D. FGrcasiu
Princeton University Princeton, New Jersey 08540
According to Hammond ( I ) , "if two states, as for example a transition state and an unstable intermediate occur consecutively during a reaction process and have nearly the same enerev ... content. their interconversion will involve only a small reorganization of the molecular structures." In other words. for a one steo endothermic orocess leadine to an unstable intermediat; as product (pbtential energy diagram depicted in nrofile A in the fia.). " .. the activated complex1 will have a structure (and geometry) very similar to that of the product ("product-like" transition state). A practical consequence of this postulate is that from the structure (exuerimentallv determinable) of an unstable intermediate,. the structkre of the transition state preceding it can be inferred. A corollary (3) of the postulate states that for a given reaction (or type of reaction) an increase in reactivity of starting materials (i.e., an increase in the reaction rate) makes the activated complex structurally (4) more similar to the starting material (more "reactant-like," or "earlier" transition state). It is this corollarv that is commonlv used to nredict the variation of the activated complex~structures with the variation of reactivitv in a reaction series (or for different. hut similar, reactions) and quoted as the Hammond postulate (5). However, the use of this corollary to predict the structure (and the variation of structure) of the activated c o m ~ l e xshould be done with caution. Thus. if the increase in rate is due to a decrease in activation energy without anv modification of the enereies of startine material and product (profile B, the fig.j; the activate2 complex should become more product-like, because the difference between the transition state and the final state decreases comparatively more than the difference between the transition state and the initial state. This trend i s seen clearly in the limiting case (profile C, fig.), where the activated complex becomes identical (energetically, whence structurally) with the product (unstable intermediate). Therefore, the comparison of activated complex structures (i.e., determination of which transition state is earlier) should be based on the evaluation of potential energies for initial, final, and transition states.2 As obvious as this point might seem, it is so often overlooked that this discussion is worthwhile.3 Thus, a recent advanced textbook (7) presents the energy profiles for the alkylation of benzene and mesitylene with PbzCH+. The energies of the initial and final (the intermediate cyclohexadienyl cation) states of both processes are presented as little different, while the lo5 greater reactivity of mesitylene is pictured to come from a lowering of the transition state energy relative to benzene. This is the same situation as presented here in going from profile A (benzene) to profile B (mesitylene), where if the energy diagrams (7) presented an accurate description of the actual process, they would indicate an earlier transition state for benzene, although this is obviously not the intention of the authors (7). In an experimental paper (a), the corollary was invoked in order to explain a difference between Hammett p constants for different reaction series, by a change along the reaction series of the degree of advancement of the 76 1 Journal of Chemical Education
The Use and Misuse of the Hammond Postulate
Potential energy profiles for reactions (reaction steps) to which the Ham-
mond postulate applies.
reaction at the transition state. The values in question were -4.10 (+0.15) ("abnormally low") and -4.63 0.20 (zk0.15) ("normal range"). (Thus, the two ranges of possible values almost overlap.) In another case, a difference in p from -4.65 to -4.54 is taken as significant in order to comply with the above mentioned corollary. However, if the position of the activated complex on the reaction coordinate changes along the reaction series (with a corresponding variation of the relief of strain in the activated complex for different reactions in the series (8)),a correlation with a or ir+ would, most probably, exhibit a curvature, except if the displacement of the transition state along the reaction coordinate is the same for the standard reaction series. (This contradicts the a c c e ~ t e d(8) . , idea that the position of transition state along the reaction coordinate is a function of reactivitv. .. which is different for the two reaction series.) Similarly, it was predicted that neighboring group participation in anchimerically assisted solvolyses should assume increasing kinetic importance as the solvent ionizing power decreases f9),4 but it was found for the 8-arylethyl system, that change of solvent ionizing power has little effect upon the kinetically significant degree of bridging in the transition state (10). The activated complex may become more reactant-like if the potential energies of both activated complex and unstable product decrease (profile D, versus profile A, fig.) (3) ( I l ) . J It is usually believed that this is the general case: Thus, Streitwieser stated: "As a reaction system is altered to stabilize the intermediate, the free energy of activation decreases . . . and the structure of the transition state resembles the reactants more." (13). A similar view was expressed about carbanion formation (14). This view-
*
'It was proposed @), that the terms "activated complex" and "transition state," usually considered synonymous, should be given different meaning. Such an approach was used in studying isotope effects (6). Criticisms expressed in this article refer only to the use of the Hammond postulate, not necessarily to other conclusions of the papers cited. The opposite prediction was made by other authors (quoted in ref. (10)).
