Chapter 6: Alkynes Nomenclature
Chapter 6: Alkynes • Nomenclature, structure, physical properties of alkynes • Reactions of alkynes--a lot like reactions of alkenes •why alkynes are less reactive •addition of hydrogen halides (stereoselectivity) •addition of halogens •addition of water •hydroboration/oxidation •addition of H2 •formation of an acetylide ion/acidity of H bonded to sp C •Retrosynthetic analysis: formation of C-C bonds
Alkynes: hydrocarbons that contain a carbon–carbon triple bond
Nomenclature Same as alkenes, but with –yne suffix Cl Br CH3CHCHC CCH2CH2CH3 1
2
3 4
5 6
7
8
CH3 CH3CHC
CCH2CH2Br
6
3 2
5
4
3-bromo-2-chloro-4-octyne not 6-bromo-7-chloro-4-octyne because 2 < 6 1-bromo-5-methyl-3-hexyne not 6-bromo-2-methyl-3-hexyne because 1 < 2
1
1
Physical properties of alkynes • Alkynes have higher boiling points than alkenes with the same number of C’s. Why? • Internal alkynes have higher boiling points than terminal alkynes. Why?
The Structure of Alkynes A triple bond is composed of a σ bond and two π bonds
Addition Reactions of Alkynes
2
Sequential Addition of HCl
An alkyne is less reactive than an alkene
Why is the transition state for an alkyne addition reaction so much less stable than the transition state for an alkene addition? Examine stabilities of the carbocation intermediates (remember: the transition state looks more like the intermediate than the reactant)
3
Relative Stabilities of Carbocations
Addition of Hydrogen Halides In the presence of 1 mole equivalent of HBr Br H
H CH3CH2C
CH
CH3CH2C
CH
CH3CH2C
CH
-
Br H CH3CH2C
CH3CH2CH CH
CH
The secondary vinylic cation is more stable
In the presence of excess HBr, a carbocation is formed by adding the electrophile to the sp2 carbon bonded to the greater number of hydrogens Br CH3CH2C
Br CH2
HBr
CH3CH2CCH3 Br
Br CH3CH2C
Br CH3
CH3CH2C
CH3
4
A secondary vinylic cation is about as stable as a primary cation, therefore a pi-complex may be the actual reaction intermediate
δ- Cl
δ+ H HC
CH
Many (but not all) alkyne addition reactions are stereoselective
Example of a stereoselective alkyne addition reaction: H3C C C CH 3
Cl
+
HCl
H3C
CH3 H
Syn or anti addition? How does this support the idea of a pi-complex?
Addition of HBr in the presence of peroxide peroxide CH3CH2C
CH
+ HBr
CH3CH2CH
CHBr
5
Addition of Halogens to Alkynes Cl Cl Cl Cl2 Cl2 CH3CH2C CCH3 CCH3 CH3CH2C CCH3 CH2Cl2 CH2Cl2 Cl Cl Cl
CH3CH2C
Br CH3C
CH
Br2 CH2Cl2
CH3C
Br Br CH Br
Br2 CH2Cl2
CH3C
CCH3
Br Br
Addition of Water
O RCH2 C
OH
tautomerization
R
keto tautomer
RCH
C
R
enol tautomer
Examples of Water Addition
6
Hg2+ is added to increase the rate of water addition to terminal alkynes
Hydroboration–Oxidation of Internal Alkynes
Hydroboration–Oxidation of Terminal Alkynes Use bulky substituents on the boron to prevent addition of 2nd equivalent of boron adding to B-substituted alkene CH3 CH3
CH3 CH3 CH3CH2C CH
+
CH3CH CH
CH3CH CH
HO-, H2O2 H H2O
HO
2
BH
B
H
CH2CH3 C
2
H C C
CH2CH3
O CH3CH2CH2CH
C H
“disamyl borane”
7
Formation of Ketone versus Aldehyde
Addition of Hydrogen Formation of Cis Alkene
Use Lindlar’s catalyst to stop at alkene…
Addition of H2 to form Trans Alkene
Radical mechanism:
8
Reason for trans addition:
Acidity of a Hydrogen Bonded to an sp Hybridized Carbon
HC pKa = 25
CH
H2 C pKa = 44
CH2 CH3CH3 pKa = 50
Electronegativities versus Acid Strengths
9
Relative base strength: CH3CH2-
>
H2C
CH-
> H2N-
>
HC
C-
> HO-
>
F-
weakest base
strongest base
Synthesis Using Acetylide Ions: Formation of C–C Bond
Designing a Synthesis Example 1
? CH3CH2C
CH
O CH3CH2CCH2CH2CH3
10
Example 2
11
Example 3
12
13
Alkynes: hydrocarbons that contain a carbon–carbon triple bond
Nomenclature Same as alkenes, but with –yne suffix Cl Br CH3CHCHC CCH2CH2CH3 1
2
3 4
5 6
7
8
CH3 CH3CHC
CCH2CH2Br
6
3 2
5
4
3-bromo-2-chloro-4-octyne not 6-bromo-7-chloro-4-octyne because 2 < 6 1-bromo-5-methyl-3-hexyne not 6-bromo-2-methyl-3-hexyne because 1 < 2
1
1
Physical properties of alkynes • Alkynes have higher boiling points than alkenes with the same number of C’s. Why? • Internal alkynes have higher boiling points than terminal alkynes. Why?
