Halides solvent effects

In fee absence of fee solvation typical of protic solvents, fee relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than do soft nucleophiles. As a result, fee relative reactivity order changes. In methanol, for example, fee relative reactivity order is N3 > 1 > CN > Br > CP, whereas in DMSO fee order becomes CN > N3 > CP > Br > P. In mefeanol, fee reactivity order is dominated by solvent effects, and fee more weakly solvated N3 and P ions are fee most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on fee azide ion is dispersed by delocalization. When fee effect of solvation is diminished in DMSO, other factors become more important. These include fee strength of fee bond being formed, which would account for fee reversed order of fee halides in fee two series. There is also evidence fiiat S( 2 transition states are better solvated in protic dipolar solvents than in protic solvents. 

In a similar manner, Jt-allyl complexes of manganese, iron, and molybdenum carbonyls have been obtained from the corresponding metal carbonyl halides [5], In the case of the reaction of dicarbonyl(r 5-cyclopentadienyl)molybdenum bromide with allyl bromide, the c-allyl derivative is obtained in 75% yield in dichloromethane, but the Jt-allyl complex is the sole product (95%), when the reaction is conducted in a watenbenzene two-phase system. Similar solvent effects are observed in the corresponding reaction of the iron compound. As with the cobalt tetracarbonyl anion, it is... 

A more familiar example is Sn2 addition of an anionic nucleophile to an alkyl halide. In the gas phase, this occurs without activation energy, and the known barrier for the process in solution is a solvent effect (see discussion in Chapter 6). Finally, reactions of electron-deficient species, including transition-metal complexes, often occur with little or no energy barrier. Processes as hydroboration and 3-hydride elimination are likely candidates. 

Solvent effects Different solvents have different effects on the nucleophilicity of a species. Solvents with acidic protons are called protic solvents, usually O—H or N—H groups. Polar protic solvents, e.g. dimethyl sulph-oxide (DMSO), dimethyl formamide (DMF), acetonitrile (CH3CN) and acetone (CH3COCH3) are often used in 8 2 reactions, since the polar reactants (nucleophile and alkyl halide) generally dissolve well in them. 

Tab. 2.8 Solvent effects on the formation of precipitates and complexes of the silver ion with halide ions (X-)...

Since these substitution reactions follow a two-term rate law, it is clear that solvent effects are very significant. Poorly coordinating solvents are benzene, carbon tetrachloride and sterically hindered alcohols and strongly coordinating solvents are water, lower alcohols, DMF, DMSO, acetonitrile and nitromethane. The first-order rate constants are greater in DMSO than in water. Since the majority of precursor platinum complexes used in synthetic and mechanistic studies are halo complexes, the replacement of halide ligands by solvent and the reversibility of this reaction are important features of platinum halide chemistry. 

The mechanism for the uncatalysed and H+-catalysed reactions of simple quinone methides with solvent and halide ions has been investigated. The observed differences in the isotope effects for addition of HX (X = Hal) and ROH are consistent with a stepwise mechanism for the H+-catalysed addition of solvent and concerted mechanism for the H+-catalysed reactions of halide ions79. 

These solvent effects can be explained in classical terms. The intrinsic (gas-phase) reactivity increases as the as the ionic radius falls. The reasons are simple as the anion becomes smaller, the surplus electron generates greater interelectronic repulsions and the reactivity rises. Dipolar aprotic solvents interact only weakly with the halides, so they do not appreciably affect this reactivity order. In protic solvents where hydrogen bonds play an important role, the smaller the ion, the more stabilized it becomes and the reactivity order is inverted. 

Figure 3.4 Order of increasing nudeophilicity of halide ions is influenced by polarizing influences such as solvent effects.

An analysis of these results in terms of solvent effects leads to the observation of similarities with Ritchie s work on the N+ relation. Thus the constant selectivities obtained in the solvolysis reactions of certain methyl derivatives (Table 9) may indicate the existence of a basic similarity between the rate-determining process in these reaction and in the electrophile-nucleophile combination reactions correlated by the IV+ relation. The failure of the methyl halides to conform to this pattern might suggest that their substitution reactions are fundamentally different, and that the free energy of activation is dependent on factors other than desolvation. 

The reaction of 9-cyanofluorenyl anion (9-CFA, prepared from 9-cyanofluorene and sodium ethoxide) with alkyl and benzyl halides in ethanol is shown to be an Sn2 reaction from a kinetic study (Bowden and Cook, 1968). Qualitative application of the Hughes-Ingold theory of solvent effects predicts that, for an SN2 reaction of this charge type, an increase in the dielectric constant of the medium will cause a small rate decrease. That should be the case if the reaction is studied in ethanol (dielectric constant, D, 24 2) and DMSO (D, 48-9) is added. The data for the 9-CFA alkylation are in accord with this prediction, as seen in Table 10. 

---