ALKYNES 219 9. 29 The alkane formed by hydrogenation of(S)-3-methyl-1-pentyne is achiral; it cannot be optically CHCH H ≡CH CH-CHLCHCH2CH3 HC (S)-3-Methyl-1-pentyne ( does not have a ster The product of hydrogenation of(S)-4-methyl-1-hexyne is optically active because a stereogenic center is present in the starting material and is carried through to the product. CH,CH, H CH3 CH2 一CHC≡CH CCH,CH, CH3 (S)-4-Methyl-l-hexyne (S)3-Methy lhexane Both(S)-3-methyl-l-pentyne and (S)-4-methyl-1-hexyne yield optically active products when their triple bonds are reduced to double bonds group. lodide is a better leaving group than chloride and is the one replaced by acetylide avr 9.30 (a) The dihaloalkane contains both a primary alkyl chloride and a primary alkyl iodide function NaC≡CH+ClCH2CH2 CHCHCH,CH,I— CICH.CH,CH,CH,CH,CHC≡CH Sodium 1-Chloro-6-jodohexane 8-Chloro-1-octyn acetylide (b) Both vicinal dibromide functions are converted to alkyne units on treatment with excess 1. excess NaNH, NH BrCH,CHCH, CH, CHCH, Br 2.H,O HC≡CCH2CH2C≡CH 1, 2. 5, 6-Tetrabromohexane 1. 5-Hexadiyne (c) The starting material is a geminal dichloride. Potassium tert-butoxide in dimethyl sulfoxide is se to convert it to an alky KOC(CH, )x DM CCH s心C=cH CI 1. 1-Dichloro1 Ethynylcyclopropane clopropylethane (d) Alkyl p-toluenesulfonates react similarly to alkyl halides in nucleophilic substitution reactions.The alkynide nucleophile displaces the p-toluenesulfonate leaving group from ethyl C≡C:+CH,-OS -CH3 ≡CCH2CH3 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website9.29 The alkane formed by hydrogenation of (S)-3-methyl-1-pentyne is achiral; it cannot be optically active. The product of hydrogenation of (S)-4-methyl-1-hexyne is optically active because a stereogenic center is present in the starting material and is carried through to the product. Both (S)-3-methyl-1-pentyne and (S)-4-methyl-1-hexyne yield optically active products when their triple bonds are reduced to double bonds. 9.30 (a) The dihaloalkane contains both a primary alkyl chloride and a primary alkyl iodide functional group. Iodide is a better leaving group than chloride and is the one replaced by acetylide. (b) Both vicinal dibromide functions are converted to alkyne units on treatment with excess sodium amide. (c) The starting material is a geminal dichloride. Potassium tert-butoxide in dimethyl sulfoxide is a sufficiently strong base to convert it to an alkyne. (d) Alkyl p-toluenesulfonates react similarly to alkyl halides in nucleophilic substitution reactions. The alkynide nucleophile displaces the p-toluenesulfonate leaving group from ethyl p-toluenesulfonate. Phenylacetylide ion C C Ethyl p-toluenesulfonate OS O O CH2 CH3 CH3 1-Phenyl-1-butyne C CCH2CH3 KOC(CH3)3, DMSO heat CCH3 Cl Cl 1,1-Dichloro-1- cyclopropylethane C CH Ethynylcyclopropane BrCH2CHCH2CH2CHCH2Br 1,2,5,6-Tetrabromohexane 1. excess NaNH2, NH3 2. H2O HC CCH2CH2C CH Br Br 1,5-Hexadiyne ClCH2CH2CH2CH2CH2CH2 NaC CH I ClCH2CH2CH2CH2CH2CH2C CH Sodium acetylide 1-Chloro-6-iodohexane 8-Chloro-1-octyne H2 Pt H H3C CH2C CH3CH2 CH (S)-4-Methyl-1-hexyne CH2CH2CH3 H H3C CH3CH2 (S)-3-Methylhexane C C H2 Pt CH3CH2CHCH2CH3 CH3 3-Methylpentane (does not have a stereogenic center; optically inactive) H H3C C CH CH3CH2 (S)-3-Methyl-1-pentyne C ALKYNES 219 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website