Alkynes, -C C- - · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical...
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Transcript of Alkynes, -C C- - · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical...
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C C H
H
HH
H
H
1.09 A
1.54 A
C C
H
H
H
H
1.08 A
1.33 A
H C C H
1.06 A
1.20 A
H C C Hπ-bondtop/bottom
σ-bond π-bondfront/back
Alkynes, -C�C-
The triple bond consists of an sp-sp σ-bond and two π-bonds. The remaining sp orbital on each carbon isoriented 180o from the former sp orbital and forms a σ-bond with another atom.
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C C
R2
H
H
R1
R1 C C R2
C C
R2
H
R1
H
Lio or NaoNH3 liquid
H2
Lindlar catalyst
Reactions of Alkynes
Reduction to an alkene ---
Geometrical isomerism possible for product, except whenalkyne is terminal (has triple bond at end).
Each of these reactions is stereoselective in that eachyields predominantly one stereoisomer of two or morepossible ones.
Reduction to an alkane ---
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R1 C C R2H2
Pt catalystR1 C C R2
H
H
H
H
R1 C C R2 R1 C C R2
X
X
X
X
X2 R1 C C R2
X
X
X2
X2 = Cl2, Br2
R1 C C R2 R1 C C R2
H
H
X
X
R1 C C R2
H
X
HX HX
HX = HCl, HBr, HItypically anti addition
Addition of halogens:
Both steps generally show anti addition.
Although triple bonds are usually less reactive than doublebonds toward electrophilic addition, this reaction can oftenbe stopped at the first step.
Addition of hydrogen halides ---
Terminal alkynes show Markovnikov addition in first step.
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R1 C C R2
R1
C C
R2HHX
R1
C C R2
H
sp2
sp
The anti addition could be explained by a vinyliccarbocation with a vacant sp2 orbital although calculationssuggest that a carbocation with a vacant p orbital (sphybrid) would be more stable.
Reaction can often be stopped after the first step eventhough alkynes are less reactive toward electrophilicaddition than alkenes. This lower reactivity of alkynes is atleast partly due to vinylic cations being less stable thancomparable alkyl cations --- 2o vinylic ~ 1o alkyl;1o vinylic ~ methyl.
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R1 C C R2
H
H
X
X
HX
R1
C C
R2H
X
R1 C C R2
H
H
X
R1 C C R2
H
H
X
X
H C C CH3H2O, H2SO4
HgSO4
H2C C
CH3
OH
H2C C
CH3
OH
an enol a ketone
Note formation of geminal dihalide in the second step. This is the result of resonance stabilization of theintermediate carbocation (and the transition state leadingto it), as shown below. Evidently, this has moreimportance than the destabilizing inductive effect of theelectronegative element.
Hydration of Alkynes - Tautomerism
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R1 C C R21) BH3/THF2) H2O2, KOH, H2O
R1 C
H
C
H
O
R2
internal alkyne: R1, R2 = H/
Usually the equilibrium favors the ketone (or CH3 - CHO, when the alkyne is acetylene).This is an acid - base equilibrium and -OH is a strongeracid than -CH3.
Structural isomers which exist in equilibrium with eachother are called tautomers. The particular type oftautomerism shown above is keto - enol tautomerism.
Hydroboration of Alkynes ---
Ketone is formed via enol.
Symmetrical alkyne gives one ketone, unsymmetricalalkyne gives a mixture of two ketones.
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C CH C
H
CH
B
C
H
CH
B
B
BH3/THF
BH3/THF
terminal alkyne
H2O2, KOH, H2O
pH=8C
H
CH
O
In the case of terminal alkynes borane will add twice to thetriple bond, in anti-Markovnikov fashion. This product canbe converted to an aldehyde by reaction with hydrogenperoxide at a pH of 8.
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Synthesis – Making Larger Alkynes from Smaller Ones
In general, it is possible to convert a terminal alkyne to ananion by removing the terminal hydrogen. This is usuallyaccomplished by using a base which is stronger than anacetylide anion:
R-C�C-H + Na+NH2- > R-C�C- Na+ + NH3
The acetylide anion can be used in synthesis, to makelarger molecules, by reacting it with alkyl halides in asubstitution reaction:
R-C�C- Na+ + ClCH2-R' > R-C�C-CH2-R' + Na+Cl-
Example problem 1 ---Synthesize 2-hexyne from starting materials which do notcontain more than 3 carbons.
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Example problem 2 ---Synthesize 4-octyne from starting materials which do notcontain more than 3 carbons.
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Example problem 3 ---Synthesize 2,2-dimethyl-3-octyne, (CH3)3C-C�C-(CH2)3CH3, from starting materials which donot contain more than 6 carbons.
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C C
H
X
C C RC C R
C C
H
X+substitution
C C
H
X
C C R C C RH+
XC C +elimination
Reaction of acetylides with alkyl halides ---Substitution vs. Elimination
So far, the reaction of acetylide anion with alkyl halide hasresulted in a substitution, producing a larger alkyne. Thisis not the only reaction which can occur between anacetylide anion and an alkyl halide; a B/L acid-basereaction leading to an elimination is also possible.
(E2) Elimination usually increases>
R-X = 1o 2o 3o
<(SN2) Substitution usually increases
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With a strong base like acetylide, substitution is successfulonly if R-X is 1o.
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