Transcript Alkynes. C2H2 CnH2n-2 H:C:::C:H H—C  C—H acetylene ethyne C3H4 CH3CCH methylacetylene propyne sp => linear, 180o nomenclature: common names: “alkylacetylene” IUPAC: parent chain = longest continuous carbon chain that contains the triple bond. alkane.

Alkynes.
C2H2
CnH2n-2
H:C:::C:H
H—C  C—H
acetylene
ethyne
C3H4
CH3CCH
methylacetylene
propyne
sp => linear, 180o
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
1-butyne
ethylacetylene
CH3
HCCCHCH2CH3
3-methyl-1-pentyne
sec-butylacetylene
physical properties:
weakly or non-polar, no H-bonding
relatively low mp/bp
water insoluble
Synthesis, alkynes:
1. dehydrohalogenation of vicinal dihalides
H
H
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—C—C—
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X
X
H
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+ KOH  — C = C —
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X
H
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—C=C—
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X
+ NaNH2  — C  C — + NaX + NH3
+ KX + H2O
H
H
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—C—C—
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X
X
+ 2 KOH  — C  C —
heat
+ KX + H2O
CH3CH2CHCH2 + KOH; then NaNH2  CH3CH2CCH
Br Br
“
+ 2 KOH, heat
X2
alkene
1. KOH
vicinal dihalide
alkyne
2. NaNH2
Br2
CH3CH=CH2
1. KOH
CH3CHCH2
CH3CCH
2. NaNH2
Br Br
Synthesis of propyne from propane
Br2, heat
CH3CH2CH3
CH3CH2CH2-Br + CH3CHCH3
Br
KOH(alc)
CH3CHCH2
Br Br
Br2
CH3CH=CH2
KOH
NaNH2
CH3CH CH
Br
CH3C CH
2. coupling of metal acetylides with 1o/CH3 alkyl halides
R-CC-Na+ + R´X  R-CC-R´ + NaX
a) SN2
b) 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!
CH3C C Na +
CH3
CH3CCH3
Br
3o
alkyl halide
CH3
CH3C CH2
+
CH3C CH
E2 elimination!
alkynes
acids
bases

 some terminal only
metals

oxid.

reduct.

halogens

terminal only
Reactions, alkynes:
1. addition of H2 (reduction)
2. addition of X2
3. addition of HX
4. addition of H2O, H+
5. as acids
6. Ag+
7. oxidation
1. Addition of H2
—CC—
+ 2 H2, Ni 
H H
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|
—C—C—
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H H
alkane
requires catalyst (Ni, Pt or Pd)
HCCH + 2 H2, Pt  CH3CH3
[ HCCH + one mole H2, Pt  CH3CH3 + CH2=CH2 + HCCH
]
Na or Li
NH3(liq)
—CC—
H2, Pd-C
Lindlar catalyst
H
\
/
C=C
/
\
H
\
/
C=C
/
\
H
H
anti-
syn-
Na or Li
NH3(liq)
CH3 H
\
/
C=C
/
\
H
CH3
anti-
trans-2-butene
CH3CCCH3
H2, Pd-C
Lindlar catalyst
H
H
\
/
C=C
/
\
CH3 CH3
cis-2-butene
syn-
2. Addition of X2
X
X X
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— C C— + X2  — C = C — + X2  — C — C —
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X
X X
CH3CCH + Br2
Br
Br Br
 CH3C=CH + Br2  CH3-C-CH
Br
Br Br
3. Addition of hydrogen halides:
H
H
X
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— C C— + HX  — C = C — + HX  — C — C —
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X
H
X
a) HX = HI, HBr, HCl
b) Markovnikov orientation
CH3CCH + HCl  CH3C=CH2
Cl
Cl
+ HCl  CH3CCH3
Cl
4. Addition of water. Hydration.
O
— C  C — + H2O, H+, HgO  — CH2 — C—
H
OH
—C=C—
“enol”
Markovnikov orientation.
keto-enol tautomerism
CH3CH2CCH + H2O, H2SO4, HgO 
1-butyne
O
CH3CH2CCH3
2-butanone
5. As acids.
terminal alkynes only!
a) with active metals
CH3CCH + Na  CH3CC-Na+ + ½ H2 
b) 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
CH3CH2CCH + LiNH2 
SA
( H2O = stronger acid! )
NH3 + CH3CH2CC-Li+
WA
6. 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.
7. Oxidation
KMnO4
R-CC-R´
hot KMnO4
RCOOH + HOOCR´
carboxylic acids
O3; then Zn, H2O
CH3CH2CCCH3
+
KMnO4  CH3CH2COOH +
HOOCCH3
CH3CCH + hot KMnO4  CH3COOH + CO2
CH3CCCH3 + O3; then Zn, H2O  2 CH3COOH
Alkynes
Nomenclature
Syntheses
1. dehydrohalogenation of vicinal dihalide
2. coupling of metal acetylides with 1o/CH3X
Reactions, alkynes:
1. addition of H2 (reduction)
2. addition of X2
3. addition of HX
4. addition of H2O, H+
5. as acids
6. Ag+
7. oxidation