When bulky groups flanking a substituent force it out of the plane of the ring, the p-orbital cannot overlap effectively with the pi system. This disrupts resonance.

Classic example: 2,6-dimethylaniline vs aniline

• In aniline, the -NH2 lone pair delocalizes into the ring (+M), reducing basicity
• In 2,6-dimethylaniline, the methyl groups twist -NH2 out of plane, blocking resonance
• Result: 2,6-dimethylaniline is MORE basic than aniline (lone pair more available)

Other examples:

• 2,6-dimethylnitrobenzene: -NO2 forced out of plane, reduced -M effect, reduced acidity of attached -OH if present
• N,N-dimethylaniline: less steric issue (lone pair still in p-orbital), but N,N-diethylaniline starts showing SIR effects

JEE favorite: Comparing basicities of substituted anilines — always check if ortho groups cause SIR.