Alkynes

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Alkynes
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Structure
sp hybridization
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Acidity of Terminal Alkynes
Stronger base
Weaker base
Other strong bases that will ionize the terminal
alkyne
Not KOH
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Important Synthetic Method Dehydrohalogenation

• 1. Dehydrohalogenation
• An alkyl halide can eliminate a hydrogen halide
  molecule, HX, to produce a pi bond.

Recall that HX can be added to a double bond to
make an alkyl halide. HX can also be removed by
strong base, called dehydrohalogenation.
Preparation of alkene
Strong base
RCHCHR HX RCHXCH2R
Or rewriting
base
RCHBrCH2R RCHCHR
Also, if we start with a vinyl halide and a very
strong base (vinyl halides are not very reactive).
NaH
RCHCHBr RCCH
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Synthetic planning (Retrosynthesis)
Work Backwards..
Trace the reactions sequence from the desired
product back to ultimate reactants.
Starting reactant
Target molecule.
Overall Sequence converts alkene ?alkyne
But typical of synthetic problems side reaction
occurs to some extent and must be taken into
account.
C
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More Sythesis Nucleophilic Substitution

• Use the acidity of a terminal alkyne to create a
  nucleophile which then initiates a substitution
  reaction.

Note that we still have an acidic hydrogen and,
thus, can react with another alkyl group in this
way to make RCCR
Alkyl halides can be obtained from alcohols
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Reactions alkyne with halogen
RCCR Br2? RBrCCBrR
No regioselectivity with Br2. Stereoselective for
trans addition.
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Reactions Addition of HX
The expected reaction sequence occurs, formation
of the more stable carbocation.
Markovnikov orientation for both additions.
Now for the mechanism.
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Mechanism
The expected reaction sequence occurs, formation
of the more stable carbocation.
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Addition of the second mole, another example of
resonance.
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Reactions Acid catalyzed Hydration (Markovnikov).
Markovnikov addition, followed by tautomerism to
yield, usually, a carbonyl compound.
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Reactions Anti Markovnikov Hydration of Alkynes,
Regioselectivity
Step 2
Step 1
Similar to formation of an anti-Markovnikov
alcohol from an alkene
Step 1, Internal Alkyne addition to the alkyne
with little or no regioselectivity issue.
Alternatively Asymmetric, terminal, alkyne if you
want to have strong regioselectivity then use a
borane with stronger selectivity for more open
site of attack.
Less exposed site.
Aldehyde not ketone.
More exposed site.
sia2BH
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Tautomerism, enol ? carbonyl
Step 2, Reaction of the alkenyl borane with H2O2,
NaOH would yield an enol. Enols are unstable and
rearrange (tautomerize) to yield either an
aldehyde or ketone.
Overall
internal alkyne ? ? ketone (possibly a mixture,
next slide)
Terminal alkyne ? ? aldehyde
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Examples
Used to insure regioselectivity.
As before, for a terminal alkyne.
But for a non-terminal alkyne frequently will get
two different ketones
Get mixture of alkenyl boranes due to low
regioselectivity.
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Reduction, Alkyne ? Alkene
1. Catalytic Hydrogenation
If you use catalysts which are also effective
for alkene hydrogenation you will get alkane.
You can use a reduced activity catalyst
(Lindlar), Pd and Pb, which stops at the alkene.
You obtain a cis alkene.
Syn addition
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Reduction - 2
2. Treatment of alkenyl borane with a
carboxylic acid to yield cis alkene.
Instead of H2O2 / NaOH
Alkenyl borane
3. Reduction by sodium or lithium in liquid
ammonia to yield the trans alkene.
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Plan a Synthetic SequenceRetrosynthesis
YES!
Synthesize butan-1-ol from ethyne. Work backward
from the target molecule.
A big alkyne can be formed via nucleophilic
substitution. This is the chance to make the C-C
bond we need.
Is read as comes from.
Catalytic Lindlar reduction
Major problem make big from small. Be alert for
when the disconnect can be done.

• BH3
• H2O2, NaOH

Convert ethyne to anion and react with EtBr.
Do a disconnect here.
Target molecule
Catalytic reduction Lindlar
Addition of HBr.
Now, fill in the forward reaction details
Can we get an alkyne from smaller molecules?
Not yet! So how can we get it?
How about joining molecules to get an alkene?
Not yet!! So how can we get an alkene?
Ask yourself! Do we know how to join any two
molecules together to yield an alcohol?