Alkynes . C n H 2n-2 C 2 H 2 H:C:::C:H H—C  C—H sp => linear, 180 o acetylene

1. Alkynes. CnH2n-2 C2H2 H:C:::C:H H—C  C—H sp => linear, 180o acetylene ethyne C3H4 CH3CCH methylacetylene propyne

2. nomenclature: common names: “alkylacetylene” IUPAC: parent chain = longest continuous carbon chain that contains the triple bond. alkane drop –ane add -yne prefix locant for the triple bond, etc. CH3CH2CCCH32-pentyne ethylmethylacetylene

3. “terminal” alkynes have the triple bond at the end of the chain: CH3 CH3CH2CCH HCCCHCH2CH3 1-butyne 3-methyl-1-pentyne ethylacetylene sec-butylacetylene

4. physical properties: weakly or non-polar, no H-bonding relatively low mp/bp water insoluble

5. Synthesis, alkynes: • dehydrohalogenation of vicinal dihalides • H H H • | | | • — C — C — + KOH  — C = C — + KX + H2O • | | | • X X X • H • | • — C = C — + NaNH2 — C  C — + NaX + NH3 • | • X

6. H H | | — C — C — + 2 KOH  — C  C — + KX + H2O | | heat X X CH3CH2CHCH2 + KOH; then NaNH2 CH3CH2CCH Br Br “ + 2 KOH, heat

7. X2 1. KOH alkene vicinal dihalide alkyne 2. NaNH2 CH3CH=CH2CH3CHCH2 CH3CCH Br Br Br2 • KOH • NaNH2

9. coupling of metal acetylides with 1o/CH3 alkyl halides • R-CC-Na+ + R´X  R-CC-R´ + NaX • SN2 • R´X must be 1o or CH3X • CH3CC-Li+ + CH3CH2-Br  CH3CCCH2CH3

10. note: R-X must be 1o or CH3 to get SN2!

11. alkynes acids bases metals oxid. reduct. halogens terminal only terminal only

12. Reactions, alkynes: • addition of H2 (reduction) • addition of X2 • addition of HX • addition of H2O, H+ • as acids • Ag+ • oxidation

13. Addition of H2 • H H • | | • — C  C — + 2 H2, Ni  — C — C — • | | • H H • alkane • requires catalyst (Ni, Pt or Pd)

14. HCCH + 2 H2, Pt  CH3CH3 [ HCCH + one mole H2, Pt  CH3CH3 + CH2=CH2 + HCCH ] H \ / Na or Li C = C anti- NH3(liq) / \ H — C  C — \ / H2, Pd-C C = C syn- Lindlar catalyst / \ H H

15. CH3 H \ / Na or Li C = C anti- NH3(liq) / \ H CH3 trans-2-butene CH3CCCH3 H H \ / H2, Pd-C C = C syn- Lindlar catalyst / \ CH3 CH3 cis-2-butene

16. Addition of X2 • X X X • | | | • — C C— + X2 — C = C — + X2 — C — C — • | | | • X X X • Br Br Br • CH3CCH + Br2 CH3C=CH + Br2  CH3-C-CH • Br Br Br

17. Addition of hydrogen halides: • H H X • | | | • — C C— + HX  — C = C — + HX  — C — C — • | | | • X H X • HX = HI, HBr, HCl • Markovnikov orientation • Cl • CH3CCH + HCl  CH3C=CH2 + HCl  CH3CCH3 • Cl Cl

18. Addition of water. Hydration. • O • — C  C — + H2O, H+, HgO  — CH2 — C— • H OH • — C = C — • “enol” keto-enol tautomerism • Markovnikov orientation.

19. CH3CH2CCH + H2O, H2SO4, HgO  1-butyne O CH3CH2CCH3 2-butanone

20. As acids. terminal alkynes only! • with active metals • CH3CCH + Na  CH3CC-Na+ + ½ H2 • with bases • CH3CCH + CH3MgBr  CH4 + CH3C CMgBr • SA SB WA WB

21. acid strength: CH4 < NH3 < HCCH < ROH < H2O < HF HC CH + NaOH  NR ( H2O = stronger acid! ) CH3CH2CCH + LiNH2 NH3 + CH3CH2CC-Li+ SA WA

22. Ag+terminal alkynes only! • CH3CH2CCH + AgNO3 CH3CH2CC-Ag+ • CH3CCCH3 + AgNO3 NR (not terminal) • formation of a precipitate is a test for terminal alkynes.

23. Oxidation • KMnO4 • R-CC-R´ hot KMnO4 RCOOH + HOOCR´ • carboxylic acids • O3; then Zn, H2O

24. CH3CH2CCCH3 + KMnO4 CH3CCH + hot KMnO4 CH3CCCH3 + O3; then Zn, H2O  CH3CH2COOH + HOOCCH3 CH3COOH + CO2 2 CH3COOH

25. Alkynes Nomenclature Syntheses 1. dehydrohalogenation of vicinal dihalide 2. coupling of metal acetylides with 1o/CH3X

26. Reactions, alkynes: • addition of H2 (reduction) • addition of X2 • addition of HX • addition of H2O, H+ • as acids • Ag+ • oxidation