The Structure of Alkynes A triple bond is composed of a σ bond and two π bonds
Addition Reactions of Alkynes
2
Sequential Addition of HCl
An alkyne is less reactive than an alkene
Why is the transition state for an alkyne addition reaction so much less stable than the transition state for an alkene addition? Examine stabilities of the carbocation intermediates (remember: the transition state looks more like the intermediate than the reactant)
3
Relative Stabilities of Carbocations
Addition of Hydrogen Halides In the presence of 1 mole equivalent of HBr Br H
H CH3CH2C
CH
CH3CH2C
CH
CH3CH2C
CH
-
Br H CH3CH2C
CH3CH2CH CH
CH
The secondary vinylic cation is more stable
In the presence of excess HBr, a carbocation is formed by adding the electrophile to the sp2 carbon bonded to the greater number of hydrogens Br CH3CH2C
Br CH2
HBr
CH3CH2CCH3 Br
Br CH3CH2C
Br CH3
CH3CH2C
CH3
4
A secondary vinylic cation is about as stable as a primary cation, therefore a pi-complex may be the actual reaction intermediate
δ- Cl
δ+ H HC
CH
Many (but not all) alkyne addition reactions are stereoselective
Example of a stereoselective alkyne addition reaction: H3C C C CH 3
Cl
+
HCl
H3C
CH3 H
Syn or anti addition? How does this support the idea of a pi-complex?
Addition of HBr in the presence of peroxide peroxide CH3CH2C
CH
+ HBr
CH3CH2CH
CHBr
5
Addition of Halogens to Alkynes Cl Cl Cl Cl2 Cl2 CH3CH2C CCH3 CCH3 CH3CH2C CCH3 CH2Cl2 CH2Cl2 Cl Cl Cl
CH3CH2C
Br CH3C
CH
Br2 CH2Cl2
CH3C
Br Br CH Br
Br2 CH2Cl2
CH3C
CCH3
Br Br
Addition of Water
O RCH2 C
OH
tautomerization
R
keto tautomer
RCH
C
R
enol tautomer
Examples of Water Addition
6
Hg2+ is added to increase the rate of water addition to terminal alkynes
Hydroboration–Oxidation of Internal Alkynes
Hydroboration–Oxidation of Terminal Alkynes Use bulky substituents on the boron to prevent addition of 2nd equivalent of boron adding to B-substituted alkene CH3 CH3
CH3 CH3 CH3CH2C CH
+
CH3CH CH
CH3CH CH
HO-, H2O2 H H2O
HO
2
BH
B
H
CH2CH3 C
2
H C C
CH2CH3
O CH3CH2CH2CH
C H
“disamyl borane”
7
Formation of Ketone versus Aldehyde
Addition of Hydrogen Formation of Cis Alkene
Use Lindlar’s catalyst to stop at alkene…
Addition of H2 to form Trans Alkene
Radical mechanism:
8
Reason for trans addition:
Acidity of a Hydrogen Bonded to an sp Hybridized Carbon
HC pKa = 25
CH
H2 C pKa = 44
CH2 CH3CH3 pKa = 50
Electronegativities versus Acid Strengths
9
Relative base strength: CH3CH2-
>
H2C
CH-
> H2N-
>
HC
C-
> HO-
>
F-
weakest base
strongest base
Synthesis Using Acetylide Ions: Formation of C–C Bond
Designing a Synthesis Example 1
? CH3CH2C
CH
O CH3CH2CCH2CH2CH3
10
Example 2
11
Example 3
12
13
