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