deposited into the kinetic energy of the product molecules and into that of surrounding molecules. For reactions that are very endothermic, it may be virtually impossible for thermal excitation to provide sufficient energy to effect reaction. In such cases, it may be possible to use a light source (i.e, photons whose energy can excite the reactant molecules)to induce reaction. When the light source causes electronic excitation of the reactants(e.g, one might excite one electron in the bound diatomic molecule discussed above from a bonding to an anti-bonding orbital), one speaks of inducing reaction by photochemical means 3. Conservation of Orbital Symmetry- the Woodward-Hoffmann Rules An example of how important it is to understand the changes in bonding that accompany a chemical reaction, let us consider a reaction in which 1, 3-butadiene is converted, via ring-closure, to form cyclobutene. Specifically, focus on the four T orbitals of 1, 3-butadiene as the molecule undergoes so-called disrotatory closing along which the plane of symmetry which bisects and is perpendicular to the C2-C3 bond preserved. The orbitals of the reactant and product can be labeled as being even-e or odd o under reflection through this symmetry plane. It is not appropriate to label the orbitals with respect to their symmetry under the plane containing the four C atoms because, although this plane is indeed a symmetry operation for the reactants and products, it does not remain a valid symmetry throughout the reaction path. That is, we symmetry label the orbtials using only those symmetry elements that are preserved throughout the reaction path being examined12 deposited into the kinetic energy of the product molecules and into that of surrounding molecules. For reactions that are very endothermic, it may be virtually impossible for thermal excitation to provide sufficient energy to effect reaction. In such cases, it may be possible to use a light source (i.e., photons whose energy can excite the reactant molecules) to induce reaction. When the light source causes electronic excitation of the reactants (e.g., one might excite one electron in the bound diatomic molecule discussed above from a bonding to an anti-bonding orbital), one speaks of inducing reaction by photochemical means. 3. Conservation of Orbital Symmetry- the Woodward-Hoffmann Rules An example of how important it is to understand the changes in bonding that accompany a chemical reaction, let us consider a reaction in which 1,3-butadiene is converted, via ring-closure, to form cyclobutene. Specifically, focus on the four p orbitals of 1,3-butadiene as the molecule undergoes so-called disrotatory closing along which the plane of symmetry which bisects and is perpendicular to the C2-C3 bond is preserved. The orbitals of the reactant and product can be labeled as being even-e or odd￾o under reflection through this symmetry plane. It is not appropriate to label the orbitals with respect to their symmetry under the plane containing the four C atoms because, although this plane is indeed a symmetry operation for the reactants and products, it does not remain a valid symmetry throughout the reaction path. That is, we symmetry label the orbtials using only those symmetry elements that are preserved throughout the reaction path being examined