ALKYNES 211 9.6 Both l-pentyne and 2-pentyne can be prepared by alkylating acetylene. All the alkylation steps involve nucleophilic substitution of a methyl or primary alkyl halide HC≡CH CH2CH2C≡CH CH3CH2C≡CCH Acetylene 2-Pentyne A third isomer, 3-methyl-1-butyne, cannot be prepared by alkylation of acetylene, because it re quires a secondary alkyl halide as the alkylating agent. The reaction that takes place is elimination. not substitution HC≡C:+CH2CHCH HC≡CH+CH,=CHCH Br n ion bromide 9.7 Each of the dibromides shown yields 3, 3-dimethyl-l-butyne when subjected to double dehydro- halogenation with strong base 1.3Na (CH3)3CCCH, or(CH3)3CCHaCHBI CH3)3 CCHCH2Br 2.O (CH3)3CC≡CH 2.2-Dibromo-3,3- 1. 1-Dibromo-3.3. 1. 2-Dibromo-3,3 3,3-Dimethyl-1-butyr dimethylbutane 9.8 (b) The first task is to convert 1-propanol to propene CH3 CH,CH,OH CH3CHECH Propene After propene is available, it is converted to 1, 2-dibromopropane and then to propyne as described in the sample solution for part(a). (c) Treat isopropyl bromide with a base to effect dehydrohalogenation (CH3)2CHBr CHaCH-CH Isopropyl bromide Next, convert propene to propyne as in parts(a)and (b) (d) The starting material contains only two carbon atoms, and so an alkylation step is needed at some point. Propyne arises by alky lation of acetylene, and so the last step in the synthesis HC≡CH CHC≡CH The designated starting material, 1, l-dichloroethane, is a geminal dihalide and can be used to prepare acetylene by a double dehydrohalogenation NaNd CH_ CHCI2 2HO HC≡CH L. 1-Dichloroethane Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website9.6 Both 1-pentyne and 2-pentyne can be prepared by alkylating acetylene. All the alkylation steps involve nucleophilic substitution of a methyl or primary alkyl halide. A third isomer, 3-methyl-1-butyne, cannot be prepared by alkylation of acetylene, because it re￾quires a secondary alkyl halide as the alkylating agent. The reaction that takes place is elimination, not substitution. 9.7 Each of the dibromides shown yields 3,3-dimethyl-1-butyne when subjected to double dehydro￾halogenation with strong base. 9.8 (b) The first task is to convert 1-propanol to propene: After propene is available, it is converted to 1,2-dibromopropane and then to propyne as described in the sample solution for part (a). (c) Treat isopropyl bromide with a base to effect dehydrohalogenation. Next, convert propene to propyne as in parts (a) and (b). (d) The starting material contains only two carbon atoms, and so an alkylation step is needed at some point. Propyne arises by alkylation of acetylene, and so the last step in the synthesis is The designated starting material, 1,1-dichloroethane, is a geminal dihalide and can be used to prepare acetylene by a double dehydrohalogenation. 1. NaNH2, NH3 2. H2O CH3CHCl2 1,1-Dichloroethane HC CH Acetylene 1. NaNH2, NH3 HC CH 2. CH3Br Acetylene CH3C CH Propyne (CH3)2CHBr NaOCH2CH3 Isopropyl bromide Propene CH3CH CH2 CH3CH2CH2OH H2SO4 heat 1-Propanol Propene CH3CH CH2 1. 3NaNH2 2. H2O (CH or or 3)3CCCH3 Br Br 2,2-Dibromo-3,3- dimethylbutane (CH3)3CCH2CHBr2 1,1-Dibromo-3,3- dimethylbutane Br (CH3)3CCHCH2Br 1,2-Dibromo-3,3- dimethylbutane (CH3)3CC CH 3,3-Dimethyl-1-butyne E2 CH3CHCH3 Br Isopropyl bromide HC CH Acetylene CH2 CHCH3 Propene HC Acetylide ion C 1. NaNH2, NH3 2. CH3CH2Br Acetylene 1-Butyne 2-Pentyne HC CH 1. NaNH2, NH3 2. CH3Br CH3CH2C CH CH3CH2C CCH3 Acetylene 1-Pentyne HC CH CH3CH2CH2C CH 1. NaNH2, NH3 2. CH3CH2CH2Br ALKYNES 211 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website