Alkynes

1
Alkynes
By
Dr/ Amani S. Awaad Professor of pharmacognosy,
King Saud University
2
Alkynes and Cycloalkynes
Cn H 2n-2

• Two sp-hybridized orbitals overlap to form a s
  bond
• One sp orbital on each C overlap with H 1s
  orbitals
• Form two CH bonds
• p orbitals overlap side-to-side to form two ?
  bonds
• spsp s bond and two pp ? bonds result in
  sharing six electrons and formation of C-C triple
  bond
• Shorter and stronger than double bond in ethylene

• Linear molecule
• SP 2p one d
• Bond angle 180?
• length 1.20 Å

Unhybridized p-orbitals
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IUPAC
HC ?CH (acetylene) C2H5C?CH
1-Butyne
CH3-CH2-CH-C?C-CH2-CH3
5-Methyl-3-heptyne CH3
CH3 H3C-C-CH2-C?CH
4,4-Dimethyl-1-pentyne CH3
Cl CH3 CH3-CH-CH-C?C-CH-
CH2-CH2 -CH3 CH3
2-Chloro-2,6-dimethyl-4-nonyne
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Physical properties
Solubility
All at room temp. Non polar compounds
C2-C4 gas C5-C17 liquids
C18 gt wax like solids Like dissolves like,
benzene, ether, CCl4
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Preparations of alkynes
1 Dehydrohalogenation of alkenyl halides
-CC- -C?C- 2HX H X
Na NH2
i) CH3CH2CHCHBr
CH3CH2C?CH
?,strong base
Na NH2
ii)
Cl
?,strong base
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2 From Calcium Carbide
3C CaO CaC2 CO
CaC2 2H2O H-C?C-H Ca(OH)2
3 Reaction of Sodium acetylide with(1?RX)
Liq. Ammon.
Acetylene (C ? C) Na
H-C?C Na H2
H- C?C-H Na H-C?C-NaR/X Na
X H- C?C- R/
R- C?C-H Na R- C?C-NaR/X
Na X R- C?C- R/
For Terminal C?C (use only
Acetylene H-C?C-H)
For Non Terminal C?C (use Monosub.
R-C?C-H)
Na
Na


NaBr
Br
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Reactions of alkynes (Addition)
Alkyne------ alkene------alkane
-C?C- CC C-C
1)Addition of H2 (Depend on catalyst )
- With (Pd /NiB) or (Pd/ CaCO3)
Cis.
- With (Na, NH3 or Liq. NH3)
Trns
CH3
CH3
H2,Pd or Ni-B
H2,Pd
H3C-C?C-CH3 CC
CH3CH2CH2CH3
H
H
Na or liq. NH3
H2,Pd or Ni-B
CH3
H
CC
CH3
H
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4 Addition of water (hydration)
H2SO4
-C?C- H2O -CC-
- C-C-
HgSO4
O
H
OH
Enol
O
H2SO4
H3C-C?CH H2O
H3CCCH3 Acetone
HgSO4
O
H2SO4
H-C?CH H2O
HCCH
HgSO4
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3 Addition of halogen
HC?CH X2 HXCCXH X2
H XX CCXXH

Cl Cl
CH3CH2C?CH Cl2 CH3CH2CCH Cl2
CH3CH2CCH

Cl Cl
Cl Cl
4 Addition of hydrogen halide
HC ? CR HX HHCCX R HX
HHHC CXXR

Br H
CH3C?CH HBr CH3CCH HBr
CH3CCH

Br H Br H
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Visual Test For Alkynes
R-C?CH AgNO3 R-C?C-Ag
(white precipitate) R-C?C- R
AgNO3 R-C?C-R (no
reaction)
AgNO3
CH3CH2CH2C?CH AgNO3

R-C?CH CuNO3 R-C?C-Cu
( Red precipitate) R-C?C R CuNO3
R-C?C-R (no reaction)