Ch 13: alkynes
conceptual points
Version 1.0:
Marc Anderson
Dept of Chemistry and Biochemistry
SF State University
[marc@sfsu.edu]
[#1] Nomenclature
• In general, for linear alkynes “ane”  “yne”
ethyne
(aka acetylene)
2-butyne
3-chloropropyne
4-bromo-4-ethyl-2-hexyne
• … but alcohols have precedence over alkynes (and alkenes!)
4-pentyne-1-ol
4-methyl-5-heptyne-2-ol
• … and alkynes can be substituents on rings!
1-bromo-3-ethynylcycloheptane
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[#2] Physical Properties of Alkynes / Ethyne
• The two carbons in an alkyne are sp hybridized, there is
a  bond and two  bonds.
 bond
 bond
 bond
• Note: C-C bond length is short! (1.2A) versus alkene bond
length (1.33A).
• Any thoughts why?
[#2] Physical Properties of Alkynes / Ethyne
• Alkynes have a high energy of combustion! Lots of energy
is present in the different bonds of alkynes.
• Thus, acetylene is commonly used as a welding
gas!
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[#2] Physical Properties of Alkynes / Ethyne
• Heat of hydrogenation of alkynes:
• Which is more stable? Internal or terminal?
• The reasoning is similar as to why substituted alkenes are more
stable (hyperconjugation!)
• Note: sp atoms are slighly electron-withdrawing!
[#2] Physical Properties of Alkynes / Ethyne
• Terminal alkynes are remarkably acidic!
• Why?
• Electronegativity of sp-hybridized alkyne carbons!
(50% s and 50% p character, draws electrons closer to nucleus!)
• …thus, terminal alkynes can be “synthetically deprotonated”
with strong base!
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[#3] 1H NMR spectroscopy of alkynes (briefly)
• Chemical shift of alkene vs alkyne protons
…recall:
next to  bond
2-3 ppm
next to  bond
2-3 ppm
deshielded
5-6 ppm
shielded
2 ppm (!!!)
• Why shielded? See fig 13-4. (not crucial, no time to address)
• Without NMR, how do you know you have an alkyne in your
molecule?
[#3] 1H NMR spectroscopy of alkynes
• Alkyne terminal CH has “long distance coupling” (and complex
splitting)
triplet
J = 2.5 Hz
triplet
J = 7 Hz
doublet triplet
J = 2.5,7 Hz
s
9H
60
50
30
t
2H
t
1H
dt
2H
40
20
10
0
2.30
2.20
2.10
2.00
1.90
1.80
1.70
1.60
1.50
1.40
1.30
1.20
1.10
1.00
0.90
0.80
0.70
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[misc] Alkynes in drugs / natural products
norethynodrel
(a component of Enovid,
the first oral contraceptive)
efavirenz (Sustivatm)
an HIV-1 reverse
transcriptase inhibitor
• “Ene-di-yne” antibiotics
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