: 200
Conjugated dienes (1,3-butadiene type) are more stable than isolated dienes by ~15 kJ/mol due to pi electron delocalization across four contiguous p-orbitals. They undergo both 1,2-addition (kinetic product, low T) and 1,4-addition (thermodynamic product, high T) through a common allylic carbocation intermediate. The Diels-Alder reaction is a concerted [4+2] cycloaddition: conjugated diene (4 pi electrons, must be in s-cis conformation) + dienophile (electron-poor alkene, 2 pi electrons) → cyclohexene. Key features: thermal (no catalyst needed), suprafacial-suprafacial (syn), stereospecific (cis-diene + cis-dienophile → cis ring junction), and endo rule (EWG on dienophile prefers endo orientation in TS for secondary orbital interactions). Retro-Diels-Alder (high T) fragments cyclohexene back into diene + dienophile. The reaction is accelerated by electron-rich dienes and electron-poor dienophiles (normal electron demand). JEE tests: identifying products from given diene/dienophile pairs, predicting stereochemistry, and applying the endo rule.