What is electro cyclic reaction ?
Electro cyclic reactions are those in which
rings are formed from a cyclic poly ene at the expense of a conjugated double
or triple bond or electrocyclic reaction-
are pericyclic reaction involving
opening or closing of a ring by conversion of sigma bond to pi bond or vice
versa.
For example, cyclo butene to 1,3-butadieneor vice versa .
What is symmetry forbidden process ?
We expect a thermal transformation to take
place only when the ground state orbitals of the reactants correlate with the
ground state orbitals of the products.
Although in the given diagram of cyclo
butene the cyclo butene ground state orbital σ correlates with the
butadiene ground state orbital Ѱ1, the π orbital of the former
does not correlate with Ѱ2 of the latter.
Instead it correlates with Ѱ3 which is
an excited state and an anti bonding orbital.
Thermal transformation of cyclo butene tobutadiene system by disrotatory process is thus symmetry
forbidden process.
symmetry forbidden process
What is symmetry allowed process?
Irradiation of cyclo butene produces the
first excited state in which an electron is promoted from π to π* orbital and in
this case σ,π and π* orbitals of cyclobutene correlate with Ѱ1, Ѱ2 and Ѱ3 orbitals of butadiene.
In other words, the first excited state of
cyclo butene correlates with the first excited state of butadiene and hence disrotatory ring-opening
and ring-closing is photo chemically a symmetry allowed
process.
What is disrotatory and conrotatory process ?
During electro cyclic reaction
of an acyclic compound, each of the terminal carbon atom and their substituent
must rotate about 90ᵒ around the bond to which they are attach to form σ-bond
of the cyclic product.
The reverse course is followed
in case of ring opening.
If the termini in an electro cyclic
reaction rotate in opposite direction, one clockwise, another anti clockwise,
the cyclisation or ring opening is said to be disrotatory process.
Thus ring closure and opening through disrotatory and conrotatory way generate stereo
chemically two distinct way of reaction.
Principle of Wood-Ward Hoffmann’s rule for electro cyclic reaction ( FMO , frontier molecular approach )
Two types of electro
cyclic cyclization are possible, namely thermal and photo chemical.
Thermal cyclization is also
known as ground state cyclization and photo chemical cyclization is known as
excited state cyclization.
In electro cyclic
reaction only HOMO is consider. In this case, thermal cyclization is
considered on the basis of HOMO having C2 axis of symmetry and
photo chemical cyclization is
considered on the basis of LUMO having σ-plane[ symmetry having 4n number
electron ]
Example of electro cyclic reaction .
Electro cyclic reaction of trans,cis,trans,2,4,6-octatriene.
The six molecular orbitals of this compound
arise from the six p atomic orbitals. The six molecular orbitals from Ψ1
to Ψ6 and their energy sequence are as follows,
Among the six molecular orbitals, Ѱ3 is
HOMO. Now, thermal ring closure by the rotation
either conrotatory or disrotatory are as
follows,
On photo chemically ring closure that
is, Ѱ3 change into Ѱ4 photo chemically and the HOMO to be considered now
therefore becomes Ѱ4.
Summary
- What is electro cyclic reaction?
- What is symmetry forbidden process?
- What is symmetry allowed process?
- What is conrotatory and disrotatory process?
