Alkynes
Structure
sp hybridization
Acidity of Terminal Alkynes
Stronger
base
Weaker
base
Other strong bases that
will ionize the terminal
alkyne:
Not KOH
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
RCH=CHR + HX
RCHXCH2R
Or rewriting
RCHBrCH2R
base
RCH=CHR
Also, if we start with a vinyl halide and a very
strong base (vinyl halides are not very reactive).
NaH
RCH=CHBr
RCCH
Synthetic planning (Retrosynthesis)
Work Backwards…..
Trace the reactions sequence from the desired product back to ultimate reactants.
Br
Br2
H
H
H
NaNH2
NaNH2
Br
CH3
Starting reactant
CH3
Br
H
prop-1-yne
CH3
CH3
Target
molecule.
Overall Sequence converts alkene alkyne
H
But typical of synthetic problems side reaction occurs to
some extent and must be taken into account.
C
H
H
H
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
Reactions: alkyne with halogen
RCCR + Br2 RBrC=CBrR
No regioselectivity with Br2.
Stereoselective for trans addition.
Reactions: Addition of HX
The expected reaction sequence occurs, formation of the more
stable carbocation.
Markovnikov orientation for both additions.
Now for the mechanism….
Mechanism
The expected reaction sequence occurs, formation of the more
stable carbocation.
Addition of the second mole, another example of resonance.
Reactions: Acid catalyzed Hydration
(Markovnikov).
Markovnikov addition, followed by tautomerism to yield, usually, a
carbonyl compound.
Reactions: Anti Markovnikov Hydration of Alkynes,
Regioselectivity
BH3
overall:
R
R'
Step 1
R
H
R'
B
H2O2, NaOH
Step 2
RCH2CR'
o
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.
More exposed site.
sia2BH
Aldehyde
not
ketone.
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.
catalyzed by
base or acid
H2O2
H
H
H
H
NaOH
OH
O
B
enol
either an aldehyde
or a ketone
Overall…
internal alkyne   ketone (possibly a mixture, next slide)
Terminal alkyne   aldehyde
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.
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
Reduction - 2
2.
Treatment of alkenyl borane with a carboxylic acid to yield cis alkene.
BH3
CH3CO2H
hex-3-yne
H
B
Instead of H2O2 / NaOH
Alkenyl borane
3. Reduction by sodium or lithium in liquid ammonia to yield the trans alkene.
Plan a Synthetic Sequence
Retrosynthesis
Synthesize butan-1-ol from ethyne. Work backward from
A big the
alkyne
target
can
molecule.
be
formed via nucleophilic
Is read as “comes from”.
substitution. This is the
chance to make
thedone.
C-C
Major problem: make big from small. Be alert for
Catalytic
when the “disconnect”
can be
1. BH3
bond we need.
Lindlar
OH
2. H2O2,
reduction
NaOH
YES!
butan-1-ol
Target
molecule
Convert ethyne to anion
Do a “disconnect” here.
and react with EtBr.
Catalytic
Br
reduction
Addition
Lindlar
of HBr.
bromoethane
Now, fill in the “forward reaction” details
Can
we
get
alkyne
from
smaller
Not
How
yet!
about
Soan
joining
how
can
molecules
we
get
it?
to get
an alkene?
Ask
yourself!
Do
we
know
how
tomolecules?
join
any two Not
yet!!
So howtogether
can we to
getyield
an alkene?
molecules
an alcohol?
ethyne