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© 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