Halides, alkyl solvolysis

The Lewis base that acts as the nucleophile often is but need not always be an anion Neutral Lewis bases can also serve as nucleophiles Common examples of substitutions involving neutral nucleophiles include solvolysis reactions Solvolysis reactions are substitutions m which the nucleophile is the solvent m which the reaction is carried out 8olvolysis m water (hydrolysis) converts an alkyl halide to an alcohol... 

In order to compare S l substitution rates in a range of alkyl halides experimental con ditions are chosen m which competing substitution by the 8 2 route is very slow One such set of conditions is solvolysis m aqueous formic acid (HCO2H)... 

Tertiary alkyl halides are so sterically hindered to nucleophilic attack that the pres ence of any anionic Lewis base favors elimination Usually substitution predominates over elimination m tertiary alkyl halides only when anionic Lewis bases are absent In the solvolysis of the tertiary bromide 2 bromo 2 methylbutane for example the ratio of substitution to elimination is 64 36 m pure ethanol but falls to 1 99 m the presence of 2 M sodium ethoxide... 

A s-sulfide accelerates the solvolysis nearly 3000-fold compared to the simple alkyl halide and approximately 15,000-fold compared to the B-chloroether. On the other hand, the y-sulfide offers no assistance apparently closure to the four-membered cyclic sulfon-ium ion cannot compete with direct displacement by the external nucleophile. 

Reference has already been made in the last chapter to the generation of carbocations, in ion pairs, as intermediates in some displacement reactions at a saturated carbon atom, e.g. the solvolysis of an alkyl halide via the SN1 mechanism. Carbocations are, however, fairly widespread in occurrence and, although their existence is often only transient, they are of considerable importance in a wide variety of chemical reactions. 

Studies on the variation in the distribution of stereoisonieric products as a function of the accessible independent variables permit deductions concerning the multiplicity of the reaction paths. A classic example is found in Hughes and Ingold s studies into the mechanism of the solvolysis of alkyl halides in aqueous-alcoholic media, an important variable, being the concentration of base or other nucleophilic reagent (56 ). The obvious variable in hydrogenation studies is the pressure of... 

Rate constants and products have been reported for solvolysis of benzhydryl chloride and /7-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)-water and-ethanol, along with additional kinetic data for solvolysis of r-butyl and other alkyl halides in 97% TFE and 97% hexafluoropropan-2-ol. The results are discussed in terms of solvent ionizing power Y and nucleophilicity N, and contributions from other solvation effects are considered. Comparisons with other 3 nI reactions show that the solvolyses of benzhydryl chloride in TFE mixtures are unexpectedly fast an additional solvation effect influences solvolysis leading to delocalized cations. 

The typical reactions of carbocation intermediates were discussed in Chapter 7. The solvolysis of alkyl halides is an example of the involvement of carbocations in the SnI mechanism, in other words, where the final outcome is a nucleophilic substitution. The first step is a heterolytic cleavage of the C—X bond. Properties of X which favor heterolytic cleavage, namely electronegativity difference with carbon (the larger, the better) and the degree of overlap of the X orbital with the spn orbital of carbon (the smaller the better), have already been elucidated (Chapter 4). The transition state has partial... 

COMPARISON OF THE HYDROLYSIS OF /-ALKYL FLUOROACETATES WITH THE SOLVOLYSIS OF /-ALKYL HALIDES AT 25°C... 

Steric hindrance is relatively unimportant in SN1 reactions because the rate is independent of the nucleophile. In fact, steric acceleration is possible in the solvolysis of highly branched alkyl halides through relief of steric compression between the alkyl groups in the halide by formation of a planar cation ... 

The steady-state approximation, and solvolysis of alkyl halides and arenesulfonates... 

Many alkyl halides and arenesulfonates (R—X) undergo first-order solvolysis in ionising solvents (SOH) and the essential experimental features maybe accommodated by the generic mechanism shown in Scheme 4.4. The mechanism includes reversible ionisation of the covalent substrate to give a contact (intimate) ion pair, R+ X-, which undergoes internal return or nucleophilic capture by the solvent (in some cases, there is evidence for the intervention of solvent-separated ion pairs, and even fully dissociated ion pairs, but the minimalist mechanism of Scheme 4.4 serves our present purpose adequately) [13]. 

Solvolysis of alkyl halides in alcohols yields ethers as the products of reaction. 

---