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Chapter 9 Alkynes

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Title: Organic Chemistry Fifth Edition Subject: Sections 9.1-9.4 Author: Frank Carey Last modified by: SHKim Created Date: 3/17/2000 6:55:59 PM Document presentation ... – PowerPoint PPT presentation

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Title: Chapter 9 Alkynes


1
Chapter 9 Alkynes
2
9.1 Sources of Alkynes
3
Acetylene
  • Industrial preparation of acetylene is by
    dehydrogenation of ethylene.

800C
CH2
H2C
CH3CH3
1150C
Cost of energy makes acetylene a more expensive
industrial chemical than ethylene.
4
Naturally Occurring Alkynes
Some alkynes occur naturally. For example,
5
Naturally Occurring Alkynes
Some alkynes occur naturally. For example,
Histrionicotoxin defensive toxin in the poison
dart frogs of Central and South America
6
9.2 Nomenclature
7
Nomenclature
Acetylene and ethyne are both acceptable IUPAC
names for
Higher alkynes are named in much the same way as
alkenes except using an -yne suffix instead of
-ene.
1-Butyne or But-1-yne
4,4-Dimethyl-2-pentyne or 4,4-Dimethyl-pent-2-yne
8
9.3 Physical Properties of Alkynes
The physical properties of alkynes are similar
to those of alkanes and alkenes.
9
9.4 Structure and Bonding in Alkynes sp
Hybridization
10
Structure
  • Linear geometry for acetylene

11
Cycloalkynes
  • Cyclononyne is the smallest cycloalkyne stable
    enough to be stored at room temperature for a
    reasonable length of time.
  • Cyclooctyne polymerizes on standing.

12
sp Hybridization in Acetylene
Mix together (hybridize) the 2s orbital and one
of the three 2p orbitals.
2p
2p
2sp
2s
13
sp Hybridization in Acetylene
Mix together (hybridize) the 2s orbital and one
of the three 2p orbitals.
2p
Each carbon has two half-filled sp
orbitals available to form ? bonds.
2sp

14
? Bonds in Acetylene
Each carbon is connected to a hydrogen by a ?
bond. The two carbons are connected to each
other by a ? bond and two ? bonds.
Figure 9.2 (a)
15
? Bonds in Acetylene
One of the two ? bonds in acetylene is shown
here. The second ? bond is at right angles to the
first.
Figure 9.2 (b)
16
? Bonds in Acetylene
This is the second of the two ? bonds in
acetylene.
Figure 9.2 (c)
17
Figure 9.3 Electrostatic Potential in Acetylene
The region of highest negative charge lies above
and below the molecular plane in ethylene.
18
Table 9.1 Structural Features of Ethane,
Ethylene, and Acetylene
Ethane Ethylene Acetylene
CC distance
153 pm
134 pm
120 pm
CH distance
111 pm
110 pm
106 pm
HCC angles
111.0
121.4
180
CC BDE
368 kJ/mol
611 kJ/mol
820 kJ/mol
CH BDE
410 kJ/mol
452 kJ/mol
536 kJ/mol
hybridization of C
sp3
sp2
sp
s character
25
33
50
pKa
62
45
26
19
9.5 Acidity of Acetylene and Terminal Alkynes
20
Acidity of Hydrocarbons
In general, hydrocarbons are exceedingly weak
acids, but alkynes are not nearly as weak as
alkanes or alkenes.
  • Compound pKa
  • 26
  • 45
  • CH4 60

21
Carbon Hybridization and Electronegativity
Electrons in an orbital with more s character are
closer to the nucleus and more strongly held.
22
Sodium Acetylide
Objective Prepare a solution containing sodium
acetylide Will treatment of acetylene with NaOH
be effective?
23
Sodium Acetylide
No. Hydroxide is not a strong enough base to
deprotonate acetylene.
In acid-base reactions, the equilibrium lies
to the side of the weaker acid.
24
Sodium Acetylide
Solution Use a stronger base. Sodium amide is
a stronger base than sodium hydroxide.
Ammonia is a weaker acid than acetylene. The
position of equilibrium lies to the right.
25
9.6 Preparation of Alkynes by Alkylation of
Acetylene and Terminal Alkynes
26
Preparation of Alkynes
There are two main methods for the preparation of
alkynes
  • Carbon-carbon bond formation alkylation of
    acetylene and terminal alkynes
  • Functional-group transformations elimination

27
Alkylation of Acetylene and Terminal Alkynes
HC
CH
RC
CH
28
Alkylation of Acetylene and Terminal Alkynes
SN2

  • The alkylating agent is an alkyl halide, and the
    reaction is nucleophilic substitution.
  • The nucleophile is sodium acetylide or the
    sodium salt of a terminal (monosubstituted)
    alkyne.

