8.6 Steric Effects in SN2 Reactions Least crowded- most reactive ea° CH3 Br CH3, Br (CH3)CHBr 666 FIGURE 8.3 Ball-and-spoke and space-filling models of alkyl bromides, showing how sub stituents shield the carbon atom that bears the leaving group from attack by a nucleophile. The ucleophile must attack from the side opposite the bond to the leaving group PROBLEM 8.6 Identify the compound in each of the following pairs that reacts with sodium iodide in acetone at the faster rate (a)1-Chlorohexane or cyclohexyl chloride (b)1-Bromopentane or 3-bromopentane (c)2-Chloropentane or 2-fluoropentane ( d)2-Bromo-2-methylhexane or 2-bromo-5-methylhexane (e)2-Bromopropane or 1-bromodecane SAMPLE SOLUTION (a)Compare the structures of the two chlorides. 1-Chloro- hexane is a primary alkyl chloride; cyclohexyl chloride is secondary. Primary alkyl halides are less crowded at the site of substitution than secondary ones and react faster in substitution by the SN2 mechanism. 1-Chlorohexane is more reactive CH3CH2 CH2 CH2 CH2 CH2 CI 1-Chlorohexane Cyclohexyl chloride primary, more reactive) (secondary, less reactive Alkyl groups at the carbon atom adjacent to the point of nucleophilic attack also decrease the rate of the SN2 reaction. Compare the rates of nucleophilic substitution in the series of primary alkyl bromides shown in Table 8.3. Taking ethyl bromide as the tandard and successively replacing its C-2 hydrogens by methyl groups, we see that each additional methyl group decreases the rate of displacement of bromide by iodide The effect is slightly smaller than for alkyl groups that are attached directly to the car bon that bears the leaving group, but it is still substantial. When C-2 is completely sub stituted by methyl groups, as it is in neopentyl bromide [(CH3)3CCH2Br], we see the unusual case of a primary alkyl halide that is practically inert to substitution by the sN2 mechanism because of steric hindrance Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.6 Steric Effects in SN2 Reactions 311 PROBLEM 8.6 Identify the compound in each of the following pairs that reacts with sodium iodide in acetone at the faster rate: (a) 1-Chlorohexane or cyclohexyl chloride (b) 1-Bromopentane or 3-bromopentane (c) 2-Chloropentane or 2-fluoropentane (d) 2-Bromo-2-methylhexane or 2-bromo-5-methylhexane (e) 2-Bromopropane or 1-bromodecane SAMPLE SOLUTION (a) Compare the structures of the two chlorides. 1-Chloro￾hexane is a primary alkyl chloride; cyclohexyl chloride is secondary. Primary alkyl halides are less crowded at the site of substitution than secondary ones and react faster in substitution by the SN2 mechanism. 1-Chlorohexane is more reactive. Alkyl groups at the carbon atom adjacent to the point of nucleophilic attack also decrease the rate of the SN2 reaction. Compare the rates of nucleophilic substitution in the series of primary alkyl bromides shown in Table 8.3. Taking ethyl bromide as the standard and successively replacing its C-2 hydrogens by methyl groups, we see that each additional methyl group decreases the rate of displacement of bromide by iodide. The effect is slightly smaller than for alkyl groups that are attached directly to the car￾bon that bears the leaving group, but it is still substantial. When C-2 is completely sub￾stituted by methyl groups, as it is in neopentyl bromide [(CH3)3CCH2Br], we see the unusual case of a primary alkyl halide that is practically inert to substitution by the SN2 mechanism because of steric hindrance. Cyclohexyl chloride (secondary, less reactive) H Cl 1-Chlorohexane (primary, more reactive) CH3CH2CH2CH2CH2CH2Cl Least crowded– most reactive Most crowded– least reactive CH3Br CH3CH2Br (CH3)2CHBr (CH3)3CBr FIGURE 8.3 Ball-and-spoke and space-filling models of alkyl bromides, showing how sub￾stituents shield the carbon atom that bears the leaving group from attack by a nucleophile. The nucleophile must attack from the side opposite the bond to the leaving group. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website