Organic Chemistry 4 th Edition Paula Yurkanis Bruice Chapter 6 Reactions of Alkynes Introduction to Multistep Synthesis Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall Alkynes are hydrocarbons that contain a carbon–carbon triple bond General formula: C n H 2n–2 (acyclic); C n H 2n–4 (cyclic) Nomenclature In common nomenclature, alkynes are named as substituted acetylenes CH 3 CHCHC Cl Br CCH 2 CH 2 CH 3 1 2 3 4 5 6 7 8 3-bromo-2-chloro-4-octyne not 6-bromo-7-chloro-4-octyne because 2 < 6 A substituent receives the lowest number if there is no functional group suffix, or if the same number for the functional group suffix is obtained in both directions CH 3 CHC CH 3 CCH 2 CH 2 Br 123456 1-bromo-5-methyl-3-hexyne not 6-bromo-2-methyl-3-hexyne because 1 < 2 The Structure of Alkynes A triple bond is composed of a σ bond and two π bonds Addition Reactions of Alkynes Sequential Addition of HCl An alkyne is less reactive than an alkene Relative Stabilities of Carbocations Addition of Hydrogen Halides In the presence of 1 mole equivalent of HBr CH 3 CH 2 C CH CH 3 CH 2 C CH H Br - CH 3 CH 2 C CH H Br CH 3 CH 2 C CH H CH 3 CH 2 CH CH The secondary vinylic cation is more stable [...]... Hydrogen Bonded to an sp Hybridized Carbon HC pKa = 25 CH H 2C pKa = 44 CH2 CH3CH3 pKa = 50 Electronegativities versus Acid Strengths relative base strength CH3CH2- strongest base > H2C CH- > H2N- > HC C- > HO- > F- weakest base The stronger the acid, the weaker its conjugate base top 252 Synthesis Using Acetylide Ions: Formation of C–C Bond Designing a Synthesis Example 1 ? CH3CH2C CH O CH3CH2CCH2CH2CH3