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

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CCH3 2-pentyne ethylmethylacetylene

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

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

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

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

X2 1. KOH alkene vicinal dihalide alkyne 2. NaNH2 CH3CH=CH2 CH3CHCH2 CH3CCH Br Br Br2 KOH NaNH2

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

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

  some alkynes acids bases terminal only metals oxid. reduct. halogens   some terminal only

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

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

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) / \ — C  C — \ / H2, Pd-C C = C syn- Lindlar catalyst / \ H H

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

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

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

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

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

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

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

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.

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

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

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

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