H1 Preparation of alkenes and alkynes ■Alkynes 1.The electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base 2.Alkylation of terminal alkynes NaNH2 heat R一C=C一R 2HBr Fig 2.Synthesis of an allyne from an alkene
College H1 Preparation of alkenes of Science and alkynes Alkynes 1. The electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base 2. Alkylation of terminal alkynes
A.Elimination of dihalides Colle C 2 equiv B C三C strong HH H X (E2) vicinal geminal dihalide dihalide 1 equiv. of base C三C HH easier more difficult NaNH2 KOH,200C BuOK NaNH2,150C KOH
College A. Elimination of dihalides of Science C X H C X H C H H C X X or 2 e quiv. st r o n g B (E 2 ) C C vi c ina l diha l i d e ge m ina l diha l i d e C X H C X H C C C X C H 1 equiv. of base 1 equiv. of base easier NaNH 2 tBuOK KOH more difficult KOH, 200ºC NaNH 2, 150ºC
Question.Starting with 2-butanol,show how to prepare 2-butyne. Solution:You know that you can convert a vicinal dihalide to an alkyne and you know you can make a dihalide from an alkene. So you just need to dehydrate the alcohol with a strong,non- nucleophilic acid like sulfuric acid,then do an addition of bromine,then do a double elimination reaction with a strong base: OH CH3一C≡C-CH3
College of Science Question. Starting with 2-butanol, show how to prepare 2-butyne. OH CH3 C C CH3 ? H2SO4, heat Br2 CCl4 Br Br 2 equivalents NaNH2 Solution: You know that you can convert a vicinal dihalide to an alkyne and you know you can make a dihalide from an alkene. So you just need to dehydrate the alcohol with a strong, nonnucleophilic acid like sulfuric acid, then do an addition of bromine, then do a double elimination reaction with a strong base:
B.Alkylation of terminal alkynes (SN2 substitution) NaNH2 H一C≡C一H 8⑧1°RX,H-C=C-R H-C≡CNa only good Nu, NaNH2 strong B R-C≡CNa C-C bond-forming reaction RX R-C三C-R Question.Starting with acetylene and any necessary alkyl halides,show how to prepare 2-hexyne. Check Answer
College B. Alkylation of terminal alkynes of Science (SN2 substitution) H C C H H C CNa H C C R NaNH2 1º RX only good Nu, strong B NaNH2 RCCNa R'X RCCR' C–C bond-forming reaction Question. Starting with acetylene and any necessary alkyl halides, show how to prepare 2-hexyne. Check Answer
Question.Starting with acetylene and any necessary alkyl halides,show how to prepare 2-hexyne. H一C≡C一H CH3一C≡C-CH2CH2CH3 Solution:Decide what carbons need to be added to each side,then convert the terminal alkyne to an acetylide ion and do the Sx2 reaction: H一C三C-H CH3CC-CH2CH2CH3 1.NaNH, 1.NaNH2 2.CH3I,DMF 2.CH3CH2CH2Br,DMF CH3-C=C-H
College of Science Question. Starting with acetylene and any necessary alkyl halides, show how to prepare 2-hexyne. H C C H CH3 C C CH2CH2CH3 Solution: Decide what carbons need to be added to each side, then convert the terminal alkyne to an acetylide ion and do the SN2 reaction: H C C H CH3 C C CH2CH2CH3 1. NaNH2 2. CH3I, DMF CH3 C C H 1. NaNH2 2. CH3CH2CH2Br, DMF