Alkynes

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Alkynes
1.54 Å 1.33 Å
1.20 Å

• Why so short for the triple bond?
• Triple bond results in good orbital overlap
• sp orbital is closer to nucleus than sp2 and sp3

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?2
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1. Acidity of Alkynes
Internal alkyne Nonacidic Terminal alkyne
Acidic
acidic hydrogen
How does the acidity compare to alkanes and
alkenes?
pKa 60 Less
acidic 44 25 More acidic
base
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sp3
base
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sp2
base
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sp
In the acetylide anion, the electrons are closer
to the nucleus due to sp hybridizaiton, and the
anion is therefore more stable, making acetylene
more acidic.
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Nevertheless, a strong base is required to
deprotonate terminal alkynes
?- ?
methyl lithium ethyl magnesium bromide sodium
amide
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2. Alkylation of Alkynes
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3. Reduction a. Hydrogenation
Alkenes are intermediates in this reaction.
Alkyne Alkene
Alkane
Question Is it possible to stop at the alkene?
Answer Yes, use a special catalyst called
Lindlaar catalyst.
Lindlaar palladium (Pd) on calcium carbonate
with poison added, usually
quinoline and lead acetate. The activity of the
catalyst is moderated such that
on the 1st pi (?) bond is reacted but the 2nd pi
(?) bond is not!!
cis isomer only!! (syn addition)
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b. Metal NH3 reduction
trans isomer only!
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4. Addition of Halogens, Br2 and Cl2
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5. Addition of HX (HCl, HBr, HI) This
reaction is only useful with a terminal alkyne.
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• Mechanism
• Markovnikov addition
• Vinyl cation intermediate

20 vinyl cation
empty p orbital

sp hybridization

Vinyl Cation not resonance stabilized.
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6. Hydration a. Water and Hg2 as catalyst
enol unstable!
ketone - stable
Markovnikov addtion
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b. Hydroboration/Oxidation
Recall
Anti-Markovnikov
10 alcohol
Anti-Markovnikov
aldehyde
Sia sec-isoamyl group
B(Sia)2H is a hindered reagent and adds only once
to the C?C bond. BH3 would add twice yielding a
different product.
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enol unstable aldehyde