Consider the three aromatic molecules (P, Q and R) whose structures have been given below : [figure] The correct order regarding the reactivity of these compounds with Ph- N N Cl under optimum but slightly acidic medium is :
- P > Q > R
- R > P > Q
- R > Q > P
- P > R > Q
Answer
(1) P > Q > R
Solution
The diazonium ion is a weak electrophile; it couples (at the para position) only with strongly activated rings such as N,N-dimethylaniline.
Activation comes from the +R effect of -NMe2, which needs the lone-pair orbital of N parallel to the ring p orbitals, i.e. the -NMe2 group in the plane of the ring.
P: no ortho substituent ⇒ -NMe2 is coplanar, full +R ⇒ most reactive.
Q: one ortho –Me clashes with the N-methyl groups ⇒ -NMe2 twists partly out of the plane (steric inhibition of resonance) ⇒ less activated.
R: two ortho –Me groups ⇒ -NMe2 is pushed almost perpendicular to the ring ⇒ resonance is nearly lost ⇒ least reactive.
∴ Reactivity towards coupling: P > Q > R
Answer: (1)