₂ is not favoured in odd number C bonds as only sigma prevails on it, while in cycle both sigma and pi bond favours the stability.

Part (b)

CH₂O + 3 C₆H₅N(CH₃)₂ ? CH(C₆H₄N(CH₃)₂)₃ + H₂O

in the above example reaction it is clearly shown the directing effect over aromatic compound, in simple words its glued with aromatic compound as its resonance favors the stability of the compound.

Part (c)

RNHX RN-N-ox Tertiary aliphatic Amine N-Nitrosoammonium salt H3C + HCl + NaNO-ERC → H20 H3C p-Nitroso-N, N-dimethyl aniline

Part (d)

The nitration normally fails in N,N-Dimethylaniline for EAS due to lone pair of electrons in the structure which prevents nitration as it destabilise the resonance structure.

Part (e)

CI Cl-Fe-CI CI R H -HC catalyst regenerated

The is FC reaction, mechanism clearly shows the molecule replacement by halogens which normally is prevented by -N(CH₃)₂ and hence acylations fails.