CHAPTER FOUR Alcohols and Alkyl Halides prepare alkyl halides, we can better appreciate the material in later chapters, where alkyl halides figure prominently in key chemical transformations. The preparation of alkyl halides also serves as a focal point to develop the principles of reaction mechanisms We'll begin with the preparation of alkyl halides from alcohols by reaction with hydro- gen halides R一OH+H一X R一X+H-OH Alcohol Hydrogen halide Alkyl halide Water The order of reactivity of the hydrogen halides parallels their acidity: HI HBr HCI>> HF. Hydrogen iodide is used infrequently, however, and the reaction of alcohols with hydrogen fluoride is not a useful method for the preparation of alkyl Among the various classes of alcohols, tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive toward hydrogen halides CH3OH RCH,OH R,CHOH R3 COH Primary Least reactive Most reactive Tertiary alcohols are converted to alkyl chlorides in high yield within minutes on reaction with hydrogen chloride at room temperature and below (CH3)3COH+ (CH3)3CCI +ho 2-Met propanol Hydrogen chloride 2-Chloro-2-methylpropane (tert-butyl chloride)(78-88%) he efficiency of a synthe Secondary and primary alcohols do not react with hydrogen chloride at rate ansted as a percenmally enough to make the preparation of the corresponding alkyl chlorides a method of vated temperatures are required in order to increase the rate of reaction en then eld, tical value. Therefore, the more reactive hydrogen HBr is use uct that could be formed if OH to any products other than those given in the equation Cyclohexanol Hydrogen bromide Bromocyclohexane(73%) Water 1-Heptano Hydrogen I-Bromoheptane The same kind of transformation may be carried out by heating an alcohol with sodium bromide and sulfuric acid CH3CH,CH,CH,OH CH CHCHCH 1-Butanol 1-Bromobutane(70-83%) Back Forward Main Menu Study Guide ToC Student OLC MHHE Websiteprepare alkyl halides, we can better appreciate the material in later chapters, where alkyl halides figure prominently in key chemical transformations. The preparation of alkyl halides also serves as a focal point to develop the principles of reaction mechanisms. We’ll begin with the preparation of alkyl halides from alcohols by reaction with hydro￾gen halides. The order of reactivity of the hydrogen halides parallels their acidity: HI HBr HCl HF. Hydrogen iodide is used infrequently, however, and the reaction of alcohols with hydrogen fluoride is not a useful method for the preparation of alkyl fluorides. Among the various classes of alcohols, tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive. Tertiary alcohols are converted to alkyl chlorides in high yield within minutes on reaction with hydrogen chloride at room temperature and below. Secondary and primary alcohols do not react with hydrogen chloride at rates fast enough to make the preparation of the corresponding alkyl chlorides a method of prac￾tical value. Therefore, the more reactive hydrogen halide HBr is used; even then, ele￾vated temperatures are required in order to increase the rate of reaction. The same kind of transformation may be carried out by heating an alcohol with sodium bromide and sulfuric acid. CH3CH2CH2CH2OH 1-Butanol (n-butyl alcohol) CH3CH2CH2CH2Br 1-Bromobutane (70–83%) (n-butyl bromide) NaBr, H2SO4 heat H2O Water Br Bromocyclohexane (73%) 80–100°C HBr Hydrogen bromide OH Cyclohexanol Water CH3(CH2)5CH2Br H2O 1-Bromoheptane (87–90%) HBr Hydrogen bromide CH3(CH2)5CH2OH 1-Heptanol 120°C (CH3)3COH 2-Methyl-2-propanol (tert-butyl alcohol) HCl Hydrogen chloride (CH3)3CCl 2-Chloro-2-methylpropane (tert-butyl chloride) (78–88%) H2O Water 25°C Increasing reactivity of alcohols toward hydrogen halides CH3OH Methyl Least reactive RCH2OH Primary R2CHOH Secondary R3COH Tertiary Most reactive R±OH Alcohol H±X Hydrogen halide R±X Alkyl halide H±OH Water 138 CHAPTER FOUR Alcohols and Alkyl Halides The efficiency of a synthetic transformation is normally expressed as a percent yield, or percentage of the theo￾retical yield. Theoretical yield is the amount of prod￾uct that could be formed if the reaction proceeded to completion and did not lead to any products other than those given in the equation. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website