- Pericyclic reactions are a class of organic reactions, different
from ionic or radical reaction mechanisms.
- Pericyclic reactions have cyclic transition state
structures, and they are concerted
reactions, i.e. all the bond-breaking and making takes place at once in a
- Cycloaddition reactions are the most
important and useful type of pericyclic reaction.
- In a cycloaddition, two conjugated
molecules come together to form a ring, by forming two new s-bonds.
Why is is that cis-1,3-butadiene
and ethene react to form cyclohexene when heated:
(Diels-Alder reaction, 4 +2 cycloaddition)
but the analogous cycloaddition of
two ethene molecules does not work?
(2+2 cycloaddition, calculated to be exothermic by 75
We can explain and understand this through the principles of conservation
of orbital symmetry
conservation of orbital symmetry