Alkynes, -C C- - · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical...

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Page 1: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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.

Page 2: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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 ---

Page 3: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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.

Page 4: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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.

Page 5: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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

Page 6: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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.

Page 7: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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.

Page 8: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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.

Page 9: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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.

Page 10: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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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

Page 11: Alkynes, -C C- -  · PDF fileReactions of Alkynes Reduction to an alkene ---Geometrical isomerism possible for product, except when alkyne is terminal ... typically anti addition

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With a strong base like acetylide, substitution is successfulonly if R-X is 1o.

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