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Chemistry · Glossary

What is Endo and exo adducts?

Also known as: endo adduct · exo adduct · endo rule

Definition 17.13 University Chemistry — Year 2 · Chapter 17 — Frontier Orbitals and Reactivity

When a cyclic diene adds to a dienophile carrying an unsaturated substituent, the endo adduct has that substituent pointing towards the newly formed double bond (under the diene in the transition state), the exo adduct away from it. The endo rule states that the endo adduct is formed faster.

Endo and exo approaches of maleic anhydride to cyclopentadiene, schematic. The forming bonds are dashed. In the endo approach the carbonyl groups lie under the diene, and their π orbitals overlap with its central carbons (dotted, secondary overlap); in the exo approach they point away.
Endo and exo approaches of maleic anhydride to cyclopentadiene, schematic. The forming bonds are dashed. In the endo approach the carbonyl groups lie under the diene, and their π\pi orbitals overlap with its central carbons (dotted, secondary overlap); in the exo approach they point away.
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