CHEM 241-601
Alkynes – How Things Work II
Hydration (addition of H2O) of alkynes:
OXYMERCURATION HYDRATION
Markovnikov
CH3CH2C
CH
O
HgSO4, H2SO4(aq)
CH3CH2CCH3
tautomerization
Hg+2
OH
CH3CH2C
H
C
H
enol
H2 O
H2 O
H
O
H
CH3CH2C
C
CH3CH2C
Hg
2vinyl carbocation SO4
H
CH3CH2C
C
Hg
SO42-
H
CH3CH2C
H
C
H
H
O
O
CH3CH2C
CH3CH2CCH3
H
C
H
H
O
CH3CH2C
H
C
H
+
OH
H
enol - keto tautomerization (intermolecular)
O
H3O
H
C
Hg
HYDROBORATION HYDRATION
Anti-Markovnikov
CH3CH2C
CH
1) disiamyl borane
2) H2O2, NaOH
O
CH3CH2CH2CH
tautomerization
B
H
(sia)2BH
H
CH3CH2C
B(sia)2
CH
H2O2, NaOH
Some problems:
C CH
1) disiamylborane, THF
2) H2O2, NaOH
HgSO4, H2SO4 (aq)
OH
CH3CH2CH=CH2
enol
HYDROGENATION
Standard catalytic hydrogenation reduces all C-C π bonds (except aromatics
H2 - 1%Pt on C
75 psi
"Poisoned" catalyst results in reduction of alkyne to alkene (syn addition)
H2 / Pd on BaSO4
quinoline
Lindlar's catalyst
5-decyne
H
H
Dissolved metal (Na or Li) - ammonia reduction of alkyne to trans alkene
CH3C
Li in NH3(l)
-78oC
CCH3
Li
H
CH3
CH3
C=C
H
H
H
Li
CH3
H
trans more stable
H
CH3
CH3C=CCH3
H
N
H
C=CCH3
NH2
C=CCH3