on Based McMurrys Organic Chemistry, 8th edition
Alkynes
Hydrocarbons that contain carbon-carbon triple bonds
Acetylene, the simplest alkyne is produced industrially
from methane and steam at high temperature
Our study of alkynes provides an introduction to organic
synthesis, the preparation of organic molecules from
simpler organic molecules
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Why this chapter?
We will use alkyne chemistry to begin looking at general
strategies used in organic synthesis
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Electronic Structure of Alkynes
Carbon-carbon triple bond results from sp orbital on each C
forming a sigma bond and unhybridized pX and py orbitals
forming bonds.
The remaining sp orbitals form bonds to other atoms at 180 to
C-C triple bond.
The bond is shorter and stronger than single or double
Breaking a bond in acetylene (HCCH) requires 318 kJ/mole
(in ethylene it is 268 kJ/mole)
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9.1 Naming Alkynes
General hydrocarbon rules apply with -yne as a
suffix indicating an alkyne
Numbering of chain with triple bond is set so that the
smallest number possible for the first carbon of the
triple bond
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9.2 Preparation of Alkynes: Elimination
Reactions of Dihalides
Treatment of a 1,2-dihalidoalkane with
KOH or NaOH produces a two-fold
elimination of HX
Vicinal dihalides are available from
addition of bromine or chlorine to an alkene
Intermediate is a vinyl halide
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9.3 Reactions of Alkynes: Addition of HX
and X2
Addition reactions of alkynes are similar to those of
alkenes
Intermediate alkene reacts further with excess reagent
Regiospecificity according to Markovnikov
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Addition of Bromine and
Chlorine
Initial addition gives trans intermediate
Product with excess reagent is tetrahalide
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Addition of HX to Alkynes Involves Vinylic
Carbocations
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Addition of H-X to alkyne should produce a vinylic carbocation
intermediate
Secondary vinyl carbocations form less readily than primary alkyl
carbocations
Primary vinyl carbocations probably do not form at all
Nonethelss, H-Br can add to an alkyne to give a vinyl bromide if the Br is not
on a primary carbon
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9.4 Hydration of Alkynes
Addition of H-OH as in alkenes
Mercury (II) catalyzes Markovinikov oriented addition
Hydroboration-oxidation gives the non-Markovnikov
product
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Mercury(II)-Catalyzed Hydration of
Alkynes
Alkynes do not react with aqueous protic acids
Mercuric ion (as the sulfate) is a Lewis acid
catalyst that promotes addition of water in
Markovnikov orientation
The immediate product is a vinylic alcohol, or
enol, which spontaneously transforms to a ketone
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Mechanism of Mercury(II)-Catalyzed
Hydration of Alkynes
Addition of Hg(II)
to alkyne gives a
vinylic cation
Water adds and
loses a proton
A proton from
aqueous acid
replaces Hg(II)
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Keto-enol Tautomerism
Isomeric compounds that can rapidily interconvert by the
movement of a proton are called tautomers and the phenomenon
is called tautomerism
Enols rearrange to the isomeric ketone by the rapid transfer of a
proton from the hydroxyl to the alkene carbon
The keto form is usually so stable compared to the enol that only
the keto form can be observed
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Hydration of Unsymmetrical Alkynes
If the alkyl groups at either end of the C-C triple bond are
not the same, both products can form and this is not
normally useful
If the triple bond is at the first carbon of the chain (then H
is what is attached to one side) this is called a terminal
alkyne
Hydration of a terminal always gives the methyl ketone,
which is useful
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Hydroboration/Oxidation of Alkynes
BH3 (borane) adds to alkynes to give a vinylic borane
Oxidation with H2O2 produces an enol that converts to the ketone
or aldehyde
Process converts alkyne to ketone or aldehyde with orientation
opposite to mercuric ion catalyzed of hydration
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Comparison Hydration of Terminal Alkynes
Hydroboration/oxidation converts terminal alkynes to
aldehydes because addition of water is non-Markovnikov
The product from the mercury(II) catalyzed hydration
converts terminal alkynes to methyl ketones
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9.5 Reduction of Alkynes
Addition of H2 over a metal catalyst (such as palladium on
carbon, Pd/C) converts alkynes to alkanes (complete
reduction)
The addition of the first equivalent of H2 produces an
alkene, which is more reactive than the alkyne so the
alkene is not observed
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Conversion of Alkynes to cis-Alkenes
Addition of H2 using chemically deactivated palladium
on calcium carbonate as a catalyst (the Lindlar
catalyst) produces a cis alkene
cis
The two hydrogens add syn (from the same side of the
triple bond)
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Conversion of Alkynes to trans-Alkenes
Anhydrous ammonia (NH3) is a liquid below -33 C
Alkali metals dissolve in liquid ammonia and function as
reducing agents
Alkynes are reduced to trans alkenes with sodium or
lithium in liquid ammonia
The reaction involves a radical anion intermediate
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9.6 Oxidative Cleavage of Alkynes
Strong oxidizing reagents (O3 or KMnO4) cleave internal
alkynes, producing two carboxylic acids
Terminal alkynes are oxidized to a carboxylic acid and
carbon dioxide
Neither process is useful in modern synthesis were used
to elucidate structures because the products indicate the
structure of the alkyne precursor
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9.7 Alkyne Acidity: Formation of Acetylide Anions
Terminal alkynes are weak Brnsted acids (alkenes and
alkanes are much less acidic (pKa ~ 25. See Table 8.1 for
comparisons))
Reaction of strong anhydrous bases with a terminal
acetylene produces an acetylide ion
The sp-hydbridization at carbon holds negative charge
relatively close to the positive nucleus (Figure 8.5 in text)
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9.8 Alkylation of Acetylide
Anions
Acetylide ions can react as nucleophiles as well as bases
(see Figure 8-6 for mechanism)
Reaction with a primary alkyl halide produces a
hydrocarbon that contains carbons from both partners,
providing a general route to larger alkynes
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Limitations of Alkyation of Acetylide Ions
Reactions only are efficient with 1 alkyl bromides and
alkyl iodides
Acetylide anions can behave as bases as well as
nucelophiles
Reactions with 2 and 3 alkyl halides gives
dehydrohalogenation, converting alkyl halide to alkene
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9.9 An Introduction to Organic Synthesis
Organic synthesis creates molecules by design
Synthesis can produce new molecules that are needed as drugs
or materials
Syntheses can be designed and tested to improve efficiency
and safety for making known molecules
Highly advanced synthesis is used to test ideas and methods,
answering challenges
Chemists who engage in synthesis may see some work as
elegant or beautiful when it uses novel ideas or combinations
of steps this is very subjective and not part of an introductory
course
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Synthesis as a Tool for Learning Organic
Chemistry
In order to propose a synthesis you must be familiar with
reactions
What they begin with
What they lead to
How they are accomplished
What the limitations are
A synthesis combines a series of proposed steps to go from a
defined set of reactants to a specified product
Questions related to synthesis can include partial information
about a reaction of series that the student completes
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Strategies for Synthesis
Compare the target and the starting material
Consider reactions that efficiently produce the outcome.
Look at the product and think of what can lead to it (Read
the practice problems in the text)
Example
Problem: prepare octane from 1-pentyne
Strategy: use acetylide coupling
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