Title: Reactions of Alkynes
1Reactions of Alkynes
2Reactions of Alkynes
- Acidity
- Hydrogenation
- Metal-Ammonia Reduction
- Addition of Hydrogen Halides
- Hydration
- Addition of Halogens
- Ozonolysis
3Hydrogenation of Alkynes
4Hydrogenation of Alkynes
cat
RCH2CH2R'
catalyst Pt, Pd, Ni, or Rh
- alkene is an intermediate
5Partial Hydrogenation
RCH2CH2R'
- Alkynes could be used to prepare alkenes if
acatalyst were available that is active enough
to catalyze the hydrogenation of alkynes, but
notactive enough for the hydrogenation of
alkenes.
6Lindlar Catalyst
RCH2CH2R'
- There is a catalyst that will catalyze the
hydrogenationof alkynes to alkenes, but not that
of alkenes to alkanes. - It is called the Lindlar catalyst and consists
ofpalladium supported on CaCO3, which has been
poisoned with lead acetate and quinoline. - syn-Hydrogenation occurs cis alkenes are formed.
7Example
H2
Lindlar Pd
8Metal-Ammonia Reductionof Alkynes
9Partial Reduction
RCH2CH2R'
RC
CR'
RCH
CHR'
- Another way to convert alkynes to alkenes isby
reduction with sodium (or lithium or
potassium)in ammonia. - trans-Alkenes are formed.
10Example
Na, NH3
11Mechanism
Metal (Li, Na, K) is reducing agent H2 is not
involved
- four steps
- (1) electron transfer
- (2) proton transfer
- (3) electron transfer
- (4) proton transfer
12Mechanism
- Step (1) Transfer of an electron from the
metalto the alkyne to give an anion radical.
13Mechanism
- Step (2) Transfer of a proton from the solvent
(liquid ammonia) to the anion radical.
.
R'
R
C
C
..
..
H
NH2
14Mechanism
- Step (3) Transfer of an electron from the
metalto the alkenyl radical to give a carbanion.
R
.
.
M
R'
C
C
H
15Mechanism
- Step (4) Transfer of a proton from the
solvent(liquid ammonia) to the carbanion .
16- Suggest efficient syntheses of (E)- and
(Z)-2-heptene from propyne and any necessary
organicor inorganic reagents.
17Strategy
18Synthesis
1. NaNH2 2. CH3CH2CH2CH2Br
Na, NH3
H2, Lindlar Pd
19Addition of Hydrogen Halidesto Alkynes
20Follows Markovnikov's Rule
HBr
(60)
- Alkynes are slightly less reactive than alkenes
21Termolecular Rate-Determining Step
CH
RC
Observed rate law rate kalkyneHX2
22Two Molar Equivalents of Hydrogen Halide
CH3CH2C
CCH2CH3
2 HF
23Free-radical Addition of HBr
HBr
peroxides
- regioselectivity opposite to Markovnikov's rule
24Hydration of Alkynes
25Hydration of Alkynes
26Enols
enol
ketone
- enols are regioisomers of ketones, and exist in
equilibrium with them - keto-enol equilibration is rapid in acidic media
- ketones are more stable than enols
andpredominate at equilibrium
27Mechanism of conversion of enol to ketone
28Mechanism of conversion of enol to ketone
..
O
H
H
H
C
C
O
H
29Mechanism of conversion of enol to ketone
30Mechanism of conversion of enol to ketone
H
..
O
H
O
H
H
C
C
31Key Carbocation Intermediate
Carbocation is stabilized by electron
delocalization (resonance)
..
O
H
C
C
32Example of Alkyne Hydration
H2O, H
33Regioselectivity
Markovnikov's rule followed in formation of enol
H2O, H2SO4
CH3(CH2)5CCH3
HgSO4
(91)
34Addition of Halogens to Alkynes
35Example
2 Cl2
36Addition is anti
CH3CH2
Br
Br2
CH2CH3
Br
(90)
37Ozonolysis of Alkynes
- gives two carboxylic acids by cleavage of triple
bond
38Example