lecture 7 - Nomenclature Alkynes Common names: prefix the...

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1 Alkynes Alkynes Chapter 7 Nomenclature ± IUPAC : use the infix - yn yn- to show the presence of a carbon-carbon triple bond 1,6-Heptadiyne 1 2 3 4 5 6 7 1 3-Methyl-1-butyne 2 3 4 6,6-Dimethyl-3-heptyne 1 2 34 5 6 7 Nomenclature Nomenclature ± Common names : prefix the substituents on the triple bond to the word “acetylene” Common name: IUPAC name: Dimethylacetylene Vinylacetylene 2-Butyne 1-Buten-3-yne Practice Physical Properties Physical Properties ± Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton 0.766 174 -36 0.746 125 -79 0.716 71 -132 0.690 40 -90 0.691 27 -32 (a gas) 8 -126 (a gas) -23 -102 (a gas) -84 -81 Density at 20°C (g/mL) Boiling Point (°C) Melting Point (°C) Formula Name 1-Decyne 1-Octyne 1-Hexyne 1-Pentyne 2-Butyne 1-Butyne Propyne Ethyne HC CH CH 3 CC H CH 3 C CCH 3 CH 3 (CH 2 ) 2 H CH 3 CH 2 H CH 3 2 ) 3 H CH 3 2 ) 5 H CH 3 2 ) 7 H
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2 Acidity Acidity (terminal alkynes) (terminal alkynes) ± The p K a of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols terminal alkynes react with sodium amide to form alkyne anions NH 2 H-C C-H H-C C: - 3 + p K a 38 (Weaker acid) p K a 25 (S tro ng er acid) + Acidity (other strong bases) terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA) Na + H Sodium hydride [(CH 3 ) 2 CH] 2 N Li + Lithium diisopropylamide (LDA) Acidity Because water is a stronger acid than terminal alkynes, the hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion pKa values for alkane and alkene hydrogens are higher than pKa values for terminal alkynes K eq = 10 -9.3 p K a 15.7 p K a 25 - - + + (Stronger acid) W(eaker acid) HC CH HC C OH H 2 O Alkylation of Alkyne Alkylation of Alkyne Anions Anions ± Alkyne anions are both strong bases and good nucleophiles ± They participate in nucleophilic substitution nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation alkylation Alkylation of Alkyne Anions Alkylation of Alkyne Anions alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes (substitution reactions) HC C - + Br Na + - Sodium acetylide + + 1-Bromobutane 1-Hexyne Alkylation of Alkyne Anions Alkylation of Alkyne Anions alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne 3 2 CC + 3 2 C H 2 3 3 2 -Br Na + - + Bromoethane 3-Hexyne + Sodium butynide
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3 Alkylation of Alkyne Alkylation of Alkyne Anions Anions because alkyne anions are also strong bases, alkylation is practical only with methyl and 1° halides with 2° and 3° halides, elimination is the major reaction HC CH HC C - Na + H Br Sodium acetylide Bromo- cyclohexane Acetylene Cyclohexene Na + Br - + + + elimin- ation (Ch 9) Preparation of Alkynes from Alkenes ± Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation dehydrohalogenation reactions (removal of H and X from adjacent
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lecture 7 - Nomenclature Alkynes Common names: prefix the...

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