9.9
Hydrogenation of Alkynes
Dr. Wolf's CHM 201 & 202
9-1
Hydrogenation of Alkynes
RC
CR'
+
2H2
cat
RCH2CH2R'
catalyst = Pt, Pd, Ni, or Rh
alkene is an intermediate
Dr. Wolf's CHM 201 & 202
9-2
Heats of hydrogenation
CH3CH2C
CH
292 kJ/mol
CH3C
CCH3
275 kJ/mol
Alkyl groups stabilize triple bonds in the
same way that they stabilize double
bonds. Internal triple bonds are more
stable than terminal ones.
Dr. Wolf's CHM 201 & 202
9-3
Partial Hydrogenation
RC
CR'
H2
cat
RCH
CHR'
H2
cat
RCH2CH2R'
Alkynes could be used to prepare alkenes if a
catalyst were available that is active enough to
catalyze the hydrogenation of alkynes, but not
active enough for the hydrogenation of alkenes.
Dr. Wolf's CHM 201 & 202
9-4
Lindlar Palladium
RC
CR'
H2
cat
RCH
CHR'
H2
cat
RCH2CH2R'
There is a catalyst that will catalyze the hydrogenation
of alkynes to alkenes, but not that of alkenes to alkanes.
It is called the Lindlar catalyst and consists of
palladium supported on CaCO3, which has been
poisoned with lead acetate and quinoline.
syn-Hydrogenation occurs; cis alkenes are formed.
Dr. Wolf's CHM 201 & 202
9-5
Example
CH3(CH2)3C
C(CH2)3CH3 + H2
Lindlar Pd
CH3(CH2)3
(CH2)3CH3
C
C
H
H
(87%)
Dr. Wolf's CHM 201 & 202
9-6
9.10
Metal-Ammonia Reduction of
Alkynes
Alkynes  trans-Alkenes
Dr. Wolf's CHM 201 & 202
9-7
Partial Reduction
RC
CR'
RCH
CHR'
RCH2CH2R'
Another way to convert alkynes to alkenes is
by reduction with sodium (or lithium or potassium)
in ammonia.
trans-Alkenes are formed.
Dr. Wolf's CHM 201 & 202
9-8
Example
CH3CH2C
CCH2CH3
Na, NH3
CH3CH2
H
C
C
CH2CH3
H
(82%)
Dr. Wolf's CHM 201 & 202
9-9
Mechanism
Metal (Li, Na, K) is reducing agent;
H2 is not involved
four steps
(1) electron transfer
(2) proton transfer
Dr. Wolf's CHM 201 & 202
(3)
electron transfer
(4)
proton transfer
9-10
Mechanism
Step (1): Transfer of an electron from the metal
to the alkyne to give an anion radical.
R
C
C
Dr. Wolf's CHM 201 & 202
R' + M .
R
M+
..–
C
.
C
R'
9-11
Mechanism
Step (2) Transfer of a proton from the solvent
(liquid ammonia) to the anion radical.
R
R
–
C
..
.
C
H
Dr. Wolf's CHM 201 & 202
R'
..
NH2
C
H
.
C
R'
..
: NH2
–
9-12
Mechanism
Step (3): Transfer of an electron from the metal
to the alkenyl radical to give a carbanion.
R
C
.
C
H
Dr. Wolf's CHM 201 & 202
R
R' + M .
C
H
M+
–
..
C
R'
9-13
Mechanism
Step (4) Transfer of a proton from the solvent
(liquid ammonia) to the carbanion .
H
R
C
..
C –
H
Dr. Wolf's CHM 201 & 202
R'
..
NH2
H
R
C
H
..
: NH2
–
C
R'
9-14
Suggest efficient syntheses of (E)- and (Z)-2heptene from propyne and any necessary organic
or inorganic reagents.
Dr. Wolf's CHM 201 & 202
9-15
1. NaNH2
2. CH3CH2CH2CH2Br
H2, Lindlar Pd
Dr. Wolf's CHM 201 & 202
Na, NH3
9-16
9.11
Addition of Hydrogen Halides
to Alkynes
Dr. Wolf's CHM 201 & 202
9-17
Follows Markovnikov's Rule
CH3(CH2)3C
HBr
CH
CH3(CH2)3C
CH2
Br
(60%)
Alkynes are slightly less reactive than alkenes
Dr. Wolf's CHM 201 & 202
9-18
Termolecular transition state
H
RC
H
..
