8.9 Carbocation Stability and SN1 Reaction Rates Neither formic acid nor water is very nucleophilic, and So SN2 substitution is suppressed The relative rates of hydrolysis of a group of alkyl bromides under these conditions are presented in Table 8.5 The relative rate order in SNI reactions is exactly the opposite of that seen in SN2 reactions ctivity: methyl primary ndary tertiary SN2 reactivity: tertiary secondary primary methyl Clearly, the steric crowding that influences reaction rates in Sn2 processes plays no role in SNl reactions. The order of alkyl halide reactivity in SNl reactions is the same as the order of carbocation stability: the more stable the carbocation, the more reactive the alkyl halide. We have seen this situation before in the reaction of alcohols with hydrogen halides(Section 4.12), in the acid-catalyzed dehydration of alcohols(Section 5.9), and in the conversion of alkyl halides to alkenes by the El mechanism(Section 5. 17). As in these other reactions, an electronic effect, specifically, the stabilization of the carboca tion intermediate by alkyl substituents, is the decisive factor PROBLEM 8.9 Identify the compound in each of the following pairs that reacts It the faster rate in an Sn1 reaction (a)Isopropyl bromide or isobutyl bromide (b)Cyclopentyl iodide or 1-methylcyclopentyl iodide (c) Cyclopentyl bromide or 1-bromo-2, 2-dimethylpropane (d) tert-Butyl chloride or tert-butyl iodide SAMPLE SOLUTION (a) Isopropyl bromide, (CH3)2 CHBr, is a secondary alkyl halide, whereas isobutyl bromide,(CH3)2 CHCH2 Br, is primary. Since the rate- determining step in an Sn1 reaction is carbocation formation and since secondary carbocations are more stable than primary carbocations, pyl bromide is eactive than isobutyl bromide in nucleophilic substitution by the n1 mechanism Primary carbocations are so high in energy that their intermediacy in nucleophilic substitution reactions is unlikely. When ethyl bromide undergoes hydrolysis in aqueous omic acid, substitution probably takes place by a direct displacement of bromide by water in an SN2-like process TABLE 8.5Reactivity of Some Alkyl Bromides Toward Substitution by the Sn1 Mechanism* Alkyl bromide Structure Relative rate thyl bromid CH3Br Unsubstituted Ethyl bromide CH3CH2 Br (CH3) Secondary tert-Butyl bromide (CH3)3CBr Tertiary 100000,000 Solvolysis in aqueous formic acid. tio of rate constant k for indicated alkyl bromide to k for methyl bromide at 25C. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.9 Carbocation Stability and SN1 Reaction Rates 317 Neither formic acid nor water is very nucleophilic, and so SN2 substitution is suppressed. The relative rates of hydrolysis of a group of alkyl bromides under these conditions are presented in Table 8.5. The relative rate order in SN1 reactions is exactly the opposite of that seen in SN2 reactions: SN1 reactivity: methyl  primary  secondary  tertiary SN2 reactivity: tertiary  secondary  primary  methyl Clearly, the steric crowding that influences reaction rates in SN2 processes plays no role in SN1 reactions. The order of alkyl halide reactivity in SN1 reactions is the same as the order of carbocation stability: the more stable the carbocation, the more reactive the alkyl halide. We have seen this situation before in the reaction of alcohols with hydrogen halides (Section 4.12), in the acid-catalyzed dehydration of alcohols (Section 5.9), and in the conversion of alkyl halides to alkenes by the E1 mechanism (Section 5.17). As in these other reactions, an electronic effect, specifically, the stabilization of the carboca￾tion intermediate by alkyl substituents, is the decisive factor. PROBLEM 8.9 Identify the compound in each of the following pairs that reacts at the faster rate in an SN1 reaction: (a) Isopropyl bromide or isobutyl bromide (b) Cyclopentyl iodide or 1-methylcyclopentyl iodide (c) Cyclopentyl bromide or 1-bromo-2,2-dimethylpropane (d) tert-Butyl chloride or tert-butyl iodide SAMPLE SOLUTION (a) Isopropyl bromide, (CH3)2CHBr, is a secondary alkyl halide, whereas isobutyl bromide, (CH3)2CHCH2Br, is primary. Since the rate￾determining step in an SN1 reaction is carbocation formation and since secondary carbocations are more stable than primary carbocations, isopropyl bromide is more reactive than isobutyl bromide in nucleophilic substitution by the SN1 mechanism. Primary carbocations are so high in energy that their intermediacy in nucleophilic substitution reactions is unlikely. When ethyl bromide undergoes hydrolysis in aqueous formic acid, substitution probably takes place by a direct displacement of bromide by water in an SN2-like process. TABLE 8.5 Reactivity of Some Alkyl Bromides Toward Substitution by the SN1 Mechanism* Alkyl bromide Methyl bromide Ethyl bromide Isopropyl bromide tert-Butyl bromide CH3Br CH3CH2Br (CH3)2CHBr (CH3)3CBr Structure Unsubstituted Primary Secondary Tertiary Class 1 2 43 100,000,000 Relative rate† *Solvolysis in aqueous formic acid. † Ratio of rate constant k for indicated alkyl bromide to k for methyl bromide at 25°C. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website