Write the correct order of rate of reaction of following compounds with $PhN_2Cl$ P: $N,N$-dimethylaniline Q: $N,N$-dimethyl-3-methylaniline R: $N,N$-dimethyl-2,6-dimethylaniline
Rate of electrophilic substitution in substituted anilines and Steric Inhibition of Resonance (SIR effect).
LOGIC: 1. Coupling with $PhN_2Cl$ is an electrophilic aromatic substitution. 2. Reactivity increases with electron density on the ring. The $-NMe_2$ group is strongly activating (+M). 3. In compound R, the two ortho-methyl groups cause 'Steric Inhibition of Resonance'. The $-NMe_2$ group rotates out of the ring plane, stopping the lone pair from entering the ring. Hence, R is least reactive. 4. In P, the resonance is maximum as there is no hindrance. 5. In Q, the meta-methyl provides a small +I effect but does not interfere with the resonance of the amino group as much as ortho groups do, though P remains more accessible. 6. Thus, the rate order is P > Q > R. Option (1).