Br :
..
CH
..
Br :
..
Observed rate law: rate = k[alkyne][HX]2
Dr. Wolf's CHM 201 & 202
9-19
Reaction with two moles of a hydrogen
halide yields a geminal dihalide
CH3CH2C
CCH2CH3
2 HF
CH3CH2
H
F
C
C
H
F
CH2CH3
(76%)
Dr. Wolf's CHM 201 & 202
9-20
Free-radical addition of HBr occurs when
peroxides are present
CH3(CH2)3C
CH
HBr
peroxides
CH3(CH2)3CH
CHBr
(79%)
regioselectivity opposite to Markovnikov's rule
Dr. Wolf's CHM 201 & 202
9-21
9.12
Hydration of Alkynes
Dr. Wolf's CHM 201 & 202
9-22
Hydration of Alkynes
expected reaction:
RC
CR'
+ H2O
H+
RCH
CR'
OH
enol
observed reaction:
RC
CR'
+ H2O
H+
RCH2CR'
O
ketone
Dr. Wolf's CHM 201 & 202
9-23
RCH
CR'
OH
enol
RCH2CR'
O
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 and
predominate at equilibrium
Dr. Wolf's CHM 201 & 202
9-24
Mechanism of conversion of enol to ketone
..
:O
H
+
:O
C
H
C
H
H
Dr. Wolf's CHM 201 & 202
9-25
Mechanism of conversion of enol to ketone
..
:O
H
+
:O
C
H
C
H
H
Dr. Wolf's CHM 201 & 202
9-26
Mechanism of conversion of enol to ketone
..
:O
H
H
C
H
C
+
: O:
H
Dr. Wolf's CHM 201 & 202
9-27
Mechanism of conversion of enol to ketone
H
..
:O
H
Dr. Wolf's CHM 201 & 202
C
H
: O:
H
C
+
9-28
Mechanism of conversion of enol to ketone
H
..
:O
H
Dr. Wolf's CHM 201 & 202
C
H
: O:
H
C
+
9-29
Mechanism of conversion of enol to ketone
H
..
:O
H
Dr. Wolf's CHM 201 & 202
C
H +O:
H
C
9-30
Key carbocation intermediate is stabilized by
electron delocalization (resonance)
..
:O
H
C
Dr. Wolf's CHM 201 & 202
C
+
..
+O
H
H
C
H
C
9-31
Example
CH3(CH2)2C
C(CH2)2CH3
via
H2O, H+
OH
Hg2+
CH3(CH2)2CH
C(CH2)2CH3
O
CH3(CH2)2CH2C(CH2)2CH3
(89%)
Dr. Wolf's CHM 201 & 202
9-32
Markovnikov's rule followed in formation of enol
O
CH3(CH2)5C
CH
H2O, H2SO4
HgSO4
CH3(CH2)5CCH3
(91%)
via
OH
CH3(CH2)5C
Dr. Wolf's CHM 201 & 202
CH2
9-33
9.13
Addition of Halogens to
Alkynes
Dr. Wolf's CHM 201 & 202
9-34
Example
Cl
HC
CCH3 + 2 Cl2
Cl2CH
C
CH3
Cl
(63%)
Dr. Wolf's CHM 201 & 202
9-35
Addition is anti
Br
CH3CH2
CH3CH2C
CCH2CH3
Br2
C
C
CH2CH3
Br
(90%)
Dr. Wolf's CHM 201 & 202
9-36
9.14
Ozonolysis of Alkynes
gives two carboxylic acids by cleavage
of triple bond
Dr. Wolf's CHM 201 & 202
9-37
Example
CH3(CH2)3C
CH
1. O3
2. H2O
O
O
CH3(CH2)3COH
+
HOCOH
(51%)
Dr. Wolf's CHM 201 & 202
9-38
End of Chapter 9
Dr. Wolf's CHM 201 & 202
9-39