point should not he considered generally valid. For instance, acid catalyzed tritiation of benzene is significantly faster than that of chlorohenzene (15) while the o-complex (henzenium ion,6 Wheland intermediate) is less stable than that of chlorobenzene (18).7 Since for this reaction the o-complex should indeed be immediately consecutive to the activated complex for the rate-determining step (20),8 the potential energy diagram should he as shown by profile E (fig.) (if the energies of reactants are not much different for the two cases). Therefore, in this case the less reactive compound will have an "earlier" transition state. As another possibility, the alteration of a reaction system can produce an increase of the potential energy of reactants. If the energies of activated complex and unstable products are not changed, the reaction rate will increase and the transition state will he reached "earlier" (profile F versus profile A, fig.).= Such a variation in the energy profile is presented by Olah, e t al. @la), in order to explain the change in the structure of activated complex for aromatic substitution (from a-complex-like to r-complex-like) with increasing reactivity (decreasing selectivity)1° of electrophiles. However, the invariance of the energy content of the a-complex, indicated in the drawing, was not substantiated in the paper (21a). On the other hand, it should be pointed out that along the reaction coordinate the structure of the other reactant changes obviously in structure and geometry from the initial state, through the r-complex, to the a-complex. The description of the transition state should aim also a t describing the structure of " E in the activated complex
.. ArH
Electrophile (1)
..
Activated complex
+ ArH R-CO ' + [ I* Electraphile Activated (111) complex
4 \\
-
Intermediate (o-complex) (11) 0
than these) exhibits a higher sensitivity of the rate to the effects of substituents, as predicted for a "later" transition state than for the other two ring systems." Since Hammett p constants are a measure of charge development a t the reaction center the difference in p values is used (7) to compare the demee of advancement of the transition state along the react& coordinate for two similar reaction series.ls This criterion is not absolute, either. (It should he always remembered that these corrdatior~s are, by their very nature, limited and approximate (27)). Caution should he particularly manifested, when the difference in p's is small. For instance, by comparing the Hammett p values for the solvolysis of N-hydruxy-piperid i e benzoates (V) (p = +0.68) and l-phenylcyclohexyl henzoates (VI) (p = +1.34), it was concluded that the transition state for the ionization of a N-0 bond occurs earlier in the hond-hreaking process than the transition state for ionization of a comparable C-0 bond (28). A s a further argument, the lower dissociation energy for the N-0 bond is mentioned (28).
