REACTIONS OF ALKYNES-6�
DR. K. K. NANDI
DEPARTMENT OF CHEMISTRY
BKC COLLEGE
9.11�Addition of Hydrogen Halides�to Alkynes
Follows Markovnikov's Rule
Alkynes are slightly less reactive than alkenes
HBr
Br
(60%)
CH3(CH2)3C
CH
CH3(CH2)3C
CH2
Termolecular Rate-Determining Step
CH
..
Br
H
:
..
RC
..
Br
H
:
..
Observed rate law: rate = k[alkyne][HX]2
Two Molar Equivalents of Hydrogen Halide
(76%)
CH3CH2C
CCH2CH3
2 HF
F
F
C
C
H
H
CH3CH2
CH2CH3
Free-radical Addition of HBr
regioselectivity opposite to Markovnikov's rule
HBr
(79%)
CH3(CH2)3C
CH
CH3(CH2)3CH
CHBr
peroxides
9.12�Hydration of Alkynes
Hydration of Alkynes
expected reaction:
enol
observed reaction:
RCH2CR'
O
H+
RC
CR'
H2O
+
H+
RC
CR'
H2O
+
OH
RCH
CR'
ketone
Enols
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
enol
OH
RCH
CR'
RCH2CR'
O
ketone
Mechanism of conversion of enol to ketone
O
H
C
C
H
+
O
H
H
:
..
:
Mechanism of conversion of enol to ketone
O
H
C
C
H
+
O
H
H
:
..
:
:
Mechanism of conversion of enol to ketone
O
H
C
C
H
H
O
:
:
H
+
..
:
Mechanism of conversion of enol to ketone
O
H
C
C
H
H
O
:
:
H
+
..
:
Mechanism of conversion of enol to ketone
O
H
C
C
H
H
O
:
H
+
..
:
Key Carbocation Intermediate
Carbocation is stabilized by �electron delocalization (resonance)
H
O
C
C
H
..
H
+
O
C
C
H
+
..
:
Example of Alkyne Hydration
H2O, H+
(89%)
via
CH3(CH2)2C
C(CH2)2CH3
Hg2+
O
CH3(CH2)2CH2C(CH2)2CH3
OH
CH3(CH2)2CH
C(CH2)2CH3
Regioselectivity
H2O, H2SO4
HgSO4
CH3(CH2)5CCH3
(91%)
via
Markovnikov's rule followed in formation of enol
CH3(CH2)5C
CH2
OH
CH3(CH2)5C
CH
O
Mechanism:
HYDROBORATION
9.13�Addition of Halogens to Alkynes
Example
+ 2 Cl2
Cl
Cl
(63%)
C
Cl2CH
CH3
HC
CCH3
Addition is anti
Br2
CH3CH2
CH2CH3
Br
Br
(90%)
CH3CH2C
CCH2CH3
C
C