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Alkynes: An Introduction
to Organic Synthesis
Based on
McMurry’s Organic Chemistry, 6th edition, Chapter 8
2
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
3
Electronic Structure of Alkynes
! Carbon-carbon triple bond result from sp orbital oneach C forming a sigma bond and unhybridized pXand py orbitals forming a ! bond
! The remaining sp orbitals form bonds to other atomsat 180º to C-C triple bond.
! The bond is shorter and stronger than single ordouble
! Breaking a ! bond in acetylene (HCCH) requires318 kJ/mole (in ethylene it is 268 kJ/mole)
4
Naming Alkynes
! General hydrocarbon rules apply wuith “-yne”as a suffix indicating an alkyne
! Numbering of chain with triple bond is set sothat the smallest number possible include thetriple bond
5
Diyines, Enynes, and Triynes
! A compound with two triple bonds is a diyine
! An enyne has a double bond and triple bond
! A triyne has three triple bonds
! Number from chain that ends nearest a
double of triple bond – double bonds is
preferred if both are present in the same
relative position
Alkynes as substituents
are called “alkynyl”H3CCH2CC1-butynyl
6
Preparation of Alkynes: Elimination
Reactions of Dihalides
! Treatment of a 1,2 dihaloalkane with KOH or NaOHproduces a two-fold elimination of HX
! Vicinal dihalides are available from addition ofbromine or chlorine to an alkene
! Intermediate is a vinyl halide
7
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
8
Addition of Bromine and Chlorine
! Initial addition gives trans intermediate
! Product with excess reagent is tetrahalide
9
Addition of HX to Alkynes Involves
Vinylic Carbocations
! 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
10
Hydration of Alkynes
! Addition of H-OH as in
alkenes
! Mercury (II) catalyzes
Markovinikov oriented
addition
! Hydroboration-
oxidation gives the
non-Markovnikov
product
Hg(II)
BH3
OH
H2O2, -OH
NaBH4
OH
O
O
H
11
Mercury(II)-Catalyzed Hydration of
Alkynes
! Alkynes do not react with aqueous proticacids
! Mercuric ion (as the sulfate) is a Lewis acidcatalyst that promotes addition of water inMarkovnikov orientation
! The immediate product is a vinylic alcohol, orenol, which spontaneously transforms to aketone
12
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)
13
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
hydration
14
Comparison of 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
15
Keto-enol Tautomerism
! Isomeric compounds that can rapidilyinterconvert by the movement of a proton arecalled tautomers and the phenomenon is calledtautomerism
! Enols rearrange to the isomeric ketone by therapid transfer of a proton from the hydroxyl to thealkene carbon
! The keto form is usually so stable compared tothe enol that only the keto form can be observed
16
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
17
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
! The two hydrogens add syn (from the same
side of the triple bond)
18
CH3CH2O-H
O
CH3CCH2-H
CH2=CH-H
CH3CH2-H
CH3C C-H
T ype of Bond pKa
16
20
25
44
51
19
Alkyne Acidity: Formation of
Acetylide Anions
! Terminal alkynes are weak Brønsted acids (alkenesand alkanes are much less acidic (pKa ~ 25)
! Reaction of strong anhydrous bases with a terminalacetylene produces an acetylide ion
! The sp-hydbridization at carbon holds negativecharge relatively close to the positive nucleus
20
Alkylation of Acetylide Anions
! Acetylide ions can react as nucleophiles aswell as bases
! Reaction with a primary alkyl halide producesa hydrocarbon that contains carbons fromboth partners, providing a general route tolarger alkynes