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chapter7 - Organic Chemistry William H Brown Christopher S...

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7- 7-1 Organic Organic Chemistry Chemistry William H. Brown William H. Brown Christopher S. Foote Christopher S. Foote Brent L. Iverson Brent L. Iverson
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7- 7-2 Alkynes Chapter 7 Chapter 7
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7- 7-3 Nomenclature Nomenclature IUPAC: use the infix - yn yn- to show the presence of a carbon-carbon triple bond Common names: prefix the substituents on the triple bond to the word “acetylene” Common name: IUPAC name: D imethylacetylene V inylacetylene 2-Butyne 1-Buten-3-yne 1 1,6-Heptadiyne 3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne 2 3 4 1 2 3 4 5 6 1 2 3 4 5 6 7 7
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7- 7-4 Cycloalkynes Cycloalkynes Cyclononyne is the smallest cycloalkyne isolated it is quite unstable and polymerizes at room temp the C-C-C bond angle about the triple bond is approximately 155°, indicating high angle strain Cyclononyne
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7- 7-5 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 C CH CH 3 C CCH 3 CH 3 ( CH 2 ) 2 C CH CH 3 CH 2 C CH CH 3 ( CH 2 ) 3 C CH CH 3 ( CH 2 ) 5 C CH CH 3 ( CH 2 ) 7 C CH
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7- 7-6 Acidity Acidity The p K a of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1) terminal alkynes react with sodium amide to form alkyne anions    NH 2 H-C C-H H-C C: - NH 3 + p K a  38 (Weaker acid) p K a  25 (Stronger acid) +
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7- 7-7 Acidity Acidity terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA) because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion Na + H [( CH 3 ) 2 CH] 2 N  Li + Sodium hydride Lithium diisopropylamide                 (LDA) 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
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7- 7-8 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 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)
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7- 7-9 Alkylation of Alkyne Anions Alkylation of Alkyne Anions alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne HC C  -   Na + Br Na + Br - Sodium acetylide + + 1-Bromobutane 1-Hexyne CH 3 CH 2 C C -   Na + CH 3 CH 2 C CCH 2 CH 3 CH 3 CH 2 -Br Na + Br - + Bromoethane 3-Hexyne + Sodium butynide
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