Tertiary halides solvolysis

Changing the solvent in which a reaction is carried out often exerts a profound effect on its rate and may, indeed, even result in a change in its mechanistic pathway. Thus for a halide that undergoes hydrolysis by the SN1 mode, increase in the polarity of the solvent (i.e. increase in e, the dielectric constant) and/or its ion-solvating ability is found to result in a very marked increase in reaction rate. Thus the rate of solvolysis of the tertiary halide, Me3CBr, is found to be 3 x 104 times faster in 50% aqueous ethanol than in ethanol alone. This occurs because, in the S,vl mode, charge is developed and concentrated in... 

All are tertiary halides so that attack by the S mode would not be expected to occur on (16) or (17) any more than it did on (8) (cf. p. 82). Sn2 attack from the back on the carbon atom carrying Br would in any case be prevented in (16) and (17) both sterically by their cagelike structure, and also by the impossibility of forcing their fairly rigid framework through transition states with the required planar distribution of bonds to the bridgehead carbon atom (cf. p. 84). Solvolysis via rate-limiting formation of the ion pair (SN1), as happens with (8) is... 

They took as their standard reaction the SN1 solvolysis of the tertiary halide, 2-chloro-2-methylpropane (46), and selected as their standard solvent 80% aqueous ethanol (80% EtOH/20% H20) ... 

Tertiary halide Favored by nonbasic nucleophiles in polar solvents Does not occur Occurs under solvolysis conditions Highly favored when bases are used... 

This solvolysis is a substitution because methoxide has replaced bromide on the tert-butyl group. It does not go through the SN2 mechanism, however. The SN2 requires a strong nucleophile and a substrate that is not too hindered. Methanol is a weak nucleophile, and ferf-butyl bromide is a hindered tertiary halide—a poor SN2 substrate. 

The tertiary halide feri-butyl bromide will undergo solvolysis by the Sj l mechanism. 

The assumptions may fail for primary halides in aqueous media and for solvolysis reactions of tertiary halides. 

To summarize, the working assumptions in this chapter have value, but are somewhat limited, and they apply only to reactions of alkyl halides and alkyl sulfonate esters. They work quite often, but may fail for primary halides in aqueous media and for solvolysis reactions of tertiary halides in particular. The take-home lesson is to use the assumptions to begin an analysis, check to be certain that they are reasonable, and then make an educated guess. Check each guess against the literature or do the experiment to determine the actual reaction products. However, even if the prediction is incorrect, the exercise forces a review of mechanism, structure, and reactivity, as well as reaction conditions. 

When 2-bromo-2-methylpropane is dissolved in methanol, it disappears rapidly. As expected, the major product, 2-methoxy-2-methylpropane, arises by solvolysis. However, there is also a signihcant amount of another compound, 2-methylpropene, the product of elimination of HBr from the original substrate. Thus, in competition with the SnI process, which leads to displacement of the leaving group, another mechanism transforms the tertiary halide, giving rise to the alkene. What is this mechanism Is it related to the SnI reaction ... 

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... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

The mechanism of this remarkable reaction, in which a halogen atom in a tertiary acetylenic halide is substituted by the nucleophilic group NH2 with high yields, is not completely dear. Probably the ethynyl hydrogen atom is abstracted in the first step by the strongly basic amide. The results from solvolysis experiments carried out with tertiary acetylenic bromides [22] suggest that the intermediate -C C-C(CH3)2C1 looses CP to give the Zwitter-ionic intermediate C=C-C+(CH3)2. This is subsequently attacked by NH3 or-NH2. 

Experimentally it has been found that primary and secondary amines react by solvolysis, while only the tertiary amines generally produce reduction, if reduction is observed. It thus seemed appropriate to study the reaction of niobium (V) halides with pyridine, where proton dissociation need not be considered and any reaction would necessarily lead to a simple adduct of pyridine or reduction of the metal halide. In this work, reduction of the niobium(V) halides was observed, and the reaction products were characterized. Elucidation of the pyridine oxidation products has permitted an interpretation of the reaction mechanism in terms of the two-electron reduction of niobium(V) by the pyridine molecule. 

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