Chapter 8: Alkynes
Naming Alkynes
Structure
Common Name
Acetylene
CH3
methyl acetylene
C
CH3
dimethyl acetylene
CH2 CH3
ethyl acetylene
HC=C- as substituent is called ethynyl.
HC
HC
H3C
HC
IUPAC
ethyne
propyne
2-butyne
1-butyne
CH
C
C
C
Name these:
H3C
H3C
CH3
CH
CH
H3C
H3C
CH3
Some things to consider:
The closest unsaturation to the end takes precedence:
CH3
H3C
H3C
CH
(4Z)-4-hexen-1-yne
Double bond takes precedence if equal
H3C
CH3
(2E)-2-hexen-4-yne
1-buten-3-yne
Reactivity of alkynes
Addition reactions:
slow
R
C
C
R
H
Br
R
+
C
CH
R
Br
-
fast
vinyl cation
Br
C
R
CH
R
The addition of HBr to an alkyne is slower than an alkene because of two factors:
• the sp π orbital is less basic than the sp2 π orbital. Increased s character.
• the vinyl cation formed is unstable compared to an alkyl cation. Here the sp hybridized C+
cannot be stabilized by hyperconjugation because of the pi bond perpendicular to it.
H2C
hybrid?
+
C
H
Stability of vinyl C+ is about the same as H3C+.
Comparison of reactivity of alkenes vs alkynes with HBr:
Alkene
Alkyne
EA1
EA2
∆H
http://www.cem.msu.edu/~reusch/VirtualText/addyne1.htm#add1
General Addition of HBr
slow
H3C
C
C
CH3
+
H3C
H
Br
1st mole
C
CH
Br
H
CH
CH3
H3C
Br
+
Br
C
2nd mole
CH2
H3C
C
H3C
vinyl cation
Br
C
Br
CH3
Br
fast
-
CH3
+
fast
C
CH2
-
Br
CH3
H3C
CH3
CH
Example:
http://www.chemistry.sci.kun.nl/onderwijs/oc1b/2002/College%20H09.pdf
General Addition of Br2
Br
slow
H3C
C
C
Br
Br Br
1st mole
CH3
C
H3C
CH3
C
Br
Br
2nd mole
C
C
H3C
Br
slow
Br
CH3
Br
H3C
trans from anti addition
Br
+
Br
CH3
Br
-
Examples:
http://www.chemistry.sci.kun.nl/onderwijs/oc1b/2002/College%20H09.pdf
Addition of Water and interconversion of the (enol) vinyl alchol to a ketone.
H
HO
H3C
C
H
CH
+
OH2
HgSO4
+
C
CH2
H2O
+
H2O
C
H3C
HO
CH2
C
H3C
H3C
CH2
H3O
+
enol
What is the role of acid in this reaction?
How many moles of water are consumed?
The enol is unstable compared to a unique rearrangement called tautonomerization. The enol
continues to react in the presence of acid.
HO
H
O
C
CH2
H
+
OH2
+
C
H3C
O
O
H2O
C
CH3
H3C
H
+
H3 O
CH3
H3C
H3C
+
CH3
This process is called keto-enol tautonomerization.
What is the role of acid in this reaction?
How many moles of water are consumed?
What is the definition of a tautonomer?
CH2
O
H
C
H3C
O
enol
H
H3C
CH2
ketone
The substitution pattern for alkynes follows Markovinikov’s rule:
——> [ H2C=CHOH ] ——> H3C-CH=O
RC≡CH + H2O + HgSO4 & H2SO4 ——> [ RC(OH)=CH2 ] ——> RC(=O)CH3
RC≡CR' + H2O + HgSO4 & H2SO4 ——>
[ RHC=C(OH)R' + RC(OH)=CHR' ] ——> RCH2-C(=O)R' + RC(=O)-CH2R'
HC≡CH + H2O + HgSO4 & H2SO4
http://www.cem.msu.edu/~reusch/VirtualText/addyne1.htm#add1
Examples:
http://www.chemistry.sci.kun.nl/onderwijs/oc1b/2002/College%20H09.pdf
H
OH
O
keto-enol tautonomerism
CH2
R
R
R
R
Hydroboration
H
R
CH
+
BR3
H2O 2
OH- (aq)
R
BH2
further alkylations
BH3
R
H
H
BH2
H
R
H
H
OH
H
note E orientation
O
keto-enol tautonomerism
CH2
R
H
R
H
The complete reaction is written like this.
O
HgSO4
R
CH
dilute H 2SO 4
R
O
1. BH 3-THF
R
CH3
CH
2. H 2O 2, aq base
RCH2
H
Usually, BH3-THF makes aldehydes and Hg catalyzed hydration makes ketones.
Reduction
H2 reduces alkynes to alkenes and then to alkanes. The fact that the reaction is exothermic and
moreso for alkynes than alkenes, the reaction is hard to stop.
The reaction takes place on the surface of Pt or Pd. Since the addition is twice, it is hard to see the
syn addition.
Reduction of alkynes to (Z) - alkenes
Lindlar’s catalyst: Pd, quinoline, Pb and CaCO3 poisons the metal catalyst, so that the H2 adds only
to the alkyne – not reactive enough for alkenes.
Reduction of alkynes to (E) – alkenes. Reaction via Na radical dissolved in liquid NH3.
Mechanism of Na/NH3 reductions:
Na
Na
-33C
+
+
R
R
+
R
H
-33C
R
H
+
H
NH2R
R
R
R
R
R
NH2
NH3 (e-)
-33C
H
H
R
NH3 (e-)
R
NH3 (e-)
H
R
H
-33C
R
NH2
-33C
H
R
NH2-
R
H
Oxidation
Like alkenes, either with O3 (ozone) or KMnO4. Because of the added unsaturation of alkynes, it is
hard to stop at an aldehyde. So, the reaction goes mostly to carboxylic acids. Terminal alkynes will
make fizzy water.
O
KMnO4
H3C
O
+
CH
OH
H3C
R
HO
+
H 2O
OH
O
O
1. O 3
H3C
CO2
+
2. H2O2 (aq)
OH
H3C
HO
R
Acid/Base
What is a suitable base to make the reaction go to the right below.
R
CH
+
B:-
R
C:
+
BH
For pKa of BH
33
NH3
HC≡CH
25
H2O
16
+
NH4
9
Alkylation
Acetylide anion RC≡C:- can react as a nucleophile and react with a 10 alkyl bromide in a
substitution reaction to make a longer alkyne. This process of adding an alkyl group and is called
alkylation.
NaNH2
Step 1
Step 2
H3C
H3C
CH
C:
H3C
+
H3C
Br
MultiStep synthesis
• Prepare cis-4-octene from 1-pentyne
Step 1: 1-pentyne to pentynyl anion
Step 2: 1-pentynyl anion to 4-octyne
Step 3: 4-octyne to (Z)-4-octene
•
Prepare trans-2-hexene from 1-pentyne
C:
H3C
CH3