igure 5. 4 The arginine molecule in a zwitterion stable structure 2. Energy Conservation In any chemical reaction as in all physical processes, total energy must be conserved Reactions in which the summation of the strengths of all the chemical bonds in the reactants exceeds the sum of the bond strengths in the products are termed endothermic. For such reactions, energy must to provided to the reacting molecules to allow the reaction to occur Exothermic reactions are those for which the bonds in the products exceed in strength those of the reactants For exothermic reactions, no net energy input is needed to allow the reaction to take place. Instead, excess energy is generated and liberated when such reactions take place. In the former(endothermic)case, the energy needed by the reaction usually comes from the kinetic energy of the reacting molecules or molecules that surround them. That is thermal energy from the environment provides the needed energy. Analogously, for exothermic reactions, the excess energy produced as the reaction proceeds is usually
11 2.546 1.740 1.705 Figure 5.4 The arginine molecule in a zwitterion stable structure. 2. Energy Conservation In any chemical reaction as in all physical processes, total energy must be conserved. Reactions in which the summation of the strengths of all the chemical bonds in the reactants exceeds the sum of the bond strengths in the products are termed endothermic. For such reactions, energy must to provided to the reacting molecules to allow the reaction to occur. Exothermic reactions are those for which the bonds in the products exceed in strength those of the reactants. For exothermic reactions, no net energy input is needed to allow the reaction to take place. Instead, excess energy is generated and liberated when such reactions take place. In the former (endothermic) case, the energy needed by the reaction usually comes from the kinetic energy of the reacting molecules or molecules that surround them. That is, thermal energy from the environment provides the needed energy. Analogously, for exothermic reactions, the excess energy produced as the reaction proceeds is usually
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 examined
12 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 oddo 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
Lowestπ orbital of 1.3- butadiene denoted 88 π orbital of π* orbital of cyclobutene cyclobutene g orbital of cyclobutene cyclobutene igure 5.5 The active valence orbitals of 1, 3-butadiene and of cyclobutene
13 Lowest p orbital of 1,3- butadiene denoted p1 p2 p3 p4 p orbital of cyclobutene p* orbital of cyclobutene s orbital of cyclobutene s* orbital of cyclobutene Figure 5.5 The active valence orbitals of 1, 3- butadiene and of cyclobutene
The four T orbitals of 1, 3-butadiene are of the following symmetries under the preserved symmetry plane( see the orbitals in Fig.55):π=e,丌2=o,π3=e,π4=0.The T and T and o and o orbitals of the product cyclobutane, which evolve from the four orbitals of the 1, 3-butadiene, are of the following symmetry and energy order o=e, I e,T=0,0=o The Woodward-Hoffmann rules instruct us to arrange the reactant and product orbitals in order of increasing energy and to then connect these orbitals by symmetry, starting with the lowest energy orbital and going through the highest energy orbital. This process gives the following so-called orbital correlation diagram T Figure 5.6 The orbital correlation diagram for the 1, 3-butadiene to cyclobutene reaction We then need to consider how the electronic configurations in which the electrons are arranged as in the ground state of the reactants evolves as the reaction occurs We notice that the lowest two orbitals of the reactants, which are those occupied by the four T electrons of the reactant, do not connect to the lowest two orbitals of the
14 The four p orbitals of 1,3-butadiene are of the following symmetries under the preserved symmetry plane (see the orbitals in Fig. 5.5): p1 = e, p2 = o, p3 =e, p4 = o. The p and p* and s and s* orbitals of the product cyclobutane, which evolve from the four orbitals of the 1,3-butadiene, are of the following symmetry and energy order: s = e, p = e, p* = o, s* = o. The Woodward-Hoffmann rules instruct us to arrange the reactant and product orbitals in order of increasing energy and to then connect these orbitals by symmetry, starting with the lowest energy orbital and going through the highest energy orbital. This process gives the following so-called orbital correlation diagram: Figure 5.6 The orbital correlation diagram for the 1,3-butadiene to cyclobutene reaction. We then need to consider how the electronic configurations in which the electrons are arranged as in the ground state of the reactants evolves as the reaction occurs. We notice that the lowest two orbitals of the reactants, which are those occupied by the four p electrons of the reactant, do not connect to the lowest two orbitals of the s * s p * p p4 p3 p2 p1
products, which are the orbitals occupied by the two o and two T electrons of the products. This causes the ground-state configuration of the reactants( I)to evolve into an excited configuration(o T*)of the products. This, in turn, produces an activation barrier for the thermal disrotatory rearrangement (in which the four active electrons occupy these lowest two orbitals)of 1, 3-butadiene to produce cyclobutene If the reactants could be prepared, for example by photolysis, in an excited state having orbital occupancy TItT2T3, then reaction along the path considered would not have any symmetry-imposed barrier because this singly excited configuration correlates to a singly-excited configuration otT*l of the products. The fact that the reactant and product configurations are of equivalent excitation level causes there to be no symmetry constraints on the photochemically induced reaction of 1, 3-butadiene to produce cyclobutene. In contrast, the thermal reaction considered first above has a symmetry imposed barrier because the orbital occupancy is forced to rearrange(by the occupancy of two electrons from I, =T*to '=I) from the ground-state wave function of the reactant to smoothly evolve into that of the product. Of course, if the reactants could be generated in an excited state having a t 3 orbital occupancy, then products could also be produced directly in their ground electronic state. However, it is difficult, if not impossible, to generate such doubly-excited electronic states, so it is rare that one encounters reactions being induced via such states It should be stressed that although these symmetry considerations may allow one to anticipate barriers on reaction potential energy surfaces, they have nothing to do with the thermodynamic energy differences of such reactions. What the above Woodward Hoffmann symmetry treatment addresses is whether there will be symmetry-imposed
15 products, which are the orbitals occupied by the two s and two p electrons of the products. This causes the ground-state configuration of the reactants (p1 2 p2 2 ) to evolve into an excited configuration (s 2 p* 2 ) of the products. This, in turn, produces an activation barrier for the thermal disrotatory rearrangement (in which the four active electrons occupy these lowest two orbitals) of 1,3-butadiene to produce cyclobutene. If the reactants could be prepared, for example by photolysis, in an excited state having orbital occupancy p1 2p2 1p3 1 , then reaction along the path considered would not have any symmetry-imposed barrier because this singly excited configuration correlates to a singly-excited configuration s2p1p*1 of the products. The fact that the reactant and product configurations are of equivalent excitation level causes there to be no symmetry constraints on the photochemically induced reaction of 1,3-butadiene to produce cyclobutene. In contrast, the thermal reaction considered first above has a symmetryimposed barrier because the orbital occupancy is forced to rearrange (by the occupancy of two electrons from p2 2 = p* 2 to p 2 = p3 2 ) from the ground-state wave function of the reactant to smoothly evolve into that of the product. Of course, if the reactants could be generated in an excited state having p1 2 p3 2 orbital occupancy, then products could also be produced directly in their ground electronic state. However, it is difficult, if not impossible, to generate such doubly-excited electronic states, so it is rare that one encounters reactions being induced via such states. It should be stressed that although these symmetry considerations may allow one to anticipate barriers on reaction potential energy surfaces, they have nothing to do with the thermodynamic energy differences of such reactions. What the above WoodwardHoffmann symmetry treatment addresses is whether there will be symmetry-imposed