6It must be noted that a decrease in energy of the final state only makes the activated complex less product-like, while s decrease in ener& of transition state only makes the activated complex more product-like. A decrease in hoth dues not normally result in an "earlier" transition state if the energy level of the transition state is lowered more than thnt of t,he final state (8s is the ease for the curves drawn in Ref. f7), discussed above). A diagram correct in this respect is given in ref. (12). However, it is not proven in ref. (12) that the curve is indeed that given, i.e., that the conclusions reached about the activated complex structures arevalid (c.f. footnote 13helow). e F ~ar discussion of nomenclature of carhocations, see (IG). It should be mentioned that the use of "carboniom ions" as s name for tricoordinated cations of carbon was also criticized hy C. D. Nenitsescu, who constantly used the name of "earhocations" (17)
'It can he armed that the rate of hvdroeen rvchanze and the
\\
"\ /C-R e,/"\H Intermediate (o-complex)
(W Moreover, if the reaction proceeds from the starting material to a less stable o-complex, without other intermediates in between (as shown in (21a)), the activated complex can he only similar to the o-complex or else the alteration of the energy profile is such that the Hammond postulate does not apply.1 Conversely, if the "r-complex" is an intermediate and the step leading to it is exothermic (as drawn in (21b)), the activated complex could be similar with the starting materials, hut not with the "r-comnlou" 12 In studies on electrophilic suhstitution a t (aromatic) heterocyclic compounds, Marino and coworkers quote the Hammond postulate to assert that for the faster reacting substrates the transition state will lie further from the Wheland intermediate (earlier) (25). Such a conclusion cannot be accepted unless proven by a complete study of the energy profile for each reaction system. Moreover, in the mentioned paper (25a), a discrepancy is found, for one case, between the conclusion based on reactivity and that based on the effect of the substituents on reactivity.13 It is admitted for the reaction type studied in Ref. (25) that a n "earlier" transition state will he less sensitive to suhstituent effects than a "late" one, with more charge developed a t the reaction center in the activated complex (7) (21a). Yet, furan, which is more reactive than thiophene or henzene (whence it should have a n "earlier" transition state
This reaction exhibits high intermolecular and intramolecular selectivity (15) so the transition state should be of henzenium ion nature by any standard (21). *The variation af energy content of the reactants will have, ohviously, a much lesser effect upon the structure o f the activated complex, than a similar variation in energy of the latter, or of the unstable products. It must be stressed that increased reactivity does not always result in decreased selectivity (see discussion in (22)). Partieularly, for reactions involving benzenium ions, it was shown for detritiation (23) and some dienol-benzene rearrangeme~ts(24) that rate increases of 10" and 108, respectively, failed to alter the e m ergy profile of the reactions. We consider application of the Hnmmond postulate to have heuristic value only when an activated complex is compared with a real intermediate, which can he (at least in principle) invrsti: gated; comparing an activated complex with a hypothetical intermediate (1) is equivalent to comparing it with itself. 12However, if the energy difference between the starting materials and the fint intermediate is small, as rompared to the barrier between them, the activated complex may well rcs~mhle none of two, and the Ilnmmond postulate does not necessa~4y apply. 13A similar discrepancy between conclusions about the structure of activated complexes, reached by two different ways, was observed in ref. (12) and was similarly dismissed. "Another example, in which two related reaction series (solvolyses of ring-substituted dimethyl-phenylcsrhinyl chlorides and p-nitmhenzoates, respectively) exhibit identical p constants (@ o+ treatment), while the rates are larger for t h e firrt series than for the second hy a factor of 105,is pointed out in ref. (lob). l5 For a different criticism of the (improper)use of the H a m ~ e r t equation in structure-reactivitycorrelations, see ref. (26). Volume 52, Number 2, February 1975
/
77
C
N-0-CO-C,
R-CHt
HA-X
u
+
R"
XC,,H,-CH(D)=O
I
xc H ~ H ( D ) (K,) ' N+ I
(VIII-H(D))
(IX-H(D)) the ru-deuterium isotope effect on equilibrium constant K* is a limiting value. The kinetic n-deuterium isotope ef. fects are nearer to or farther from the limitinn- value by as much as the activated complex is more or less productlike. The authors concluded that the position of activated complex along the reaction coordinaie changes with the suhstituent X, because k111k~changes with X. However, the authors (39) did not verify whether or not KIIIKII changes with X, hut used the value of KIIIKHfor X = H as a general reference (limiting value). Moreover, (Kn/ KII)X=H was measured for addition of hydroxylamine and compared with (kn/k& for addition of semicarbazide and phenylhydrazine, without checking whether or not the entering group (nucleophile) has any effect on K I I I K I ~ ~ S (as it does on k ~ l k ~ tTherefore, ). the conclusions reached about the structural variation of transition states (39) do not appear convincing. Yet, if the relationship (39) I
