" But what can we say about an SN1 reaction? Here, the rate-determining step is simple heterolysis - bond breaking without apparently bond making to balance it. In the gas phase, bond dissociation energies show, heterolysis of an alkyl halide would require a greate deal of energy : 149 kcal/mol for tert-butyl bromide, and even more for other substrates. Yet in an SN1 reaction heterolysis occurs at moderate temperatures with an Eact of only 20 to 30 kcal/mol. This leaves a difference of 130 kcal or more to be provied. Where does this very large amount of energy come from?
The answer is, once again, from bond formation: not formation of one bond as in SN2 reaction , but formation of many bonds- bonds between the ions produced and the solvent. The ions are not generated as naked particles in the near emptiness of the gas phase; instead , they are generated as solvated ions..
The reactant has a dipole moment, and forms dipole-dipole bonds to solvent molecules. The transition state, we have seen has a stretched carbon-halogen bond and well-developed positive and negative charges. It has much greater dipole moment than the reactant and forms much stronger dipole dipole bonds to the solvent. The solvent thus stabilizes the transition state more than it does the reactant, lowers the Eact and speeds up the reaction.
... On the basis of this, then, the ionizing power of the solvent depends upon how well it solvates the ions. In turn, the ability of solvate ions depends in part on the polarity of the solvent- other things being equal, the more polar the solvent, the stronger the ion-dipole bonds. Thus, SN1 reactions of netural substrates go faster in water than in ethanol; they go faster in say 20% ethanol (20:80 ethanol:water mixture) than in 80% ethanol .
... Cations are solvated chiefly through unshared pairs of electrons anions are solvated chiefly through hydrogen bonding. Now , here, the cations are carbocations because of their dispersed charge they form weaker ion-dipole bonds than smaller metal cations. In the ionization of these organic substrates , therefore, solvation of the cation is relatively weak; it is the solvation of the anion that is particularly important. For this we want solvents capable of hydrogen bonding, that is protic solvants. Thus SN1 reactions proceed more rapidly in water, alcohols and mixtures of water and alcohol than in aprotic solvents like DMF , DMSO and HMPT "
I think the above text means tertiary halides get solvated by water molecules?
The text is from Morrison and Boyd.
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Topic: Alkenes soluble in cold sulphuric acid, alkyl halides are not -why? (Read 21525 times)