8.2 Relative Reactivity of Halide Leaving Groups TABLE 8.1 Representative Functional Group Transformations by Nucleophilic Substitution Reactions of Alkyl Halides(Continued) Nucleophile and comments General equation and specific example lodide ion (:: )Alkyl chlorides and bromides are converted to alkyl dide in acetone. Nal is soluble in lodide ion Alkyl chloride Alkyl iodide Chloride or acetone, but NaCl and NaBr are bromide io soluble and crystallize from the reaction mixture, driving the reac- CH3 CHCH3 CH3CHCH3 NaBr (solid) tion to complet 2-Bromopropane Sodium 2-lodopropand PROBLEM 8.1 Write a structural formula for the principal organic product formed in the reaction of methyl bromide with each of the following compounds (a)NaOH (sodium hydroxide (b)KOCH2 CH3(potassium ethoxide NaoC (sodium benzoate) (d)LiN3(lithium azide) (e)KCN (potassium cyanide NaSH (sodium hydrogen sulfid (g)Nal (sodium iodide) SAMPLE SOLUTION (a)The nucleophile in sodium hydroxide is the negatively charged hydroxide ion. The reaction that occurs is nucleophilic substitution of bro- mide by hydroxide. the product is methyl alcohol CH3--Br CH3OH+ Hydroxide ion Methyl bromide Methyl alcohol Bromide ion (substrate) product) (leaving group) With this as background, you can begin to see how useful alkyl halides are in syn- thetic organic chemistry. Alkyl halides may be prepared from alcohols by nucleophilic substitution, from alkanes by free-radical halogenation, and from alkenes by addition of hydrogen halides. They then become available as starting materials for the preparation of other functionally substituted organic compounds by replacement of the halide leav ing group with a nucleophile. The range of compounds that can be prepared by nucle- ophilic substitution reactions of alkyl halides is quite large; the examples shown in Table 8. 1 illustrate only a few of them. Numerous other examples will be added to the list in is and subsequent chapters 8.2 RELATIVE REACTIVITY OF HALIDE LEAVING GROUPS Among alkyl halides, alkyl iodides undergo nucleophilic substitution at the fastest rate, alkyl fluorides the slowest. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.2 Relative Reactivity of Halide Leaving Groups 305 TABLE 8.1 Representative Functional Group Transformations by Nucleophilic Substitution Reactions of Alkyl Halides (Continued) Nucleophile and comments Iodide ion (:I: ) Alkyl chlorides and bromides are converted to alkyl iodides by treatment with sodium iodide in acetone. NaI is soluble in acetone, but NaCl and NaBr are insoluble and crystallize from the reaction mixture, driving the reac￾tion to completion. : : General equation and specific example 2-Bromopropane CH3CHCH3 W Br Sodium iodide NaI 2-Iodopropane (63%) CH3CHCH3 I W Sodium bromide NaBr (solid) acetone Chloride or bromide ion X Iodide ion I Alkyl chloride or bromide R X Alkyl iodide R I acetone PROBLEM 8.1 Write a structural formula for the principal organic product formed in the reaction of methyl bromide with each of the following compounds: (a) NaOH (sodium hydroxide) (b) KOCH2CH3 (potassium ethoxide) (c) (d) LiN3 (lithium azide) (e) KCN (potassium cyanide) (f) NaSH (sodium hydrogen sulfide) (g) NaI (sodium iodide) SAMPLE SOLUTION (a) The nucleophile in sodium hydroxide is the negatively charged hydroxide ion. The reaction that occurs is nucleophilic substitution of bro￾mide by hydroxide. The product is methyl alcohol. With this as background, you can begin to see how useful alkyl halides are in syn￾thetic organic chemistry. Alkyl halides may be prepared from alcohols by nucleophilic substitution, from alkanes by free-radical halogenation, and from alkenes by addition of hydrogen halides. They then become available as starting materials for the preparation of other functionally substituted organic compounds by replacement of the halide leav￾ing group with a nucleophile. The range of compounds that can be prepared by nucle￾ophilic substitution reactions of alkyl halides is quite large; the examples shown in Table 8.1 illustrate only a few of them. Numerous other examples will be added to the list in this and subsequent chapters. 8.2 RELATIVE REACTIVITY OF HALIDE LEAVING GROUPS Among alkyl halides, alkyl iodides undergo nucleophilic substitution at the fastest rate, alkyl fluorides the slowest. Hydroxide ion (nucleophile) HO Methyl bromide (substrate) CH3 Br Bromide ion (leaving group) Br Methyl alcohol (product) CH3 OH NaOC O (sodium benzoate) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website