A General Comment on Mechanism

The preceding Sections illustrate several experimental features of heteroaromatic substitutions. It is now intended to comment on some of these features which are most significant in terms of reaction mechanism. As stated in the Introduction, a possible mechanism of nucleophilic bimolecular aromatic substitution reactions is that represented by Eq. (14), where an intermediate of some stability 

The latter mechanism is characterized by a transition-state structure of type 64 and by its being (in part) analogous to the mechanism at a saturated carbon. The preference of a two-step mechanism to the apparently simpler one-step mechanism is suggested by the isolation of Meisenheimer complexes and by the kinetics of their formation. The experimental evidence on these 

Streitwieser, Jr., Solvolytic Displacement Reactions. McGraw-Hill, 

Second, for both classes of aromatic compounds such values show a surprisingly small dependence on the nature of the attacking reagent, probably indicating the predominant role of the reorganization of the substrate toward a new state represented by structure 63 or 65. FinaUy, it may not be fortuitous that a correspondence is found between structural effects on substitution rates and on ionization constants (Section IV,C, l,a). Bond-making would in fact be the essential analogy between these phenomena [Eqs. (16) and (17)], and 

The above evidence shows that a high degree of bond-making in the rate-determining step of bimolecular nucleophilic substitutions seems 

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We conclude this section with a general comment on interstitial models. The study of such models is useful and quite rewarding in providing us insight into the possible mechanism by which water exhibits its anomalous behavior. One should be careful not to conclude that the numerical results obtained from the model are an indication of the extent of the reality of the model. It is possible, by a judicious choice of the molecular parameters, to obtain thermodynamic results which are in agreement with experimental values measured for real water. Such agreement can be achieved by quite different models. The important point is not the quantitative results of the model but the qualitative explanation that the model offers for the various properties of water. We shall use the same model in Sec. 3.6 to explain some aspects of aqueous solutions of simple solutes. 

One final comment on mechanisms in general There exists a common sort of shorthand way of writing organic reaction mechanisms. First, each step is written separately. Second, bonding changes in each step are indicated by arrows that represent the movement of pairs of electrons. For the SN2 mechanism, a one-step process, we have ... 

Stylistic aspects. A few comments on stylistic aspects of the book might be helpful. Perhaps foremost among them is the use of color in our reaction mechanisms, which include blue, black, red, and green. In general, the first nucleophile in a given mechanism is always indicated in blue and the first electrophile in black. Later in the mechanism, if the atoms originating in the initial nucleophile take on a different role, such as that of an electrophile. 

Another way in which the general message that we endorse might be expressed is through clarification not of the notion of accommodation but instead of that of prediction . In fact scientists often use the notion of prediction in an atemporal sense—that is, one which carries no implicit requirement that predicted events have been hitherto unobserved. Here, for example, is a comment on Newton s theory from French s excellent textbook on Newtonian Mechanics (French, 1971, pp, 5-6 emphases added) ... 

The most common alkenes employed in the Pd-catalysed synthesis of alternating polyketones are ethene, styrene, propene and cyclic alkenes such as norbomene and norbornadiene. Even though the mechanism does not vary substantially with the alkene, the reactions of the various co-monomers are here reported and commented on separately, starting with the ethene/CO copolymerisation, which is still the most studied process. As a general scheme, the proposed catalytic cycles are presented first, then the spectroscopic experiments that have allowed one to elucidate each single mechanistic step. 

The other point concerns the persistent controversy about the detailed mechanisms of general reactions of this type—i.e., the deoxymercuration reactions, not only of methoxycarbonyl compounds but also of the corresponding olefin adducts. These are clearly concerted processes that are aided in some way by coordination of X- on the mercury and coordination of H+ on the oxygen. There has been controversy, in which Wright and others have participated, as to whether the detailed mechanism involves a cyclic intermediate, in other words whether undissociated HX participates in the reaction or whether the assistance at the metal and oxygen centers are independent processes. Have you any comments on this ... 

The excited states arising in the photolysis of aromatic compounds provide a good starting point. It should be noted in this connection that the expectation that benzenoid excited states differ from the parent ground states both in electronic structure and reactivity has been commented on previously in the literature (2.3) however, a general link between quantum mechanical description and experimental photochemistry lias been needed. 

When SO is added to the reaction mixture for the acid-catalysed hydrolysis of acetylcholine, (CH3)3N+CH2CH20COCH3 (HE+), with H30+, added catalysis is observed. This can be interpreted as anion-catalysed hydrolysis by SO4-, or as general acid catalysis by HSO4. Predict the effect of ionic strength on the rate constants describing each mechanism, and comment on the result. Does this represent a means of distinguishing between the two mechanisms ... 

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