Hammond Principle

Observations of reactivity are concerned with rate determining processes and require the knowledge of the structure and energy of the activated complexes. Up to now, the Hammond principle has been employed (see part 3.2) and reactive intermediates (cationic chain ends) have been used as models for the activated complexes. This was not successful in every case, therefore models of activated complexes related to the matter at hand were constructed, calculated and compared. For example, such models were used to explain the high reactivity of the vinyl ethers19 80). These types of obser-... 

State is that assembly of atoms or moieties that closely resembles the reactant(s), such that only a relatively small reorganization will generate the reactant(s). Analogously, a late transition state more closely resembles the structure of the reaction product(s). See Chemical Kinetics Transition State Theory Potential Energy Surface Hammond Principle Transition Structure... 

The terms early and late transition states are qualitative descriptions related to concepts such as those advanced by the Hammond principle. An early transition... 

ACIDITY FUNCTION BUNNETT-OLSEN EQUATIONS DECREE OF DISSOCIATION HAMMETT EQUATION HAMMOND PRINCIPLE/POSTULATE LEFFLER S ASSUMPTION REACTING BOND RULES HANES PLOT... 

TRANSITION STATE THEORY POTENTIAL ENERGY SUREACE HAMMOND PRINCIPLE TRANSITION STRUCTURE CHEMICAL KINETICS TRANSITION-STATE ANALOGUES MOLECULAR SIMILARITY... 

Because of kinetic control, the intermediate with the lowest-enthalpy transition state (TS) is formed in the greatest amount. Since this step is endothermic, the Hammond principle says that the intermediate resembles the TS. We then evaluate the relative energies of the intermediates op vs. m) and predict that the one with the lowest enthalpy has the lowest AH and is formed in the greatest yield. 

Problem 16.33 Use the Hammond principle (Problem 11.8) to explain why strong bases such as OR (from esters) and NH, (from amides) can be leaving groups in nucleophilic transacylations. ... 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

Problem 11.8 (a) Give the delocalized structure (Problem 11.1) for the 3 benzenonium ions resulting from the common ground state for electrophilic substitution, C6H5G + E+. (b) Give resonance structures for the para-benzenonium ion when G is OH. (c) Which ions have G attached to a positively charged C (d) If the products from this reaction are usually determined by rate control (Section 8.5), how can the Hammond principle be used to predict the relative yields of op (i.e., the mixture of ortho and para) as against m (meta) products (e) In terms of electronic effects, what kind of G is a (i) op-director, (ii) m-director (/) Classify G in terms of its structure and its electronic effect. ... 

Flaloform reaction, 273, 311, 335 Halogen exchange, 119 Halogenation of alkanes, 56 of arenes, 202 Halohydrin, 100 Hammond principle, 209 Hard bases, 121 H-bonding, 257, 333, 401 H-counts in nmr, 241 Heat of combustion and stability, 90 Hell-Volhard-Zelinsky reaction, 340 Hemiacetal, 317 Hertz, 230... 

The curve acts as a reference line, but it also shows that the slope, a, in Equation (1.30) has a value near one for AG° near zero, and approaches zero as AG ° becomes a large negative number. Such behavior is an expected consequence of the Bell-Evans-Polanyi-Leffler-Hammond principle.It corresponds to a late transition state for the more difficult reactions and a progressively earlier transition state for more exergonic reactions. However, the curve appears to level off at an n value near 8, which corresponds to a second-order rate constant of about 1 s at 25 °C, far from the diffusion-controlled limit. 

The considerable exothermicities associated with reactions (3) and (4) reflect the unsaturated nature of 1. In accordance with the Hammond principle (54), we can expect low or nonexistent activation energy barriers. For M = Ir, no transition state or precursor complex can be located for reaction (4) with any of the computational methods applied here. For reaction (3) it is possible to locate... 

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