© Prentice Hall 2001 Chapter 5 1

Hydrogen Halide Addition

The addition of a hydrogen halide to an alkyne follows Markovnikov’s rule because a secondary vinylic cation is more stable than a primary vinylic cation

CH3CH2C CH

H

CH3CH2C CH2

Br

CH3CH2C CH2

Br

© Prentice Hall 2001 Chapter 5 2

Hydrogen Halide Addition When excess hydrogen halide is present,

a second equivalent is added Markovnikov’s Rule also is followed for the

second addition, forming a geminal dihalide

CH3CH2C CH2

Br

HBr

CH3CH2CCH3

Br

Br

© Prentice Hall 2001 Chapter 5 3

Hydrogen Halide Addition in the presence of Peroxide

Hydrogen peroxide has same effect on hydrogen bromide addition to an alkyne as to an alkene

© Prentice Hall 2001 Chapter 5 4

Halogen Addition to Alkynes Halogens add to alkynes as well as to

alkenes Excess halogen leads to the addition of a

second equivalent

CH3CH2C CCH3

Cl2CH3CH2C CCH3

Cl

Cl

Cl2

CH2Cl2 CH2Cl2CH3CH2C CCH3

Cl Cl

Cl Cl

CH3C CH

Br2

CH3C CH

Br

Br

Br2

CH2Cl2 CH2Cl2

CH3C CH

Br

Br

Br

Br

© Prentice Hall 2001 Chapter 5 5

Addition of Water to Alkynes Water adds to alkynes in the presence

of acid to yield an enol

© Prentice Hall 2001 Chapter 5 6

Addition of Water to Alkynes

However the initially formed enol reacts further to produce a ketone

Such isomers, differing only in the placement of a hydrogen atom, are called tautomers

© Prentice Hall 2001 Chapter 5 7

Mercuric-Ion-Catalyzed Addition of Water to Alkynes

© Prentice Hall 2001 Chapter 5 8

Hydroboration–Oxidation Hydroboration of an internal alkyne

leads to a ketone

© Prentice Hall 2001 Chapter 5 9

Addition of Water to an Alkyne

© Prentice Hall 2001 Chapter 5 10

Addition of Hydrogen to an Alkyne

© Prentice Hall 2001 Chapter 5 11

Addition of Hydrogen to an Alkyne

© Prentice Hall 2001 Chapter 5 12

Acidity of a Hydrogen Bonded to an sp Carbon

The acidities of hydrocarbons can be compared with the acidities of compounds having hydrogen attached to other second-row elements

© Prentice Hall 2001 Chapter 5 13

Acidity of a Hydrogen Bonded to an sp Carbon The conjugate bases have the following

relative base strength because the stronger the acid, the weaker the conjugate base

© Prentice Hall 2001 Chapter 5 14

Acidity of a Hydrogen Bonded to an sp Carbon

Relative electronegativities of carbon

sp > sp2 > sp3

The amide ion is strong enough to remove the proton from an sp carbon

© Prentice Hall 2001 Chapter 5 15

Acidity of a Hydrogen Bonded to an sp Carbon

Hydroxide ion is too weak to remove a proton from an sp carbon

© Prentice Hall 2001 Chapter 5 16

Syntheses Using Acetylide Ions

Alkylation reactions work best with primary alkyl halides and methyl halides

© Prentice Hall 2001 Chapter 5 17

Starting with 1-butyne, how would you prepare the following ketone?

CH3CH2C CH

O

CH3CH2CCH2CH2CH3

?

At this point the only way we know to form a ketone is to add water to an alkyne in the presence of a catalyst

CH3CH2C CCH2CH3H2O

H2SO4

CH3CH2C CHCH2CH3

OH O

CH3CH2CCH2CH2CH3

Introduction to Multistep Introduction to Multistep SynthesisSynthesis

© Prentice Hall 2001 Chapter 5 18

If the alkyne used has identical substituents on both sp carbons, only one ketone will result

3-Hexyne can be obtained from the starting material by removing the hydrogen bonded to the sp carbon and reacting with an alkyl halide

CH3CH2C CH1. NaNH2

2. CH3CH2BrCH3CH2C CCH2CH3

Introduction to Multistep Introduction to Multistep SynthesisSynthesis

© Prentice Hall 2001 Chapter 5 19

Introduction to Multistep Introduction to Multistep SynthesisSynthesis Overall we have a two-step synthesis

© Prentice Hall 2001 Chapter 5 20

In designing a synthesis it is best to start from the product and work backwards as we did in this example

Introduction to Multistep Introduction to Multistep SynthesisSynthesis

© Prentice Hall 2001 Chapter 5 21

The thought process is known as retrosynthetic analysis and is indicated by using open arrows

Introduction to Multistep Introduction to Multistep SynthesisSynthesis