ALCOHOLS AND ALKYL HALIDES 4.14 1-Butanol is a primary alcohol; 2-butanol is a secondary alcohol. A carbocation intermediate is possible in the reaction of 2-butanol with hydrogen bromide but not in the corresponding reaction of 1-butanol The mechanism of the reaction of 1-butanol with hydrogen bromide proceeds by displacement of water by bromide ion from the protonated form of the alcohol(the alkyloxonium ion) Protonation of the alcohol CH CH, CH,CH,O:+ H →CH2CH CHO++ Hydroge Bromide Displacement of water by bromide: CHCHCH CH,O+ CH CH Br o The slow step, displacement of water by bromide from the oxonium ion, is bimolecular. The reaction of 1-butanol with hydrogen bromide follows the Sn2 mechanism. The reaction of 2-butanol with hydrogen bromide involves a carbocation intermediate Protonation of the alcohol CH3 CH CHCH3+ CH3,CHCH3 Br: 2-Butanol sec-Butyloxonium ion Bromide io Dissociation of the oxonium ion CH3, CHCH3 CHaCH,CHCH Butyloxonium io sec-Butyl cation Water Capture of sec-butyl cation by bromide: CHCH Br CHCH2—CH3CH2CHCH3 Bromide ion sec-Butyl cation 2-Bromobutane The slow step, dissociation of the oxonium ion, is unimolecular. The reaction of 2-butanol with 4.15 The most stable alkyl free radicals are tertiary. The tertiary free radical having the formula C has the same skeleton as the carbocation in Problem 4.13 CH CH CH2-C Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website4.14 1-Butanol is a primary alcohol; 2-butanol is a secondary alcohol. A carbocation intermediate is possible in the reaction of 2-butanol with hydrogen bromide but not in the corresponding reaction of 1-butanol. The mechanism of the reaction of 1-butanol with hydrogen bromide proceeds by displacement of water by bromide ion from the protonated form of the alcohol (the alkyloxonium ion). Protonation of the alcohol: Displacement of water by bromide: The slow step, displacement of water by bromide from the oxonium ion, is bimolecular. The reaction of 1-butanol with hydrogen bromide follows the SN2 mechanism. The reaction of 2-butanol with hydrogen bromide involves a carbocation intermediate. Protonation of the alcohol: Dissociation of the oxonium ion: Capture of sec-butyl cation by bromide: The slow step, dissociation of the oxonium ion, is unimolecular. The reaction of 2-butanol with hydrogen bromide follows the SN1 mechanism. 4.15 The most stable alkyl free radicals are tertiary. The tertiary free radical having the formula C5H11 has the same skeleton as the carbocation in Problem 4.13. CH3CH2 C CH3 CH3 Bromide ion sec-Butyl cation 2-Bromobutane CH3CH2CHCH3 Br CHCH3 CH3CH2 Br CH3CH2CHCH3 O H H CH3CH2CHCH3 O H H sec-Butyloxonium ion sec-Butyl cation Water slow CH3CH2CHCH3 O H Br 2-Butanol Hydrogen bromide sec-Butyloxonium ion Bromide ion H Br CH3CH2CHCH3 O H H CH3CH2CH2CH2Br 1-Bromobutane O Water slow Br Bromide ion CH3CH2CH2 CH2 O Butyloxonium ion H H H H CH3CH2CH2CH2O H 1-Butanol H Br Hydrogen bromide Br Butyloxonium ion Bromide CH3CH2CH2CH2O H H 70 ALCOHOLS AND ALKYL HALIDES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website