8.9 Carbocation Stability and SN1 Reaction Rates 315 increases the rate of substitution. Among related atoms, polarizability increases with increasing size. Thus iodide is the most polarizable and most nucleophilic halide ion, fluoride the least PROBLEM 8.7 Sodium nitrite(NaNO) reacted with 2-iodooctane to give a mix ture of two constitutionally isomeric compounds of molecular formula CgH17NO2 in a combined yield of 88%. Suggest reasonable structures for these two isomers. 8.8 THE SN1 MECHANISM OF NUCLEOPHILIC SUBSTITUTION laving just learned that tertiary alkyl halides are practically inert to substitution by the SN2 mechanism because of steric hindrance, we might wonder whether they undergo nucleophilic substitution at all. We'll see in this section that they do, but by a mecha nism different from sy2 Hughes and Ingold observed that the hydrolysis of tert-butyl bromide, which occurs readily, is characterized by a first-order rate law (CH3)3CBr H,0->(CH3)3COH+ tert-Butyl bromide Water tert-Butyl alcohol Hydrogen bromide Rate=k[(CH3)3CBr They found that the rate of hydrolysis depends only on the concentration of tert-butyl bromide. Adding the stronger nucleophile hydroxide ion, moreover, causes no change in the rate of substitution, nor does this rate depend on the concentration of hydroxide. Just as second-order kinetics was interpreted as indicating a bimolecular rate-determining step, first-order kinetics was interpreted as evidence for a unimolecular rate-determinin step-a step that involves only the alkyl halide The proposed mechanism is outlined in Figure 8.5 and is called SNl, standing for substitution nucleophilic unimolecular. The first step, a unimolecular dissociation of lier introduce se as ear The Sx1 mechanism was ear energy diagram for the process is shown in Figure 8 ediate, is rate-determining. An the alkyl halide to form a carbocation as the key interm PROBLEM 8.8 Suggest a structure for the product of nucleophilic substitution obtained on solvolysis of tert-butyl bromide in methanol, and outline a reason- able mechanism for its formation The SNI mechanism is an ionization mechanism. The nucleophile does not participate until after the rate-determining step has taken place. Thus, the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions pro- ceeding by the Sn2 pathway. How the structure of the alkyl halide affects the rate of SNI reactions is the topic of the next section 8.9 CARBOCATION STABILITY AND SN1 REACTION RATES In order to compare SNI substitution rates in a range of alkyl halides, experimental con- ditions are chosen in which competing substitution by the Sn2 route is very slow. One such set of conditions is solvolysis in aqueous formic acid(HCO2H) RX + HO HX Alkyl halide Water Alcohol Hydrogen halide Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.9 Carbocation Stability and SN1 Reaction Rates 315 increases the rate of substitution. Among related atoms, polarizability increases with increasing size. Thus iodide is the most polarizable and most nucleophilic halide ion, fluoride the least. PROBLEM 8.7 Sodium nitrite (NaNO2) reacted with 2-iodooctane to give a mix￾ture of two constitutionally isomeric compounds of molecular formula C8H17NO2 in a combined yield of 88%. Suggest reasonable structures for these two isomers. 8.8 THE SN1 MECHANISM OF NUCLEOPHILIC SUBSTITUTION Having just learned that tertiary alkyl halides are practically inert to substitution by the SN2 mechanism because of steric hindrance, we might wonder whether they undergo nucleophilic substitution at all. We’ll see in this section that they do, but by a mecha￾nism different from SN2. Hughes and Ingold observed that the hydrolysis of tert-butyl bromide, which occurs readily, is characterized by a first-order rate law: Rate  k[(CH3)3CBr] They found that the rate of hydrolysis depends only on the concentration of tert-butyl bromide. Adding the stronger nucleophile hydroxide ion, moreover, causes no change in the rate of substitution, nor does this rate depend on the concentration of hydroxide. Just as second-order kinetics was interpreted as indicating a bimolecular rate-determining step, first-order kinetics was interpreted as evidence for a unimolecular rate-determining step—a step that involves only the alkyl halide. The proposed mechanism is outlined in Figure 8.5 and is called SN1, standing for substitution nucleophilic unimolecular. The first step, a unimolecular dissociation of the alkyl halide to form a carbocation as the key intermediate, is rate-determining. An energy diagram for the process is shown in Figure 8.6. PROBLEM 8.8 Suggest a structure for the product of nucleophilic substitution obtained on solvolysis of tert-butyl bromide in methanol, and outline a reason￾able mechanism for its formation. The SN1 mechanism is an ionization mechanism. The nucleophile does not participate until after the rate-determining step has taken place. Thus, the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions pro￾ceeding by the SN2 pathway. How the structure of the alkyl halide affects the rate of SN1 reactions is the topic of the next section. 8.9 CARBOCATION STABILITY AND SN1 REACTION RATES In order to compare SN1 substitution rates in a range of alkyl halides, experimental con￾ditions are chosen in which competing substitution by the SN2 route is very slow. One such set of conditions is solvolysis in aqueous formic acid (HCO2H): Alkyl halide RX Water H2O Hydrogen halide HX Alcohol ROH formic acid tert-Butyl bromide (CH3)3CBr Water H2O Hydrogen bromide HBr tert-Butyl alcohol (CH3)3COH The SN1 mechanism was ear￾lier introduced in Section 4.11. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website