Bond breaking in

The orbitals from which electrons are removed and those into which electrons are excited can be restricted to focus attention on correlations among certain orbitals. For example, if excitations out of core electrons are excluded, one computes a total energy that contains no correlation corrections for these core orbitals. Often it is possible to so limit the nature of the orbital excitations to focus on the energetic quantities of interest (e.g., the CC bond breaking in ethane requires correlation of the acc orbital but the 1 s Carbon core orbitals and the CH bond orbitals may be treated in a non-correlated manner). 

What are the implications of the effects of the various halide leaving groups Because the halogen with the weakest bond to carbon reacts fastest. Ingold concluded that the carbon-halogen bond breaks in the rate-detennining step. The weaker the carbon-halogen bond, the easier it breaks. 

The E2 mechanism is a concerted process in which the carbon-hydrogen and car bon-halogen bonds both break in the sane elementary step. What if these bonds break in separate steps ... 

Because the carbon-halogen bond breaks in the slow step, the rate of the reaction depends on the leaving group. Alkyl iodides have the weakest carbon-halogen bond and are the most reactive alkyl fluorides have the strongest carbon-halogen bond and are the least reactive. 

Both of the reactions, radical combination and Diels-Alder cycloaddition, cause new bonds to be made. Bond making normally releases energy. Why then are the barriers for the two reactions so different (Hint Consider the nel bond making/bond breaking in the two reactions.)... 

The main symbols of both equations are similar to the ones whose physico-chemical sense was explained previously. AGc the change of free energy at chemical bond break in cycle and noncyclic bonds formation. Gid is equal for initial and end states. 

Separate experiments on the iodine-catalysed bromination of these compounds revealed a rate maximum at [I2]/[Br2] = 0.35, from which it follows that the concentrations of molecular bromine and iodine monobromide are equal, i.e. the latter catalyses bond-breaking in the former in the intermediate. Since iodine monobromide is dissociated into iodine and bromine, dissociation constant K, [Br2]VAT is proportional to [IBr] and hence equation (152) may be rewritten in the form... 

As has been indicated, since there is a ring isotope effect there must be a degree of C-H bond breaking in the transition state of the rate-determining stage. Clearly further work is required in this system before a definitive mechanism can be established for the intramolecular rearrangement. 

If a bond breaks in such a way that each fragment gets one electron, free radicals are formed and such reactions are said to take place by homolytic or free-radical mechanisms. 

To estimate the amount of energy absorbed or released in this reaction, we must compile an inventoiy of all the bonds that break and all the bonds that form. A ball-and-stick model shows that propane contains 8 C—H bonds and 2 C—C bonds. These bonds break in each propane molecule, and one ODO bond breaks in each oxygen molecule. Two CDO bonds form in each CO2 molecule, and two O—H bonds form in each H2 O molecule. In summary ... 

As demonstrated in Section 2.2, the energy of activation of simple electron transfer reactions is determined by the energy of reorganization of the solvent, which is typically about 0.5-1 eV. Thus, these reactions are typically much faster than bondbreaking reactions, and do not require catalysis by a J-band. However, before considering the catalysis of bond breaking in detail, it is instructive to apply the ideas of the preceding section to simple electron transfer, and see what effects the abandomnent of the wide band approximation has. 

Mechanistic studies have been designed to determine if the concerted cyclic TS provides a good representation of the reaction. A systematic study of all the E- and Z-decene isomers with maleic anhydride showed that the stereochemistry of the reaction could be accounted for by a concerted cyclic mechanism.19 The reaction is only moderately sensitive to electronic effects or solvent polarity. The p value for reaction of diethyl oxomalonate with a series of 1-arylcyclopentenes is —1.2, which would indicate that there is little charge development in the TS.20 The reaction shows a primary kinetic isotope effect indicative of C—H bond breaking in the rate-determining step.21 There is good agreement between measured isotope effects and those calculated on the basis of TS structure.22 These observations are consistent with a concerted process. 

Rate studies show that chlorination is subject to acid catalysis, although the kinetics are frequently complex.13 The proton is believed to assist Cl-Cl bond breaking in a reactant-Cl2 complex. Chlorination is much more rapid in polar than in nonpolar solvents.14 Bromination exhibits similar mechanistic features. 

Csajka, F. S. Chandler, D., Transition pathways in a many-body system application to hydrogen-bond breaking in water, J. Chem. Phys. 1998,109, 1125-1133... 

Hydrolyses of alkyl halides and arenesulfonates have long been known to be micelle-inhibited (Gani et al., 1973 Lapinte and Viout, 1973, 1979) but now k+/k < 1, except for hydrolysis of methyl benzenesulfonate which involves extensive bond making in the transition state (Al-Lohedan et al., 1982b Bunton and Ljunggren, 1984). Thus values of k+/k < 1 seem to be characteristic of hydrolyses in the SN1-SN2 mechanistic spectrum which involve considerable bond breaking in the transition state, and k+/k is very low for hydrolyses of diphenylmethyl halides where the transition state has considerable carbocation character (Table 8). 

A mechanistic study of acid and metal ion (Ni2+, Cu2+, Zn2+) promoted hydrolysis of [N-(2-carboxyphenyl)iminodiacetate](picolinato)chromate (III) indicated parallel H+- or M2+-dependent and -independent pathways. Solvent isotope effects indicate that the H+-dependent path involves rapid pre-equilibrium protonation followed by rate-limiting ring opening. Similarly, the M2+-dependent path involves rate-determining Cr-0 bond breaking in a rapidly formed binuclear intermediate. The relative catalytic efficiencies of the three metal ions reflect the Irving-Williams stability order (88). 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

---