Page 293 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg
P. 293

274               that exist along the reaction pathway. The sequence of reaction steps and interme-
                       diates involved in the overall transformation is the reaction mechanism. Reactants,
     CHAPTER 3         intermediates, and products correspond to energy minima on the energy diagram and
     Structural Effects on  are separated by activated complexes or transition states, which are the molecular
     Stability and Reactivity
                       arrangements having the maximum energy for each successive step of the reaction
                       mechanism. Figure 3.2 gives reaction energy diagrams for hypothetical reactions that
                       proceed in one, two, or three steps through zero, one, or two intermediates. For the
                       example in Figure 3.2. these are:

                                (a)     A  +  B      C  +  D

                                                          D
                                (b)     A       B  +  C         E + F
                                                                        F
                                (c)     A  +  B      C          D  +  E       G +   E

                       The diagram in (a), for example, might correspond to an S 2 displacement reaction.
                                                                       N
                                                         H
                                                   δ −       δ −
                               Nu: –  +  CH 3  X    Nu   C   X        Nu  CH 3   +   X –
                                                       H   H
                               A        B                                C         D

                       The diagram in (b) could pertain to an S 1 reaction with a solvent D proceeding
                                                          N
                       through a carbocation intermediate (B).
                                                slow
                                        R C  X       R C +  +  X –
                                         3
                                                      3
                                           A          B      C
                                                        fast
                                        R C +  +  Nu  H       R 3 C  Nu  +  HX
                                         3
                                           B     D               E       F
                       The diagram in (c) might apply to an acid-catalyzed ionization mechanism, such as
                       might occur in a reaction of a secondary benzylic alcohol with HBr.

                                 PhCHR        fast  PhCHR  slow    PhCHR
                                        +   H +                      +     +   H O
                                                                                2
                                                       +
                                   OH                 O H 2
                                    A     B                C         D         E
                                               fast  PhCHR
                                 PhCHR +  Br –
                                    +                 Br
                                     D    F              G
                           Such diagrams make clear the difference between an intermediate and a transition
                       state. An intermediate lies in a depression on the potential energy curve. Thus, it has
                       a finite lifetime, which depends on the depth of the depression. A shallow depression
                       implies a low activation energy for the subsequent step, and therefore a short lifetime.
                       The deeper the depression, the longer the lifetime of the intermediate. The situation at
                       a transition state is quite different. This arrangement has only fleeting existence and
                       represents an energy maximum on the reaction path.
   288   289   290   291   292   293   294   295   296   297   298