ORGANIC CHEMISTRY
Chapter 7
7.11 The Acidity of Terminal Alkynes
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
sp
CH CH
pKa
25
Relative Basicity
CH2
sp2
CH2
44
sp3
CH3 CH3
50
CH C < CH2 CH < CH3 CH2
Relative Acidity
H2O > ROH > RC CH > NH3 > CH2 CH2 > CH3 CH3
pKa 15.7 16-17
25
38
44
50
Relative Basicity
OH < RO < RC C < NH2 < CH2 CH < CH3 CH2
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
Acetylenic hydrogens can be deprotonated with
relatively strong bases (sodium amide is typical)
2Na + NH3(liq.)
−40
CH CH + NaNH2
2NaNH2 + H2
NH3(l)
−33
CH3C CH + NaNH2
CH CNa
NH3(l)
−33
NaNH2
NaC CNa
CH3C CNa
The products are called alkynides
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ORGANIC CHEMISTRY
RC CH + HOBr
Chapter 7
RC CBr + H2O
CH CH + Ag(NH3)+
AgC CAg white
CH CH + Cu(NH3)+
CuC CCu
red
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
7.12 Replacement of the Acetylenic
Hydrogen Atom of Terminal Alkynes
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
CH CNa
(1) NaNH2
Chapter 7
CH3CH2CH2Br
CH CCH2CH2CH3
CH3C CCH2CH2CH3
(2) CH3I
Only primary alkyl halides can be used or else elimination
reactions predominate
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
7.13 Hydrogenation of Alkenes
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
Pd, Pt or Ni
CH2 CH2 + H2
CH3CH CH2 + H2
CH3 CH3
25°C
Pd, Pt or Ni
25°C
CH3CH2 CH3
Catalytic hydrogenation
An unsaturated compound
A saturated compound
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
7.14 Hydrogenation: The Function of
the Catalyst
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
Hydrogenation of an alkene is an exothermic reaction.
R CH CH R + H2
Pt
R CH2 CH R + heat
Both hydrogen atoms usually add from the same side
of the molecule — syn addition.
C C
+
H H
Pt
C C
C C
+
X
Y
X Y
A syn addition (顺式加成)
C C
H
H
C C
+
X Y
Y
C C
X
An anti addition (反式加成)
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
In heterogeneous catalysis the hydrogen and alkene
adsorb to the catalyst surface and then a step-wise
formation of C-H bonds occurs
H2
H H
H H
C C
H
H
C C
H H
H H
Surface of
metal catalyst
C
HH
C
HH
+
C C
H
H
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
7.15 Hydrogenation of Alkynes
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
CH3C CCH3
Chapter 7
H2
Pt
CH3CH CHCH3
H2
Pt
CH3CH2 CH2CH3
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ORGANIC CHEMISTRY
Chapter 7
7.15A Syn Addition of Hydrogen:
Synthesis of cis-Alkenes
P-2 catalyst
NaBH4
(CH3CO2)2Ni
C2H5OH
Lindlar's catalyst
Pd/CaCO3-quinoline
Ni2B
P-2
Pd/BaSO4-quinoline
H2 / Ni2B(P-2)
CH3CH2C CCH2CH3
CH3CH2
CH2CH3
C
or Lindlar's catalyst
H
C
H
Syn addition
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
7.15B Anti Addition of Hydrogen:
Synthesis of trans-Alkenes
A dissolving metal reaction which uses lithium or
sodium metal in low temperature ammonia or amine
solvent produces trans-alkenes
CH3(CH2)2C C(CH2)2CH3
Li/ C2H5NH2, −78°C
or Na / NH3(l)
H
CH3(CH2)2
C
H
C
anti addition
(CH2)2CH3
Dissolving metal reduction (溶化金属还原)
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
The mechanism is a free radical reaction with two electron transfer reactions from
the metal
The vinylic anion prefers to be trans and this determines the trans stereochemistry
of the product
Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY
Chapter 7
7.16 Molecular Formulas of
Hydrocarbons: The Index of Hydrogen
Deficiency
Department of Chemistry, Xiamen University
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Chapter 7
General formula
Alkanes
Alkenes and cycloalkanes
CnH2n+2
CnH2n
Alkynes and cycloalkenes
CnH2n-2
Index of hydrogen deficiency — the number of pairs of hydrogen
atoms that must be subtracted from the molecular formula of the
corresponding alkane to give the molecular formula of the
compound under consideration.
Ω=
2n+2 − X1 + N3
2
Department of Chemistry, Xiamen University