Transition product-like

The significance of the concept incorporated in Hammond s postulate is that, in appropriate cases, it permits discussion of transition-state structure in terms of the reactants, inteimediates, or products in a multistep reaction sequence. The postulate indicates that the cases in which such comparison is appropriate are those in which the transition state is close in energy to the reactant, intermediate, or product. Chemists sometimes speak of early or late transition states. An early transition state is reactant-like whereas a late transition state is product-like. 

Compare the bonding surface in the transition state to those of the reactant and the products. The CO single bond of the reactant is clearly broken in the transition state. Also, the migrating hydrogen seems more tightly bound to oxygen (as in the product) than to carbon (as in the reactant). It can be concluded that the transition state more closely resembles the products than the reactants, and this provides an example of what chemists call a late or product-like transition state. 

Obtain the energies of propene, dimethylborane, and 1-propyldimethyl borane, and calculate AH n for dimethylborane addition. Is this reaction exothermic or endothermic Use this result and the Hammond Postulate to predict whether the transition state will be more reactant like or more product like . Compare the geometry of the transition state to that of the reactants and products. Does the Hammond Postulate correctly anticipate the structure of the transition state Explain. 

The transition state for the nucleophilic addition reaction to a carbonyl compound is essentially reactant-hke , rather than product-like . 

If, on the other hand, the transition state is product-like ( late ), the Nu —Np distance is short repulsion between Nu and Ar is stronger than that between the Np lone pair and Ar, and the transition state 7.15 leading to the (E)-azo compound 7.16 is favored. 

This correlation between /7-values for rates and equilibria reflects a long-established principle of physical organic chemistry, the so-called Hammond postulate (Hammond, 1955 see also Farcasiu, 1975). This postulate states that in a series of related reactions the transition state becomes more product-like as the positive enthalpy differences between reagents and products increase. 

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

Transition states (Continued) in hydrogen abstraction, 25 in phosphodiester hydrolysis, 190 reactant-like vs product-like, 96 solvation energy of, 211, 213,214 solvent effects on, 46 stabilization of charge distribution, 91, 225-227... 

Carboxylates, which are chiral in the a-position totally lose their optical activity in mixed Kolbe electrolyses [93, 94]. This racemization supports either a free radical or its fast dynamic desorption-adsorption at the electrode. A clearer distinction can be made by looking at the diastereoselectivity of the coupling reaction. Adsorbed radicals should be stabilized and thus react via a more product like transition state... 

Figure 2.5 (A) Substrate, transition state-like intermediate state and product of the reaction cat-...

These results suggest that the transition states leading to the formation of the cyclo-adducts (33) and (34) are product-like and that the greater than statistical formation of adducts (34) is due to the increased thermodynamic stability of a trisubstituted double bond. In agreement with this explanation is the fact that in reactions with for example p-xylene and durene (1,2,4,5-tetramethylbenzene) only the adducts (35) and (36) were obtained 54-59). Also as expected, two adducts were obtained with tetralin but only the compound (37) was obtained using 5,8-dimethyltetralin, which we may regard as a 1,2,3,4-tetra-alkylben-zene 54>. 

The observation that the transition state volumes in many Diels-Alder reactions are product-like, has been regarded as an indication of a concerted mechanism. In order to test this hypothesis and to gain further insight into the often more complex mechanism of Diels-Alder reactions, the effect of pressure on competing [4 + 2] and [2 + 2] or [4 + 4] cycloadditions has been investigated. In competitive reactions the difference between the activation volumes, and hence the transition state volumes, is derived directly from the pressure dependence of the product ratio, [4 + 2]/[2 + 2]p = [4 + 2]/[2 + 2]p=i exp —< AF (p — 1)/RT. All [2 + 2] or [4 + 4] cycloadditions listed in Tables 3 and 4 doubtlessly occur in two steps via diradical intermediates and can therefore be used as internal standards of activation volumes expected for stepwise processes. Thus, a relatively simple measurement of the pressure dependence of the product ratio can give important information about the mechanism of Diels-Alder reactions. 

The small Hammett p value of +0.16 observed for a series of related meta- and para-substituted mandelic acids indicates that there is a very small negative charge development on the benzyl carbon in the transition state of the rate-determining step of the pyridine catalysed oxidation of mandelic acid. The large positive AS value (+24 e.u./mol) found for the catalysed reaction led Banetjee and coworkers to conclude that the transition state (Figure 5) is product-like . This conclusion is consistent with the small f n/f D that is observed in this reaction164. The Pb—O bond is shown to rupture in a heterolytic fashion because Partch and Monthony185 have demonstrated that pyridine diverts the reaction from a homolytic to a heterolytic mechanism. 

In the common parlance of physical organic chemists such phrases as product-like or reactant-like transition states are common. The degree of resemblance of transition states to either reactants or products is usually assessed for reaction series obeying the linear-free-energy principle on the basis of suitable reaction constants, such as Bronsted a- and (3-values, and Hammett reaction constants p. The question is inherently more complex for cyclisation reactions, since they are not expected to follow the linear-free-energy principle. 

As wR(s) and wp(s) vary between these limits, they necessarily cross in the neighborhood of the transition state. The TS complex therefore corresponds to a resonance hybrid of nearly equal reactant-like and product-like contributions,92... 

The different reactivities of the carbon—boron and carbon—zirconium bonds toward electrophiles are a consequence of the different bond polarities and the different electronegativities of boron and zirconium. Moreover, zirconium is a transition metal, while boron exhibits intriguing transition metal-like chemistry [55], It is thus reasonable to presume that the combined use of boron and zirconium in organic chemistry should be synergistic, affording products and chemistry not attainable with the individual organo-metallics alone. 

A primary isotope effect results when the breaking of a carbon-hydrogen versus a carbon-deuterium bond is the rate-limiting step in the reaction. It is expressed simply as the ratio of rate constants, i wlky,. The full expression of k /kn measures the intrinsic primary deuterium isotope for the reaction under consideration, and its magnitude is a measure of the symmetry of the transition state, e.g., -C- H- 0-Fe+3 the more symmetrical the transition state, the larger the primary isotope effect. The theoretical maximum for a primary deuterium isotope effect at 37°C is 9. The less symmetrical the transition state, the more product-like or the more substrate-like the smaller the intrinsic isotope effect will be. 

Product-like transition states would be expected to have inverse k"lk" incoming nucleophile carbon KIEs. The authors were unable to explain why all the klllk14 KIEs for these reactions were normal, i.e. between 1.0105 and 1.0070. 

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