Bond breaking, position

All the evidence to date fits the hypothesis that micellar charge effects are related to mechanism, but the results are not so easy to explain. In a reaction dominated by bond breaking, positive charge developing at the reaction center should interact favorably with an anionic head group and the... 

In this section, permanganate [principally potassium permanganate (KMnOq)] will be used to oxidize alkenes to 1,2-diols. It is apparent from preceding discussions that alkenes can donate an electron pair to an electrophilic center. The structure of permanganate (MnOq", 239) is best viewed as the 1,3-dipolar molecule (b O and an negative O). Reaction occurs between an alkene, which donates electrons to the electrophilic oxygen of the dipole. As the Jt-bond breaks, positive character develops on one carbon, which is attacked by... 

Other solvents can be divided into several classes. In hydrogen bond-breaking solvents (dipolar aprotics), the simple amino, hydroxy and mercapto heterocycles all dissolve. In the hydrophobic solvents, hydrogen bonding substituents greatly decrease the solubility. Ethanol and other alcohols take up a position intermediate between water and the hydro-phobic solvents (63PMH 1)177). 

For many secondary sulfonates, nucleophilic substitution seems to be best explained by a concerted mechanism with a high degree of carbocation character at the transition state. This has been described as an exploded transition state. Both the breaking and forming bonds are relatively weak so that the carbon has a substantial positive charge. However, the carbocation per se has no lifetime because bond breaking and fonnadon occur concurrently."... 

Bond breaking can occur at any position along the hydrocarbon chain. Because the aromatization reactions mentioned earlier produce hydrogen and are favored at high temperatures, some hydrocracking occurs also under these conditions. However, hydrocracking long-chain molecules can produce Ce, C7, and Cg hydrocarbons that are suitable for hydrode-cyclization to aromatics. 

All bond enthalpies are positive because heat must be supplied to break a bond. In other words, bond breaking is always endothermic and bond formation is always exothermic. Table 6.7 lists bond enthalpies of some diatomic molecules. 

STRATEGY Decide which bonds are broken and which bonds are formed. Use the mean bond enthalpies in Table 6.8 to estimate the change in enthalpy when the reactant bonds break and the change in enthalpy when the new product bonds form. For diatomic molecules, use the information in Table 6.7 for the specific molecule. Finally, add the enthalpy change required to break the reactant bonds (a positive value) to the enthalpy change that occurs when the product bonds form (a negative value). 

Deuterium isotope effects have been found even where it is certain that the C—H bond does not break at all in the reaction. Such effects are called secondary isotope effectsf" the term primary isotope effect being reserved for the type discussed previously. Secondary isotope effects can be divided into a and P effects. In a P secondary isotope effect, substitution of deuterium for hydrogen p to the position of bond breaking slows the reaction. An example is solvolysis of isopropyl bromide ... 

Figure 41. Selective bond breaking of H2O by means of the quadratically chirped pulses with the initial wave packets described in the text. The dynamics of the wavepacket moving on the excited potential energy surface is illustrated by the density, (a) The initail wave packet is the ground vibrational eigen state at the equilibrium position, (b) The initial wave packet has the same shape as that of (a), but shifted to the right, (c) The initail wave packet is at the equilibrium position but with a directed momentum toward x direction. Taken from Ref. [37]. (See color insert.)...

The CO-X bond breaking is the result of an electrophilic attack (on the carbonyl oxygen atom, hence the catalytic role of acids in these rupture reactions) or a nucleophilic one (on the carbonyl carbon atom whose positive property is due to the X electron-withdrawing property). The dangers of this type of reaction come from its speed and high exothermicity and/or instability of the products obtained in some cases. The accidents that are described below can make one believe that acid anhydrides in general and acetic anhydride in particular represent greater risks than acid chlorides since they constitute the accident factor of almost all accidents described. This is obviously related to their frequent use in synthesis rather than acid chlorides, that are rarely used. 

For bond breaking AH° is positive and for bond formation A77° is negative. 

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