29
Example Alkylation of Acetylene
NaNH2
NH3
30
Example Alkylation of a Terminal Alkyne
NaNH2, NH3
31
Example Dialkylation of Acetylene
32
Limitation
  • Effective only with primary alkyl halides
  • Secondary and tertiary alkyl halides undergo
    elimination

33
Acetylide Ion as a Base
E2 predominates over SN2 when alkyl halide is
secondary or tertiary.
34
9.7 Preparation of Alkynes by Elimination
Reactions
35
Preparation of Alkynes by Double
Dehydrohalogenation
Geminal dihalide
Vicinal dihalide
The most frequent applications are in preparation
of terminal alkynes.
36
Geminal dihalide ? Alkyne
(CH3)3CCH2CHCl2
37
Geminal dihalide ? Alkyne
(CH3)3CCH2CHCl2
(slow)
NaNH2, NH3
38
Vicinal dihalide ? Alkyne
39
9.8 Reactions of Alkynes
40
Reactions of Alkynes
  • Acidity (Section 9.5)
  • Hydrogenation (Section 9.9)
  • Metal-Ammonia Reduction (Section 9.10)
  • Addition of Hydrogen Halides (Section 9.11)
  • Hydration (Section 9.12)
  • Addition of Halogens (Section 9.13)
  • Ozonolysis (Section 9.14)

41
9.9 Hydrogenation of Alkynes
42
Hydrogenation of Alkynes
cat
RCH2CH2R'
catalyst Pt, Pd, Ni, or Rh
  • Alkene is an intermediate.

43
Heats of Hydrogenation
292 kJ/mol
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.
44
Partial Hydrogenation
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.

45
Lindlar Catalyst
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.

46
Example
H2
Lindlar Pd
47
9.10 Metal-Ammonia Reduction of Alkynes
  • Alkynes ? trans-Alkenes

48
Partial Reduction
RCH2CH2R'
  • Another way to convert alkynes to alkenes is by
    reduction with sodium (or lithium or
    potassium) in ammonia.
  • trans-Alkenes are formed.

49
Example
Na, NH3
50
Mechanism
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

51
Mechanism
  • Step (1) Transfer of an electron from the
    metal to the alkyne to give an anion radical.

52
Mechanism
  • Step (2) Transfer of a proton from the solvent
    (liquid ammonia) to the anion radical.

.

R'
R
C
C
..
..
H
NH2
53
Mechanism
  • Step (3) Transfer of an electron from the
    metal to the alkenyl radical to give a carbanion.

R
.
.

M
R'
C
C
H
54
Mechanism
  • Step (4) Transfer of a proton from the
    solvent (liquid ammonia) to the carbanion.

55
  • Suggest efficient syntheses of (E)- and
    (Z)-2- heptene from propyne and any necessary
    organic or inorganic reagents.

56
Strategy
57
Synthesis
1. NaNH2 2. CH3CH2CH2CH2Br
Na, NH3
H2, Lindlar Pd
58
9.11 Addition of Hydrogen Halides to Alkynes
59
Follows Markovnikov's Rule
HBr
(60)
  • Alkynes are slightly less reactive than alkenes.

60
Termolecular Rate-determining Step
CH
RC
Observed rate law rate kalkyneHX2
61
Two Molar Equivalents of Hydrogen Halide
CH3CH2C
CCH2CH3
2 HF
62
Free-radical Addition of HBr
HBr
peroxides
  • regioselectivity opposite to Markovnikov's rule

63
9.12 Hydration of Alkynes
64
Hydration of Alkynes
  • expected reaction

enol
65
Enols
enol
ketone
  • Enols are tautomers 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.

66
Mechanism of Conversion of Enol to Ketone
67
Mechanism of Conversion of Enol to Ketone
..

O
H
H
H
C
C



O
H
68
Mechanism of Conversion of Enol to Ketone
69
Mechanism of Conversion of Enol to Ketone
H
..

O
H


O
H
H
C
C
70
Key Carbocation Intermediate
Carbocation is stabilized by electron
delocalization (resonance).
..

O
H
C
C

71
Example of Alkyne Hydration
H2O, H
72
Regioselectivity
Markovnikov's rule followed in formation of enol
H2O, H2SO4
CH3(CH2)5CCH3
HgSO4
(91)
73
9.13 Addition of Halogens to Alkynes
74
Example
2Cl2
75
Addition is anti
CH3CH2
Br
Br2
CH2CH3
Br
(90)
76
9.14 Ozonolysis of Alkynes
  • gives two carboxylic acids by cleavage of triple
    bond

77
Example
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