Understanding Alkynes in Organic Chemistry

 
Unsaturated Hydrocarbons
Alkynes
 
1
 
 
1435-1436
2014-2015
 
Learning Objectives
Learning Objectives
 
Chapter three discusses the following topics which have to be understood
Chapter three discusses the following topics which have to be understood
and memorized :
and memorized :
 
 The structure, hybridization and Bonding in alkynes
 The structure, hybridization and Bonding in alkynes
 
 Common and IUPAC naming of alkynes
 Common and IUPAC naming of alkynes
 
 Physical properties of alkynes
 Physical properties of alkynes
 
 Preparation of alkynes
 Preparation of alkynes
 
 Reactions of alkynes: addition reactions and acidity
 Reactions of alkynes: addition reactions and acidity
 
Alkynes
 
Alkynes: Molecular And Structural Formulae
 
The 
The 
alkynes
alkynes
 comprise a series of carbon- and hydrogen- based compounds that
 comprise a series of carbon- and hydrogen- based compounds that
contain at least one triple bond. This group of compounds is a homologous
contain at least one triple bond. This group of compounds is a homologous
series with the general molecular formula of 
series with the general molecular formula of 
C
C
n
n
 
 
H
H
2
2
n
n
—2
—2
The alkyne triple bond is composed of 
The alkyne triple bond is composed of 
one σ and two 2 
one σ and two 2 
 covalent bonds
 covalent bonds
, the
, the
triple bond can be terminal or internal.
triple bond can be terminal or internal.
 
 
 
 
 
The simplest alkyne, 
The simplest alkyne, 
ethyne
ethyne
 (also known as 
 (also known as 
acetylene
acetylene
), has two carbon atoms
), has two carbon atoms
and the molecular formula of 
and the molecular formula of 
C
C
2
2
H
H
2
2
. The structural formula for ethyne is:
. The structural formula for ethyne is:
 
3
 
Alkynes
 
sp Hybridization Of Alkynes
sp Hybridization Of Alkynes
 
 This involves the mixing of one s- and one p-orbital forming two sp-
 This involves the mixing of one s- and one p-orbital forming two sp-
hybrid orbitals of equivalent energy. The two sp-hybrid orbitals are
hybrid orbitals of equivalent energy. The two sp-hybrid orbitals are
oriented in a linear arrangement and bond angle is 180° to minimize the
oriented in a linear arrangement and bond angle is 180° to minimize the
repulsion between them.
repulsion between them.
 The remaining two p orbitals (py and Pz) are unaltered
 The remaining two p orbitals (py and Pz) are unaltered
 
4
 
Alkynes
 
Molecular formula of ethyne is C
Molecular formula of ethyne is C
2
2
H
H
2
2
.
.
In ethyne, each carbon atom is sp-hybridized.
In ethyne, each carbon atom is sp-hybridized.
 In this way, four sp-orbital are generated.
 In this way, four sp-orbital are generated.
 One sp- orbital  of each carbon atom by overlapping forms a sigma
 One sp- orbital  of each carbon atom by overlapping forms a sigma
bond between carbon atoms.
bond between carbon atoms.
Remaining one sp-orbital of each carbon atom overlap with 1s-
Remaining one sp-orbital of each carbon atom overlap with 1s-
orbital of a hydrogen atom to produce two sigma bonds.
orbital of a hydrogen atom to produce two sigma bonds.
P
P
y
y
-orbital and P
-orbital and P
z
z
-orbitals of each carbon by parallel overlapping
-orbitals of each carbon by parallel overlapping
form two pi-bonds between the two carbon atoms.
form two pi-bonds between the two carbon atoms.
 Geometry (shape) of ethyne molecule is linear in which bond
 Geometry (shape) of ethyne molecule is linear in which bond
angles are 180
angles are 180
o 
o 
and 
and 
C=C bond length is 
C=C bond length is 
1.20 Å
1.20 Å
 
5
 
Orbital Overlap IN Ethyne
Orbital Overlap IN Ethyne
 
Alkynes
 
2
s
2
2
p
2
 
2
s
1
2
p
3
 
2
 
x
 
s
p
 
2
p
 
s
s
p
p
 
 
H
H
Y
Y
B
B
R
R
I
I
D
D
I
I
S
S
A
A
T
T
I
I
O
O
N
N
 
 
O
O
F
F
 
 
O
O
R
R
B
B
I
I
T
T
A
A
L
L
S
S
 
 
i
i
n
n
 
 
A
A
L
L
K
K
Y
Y
N
N
E
E
S
S
 
T
h
e
 
e
l
e
c
t
r
o
n
i
c
 
c
o
n
f
i
g
u
r
a
t
i
o
n
 
o
f
 
a
 
c
a
r
b
o
n
 
a
t
o
m
 
i
s
 
1
s
2
2
s
2
2
p
2
 
promotion
 
hybridization
 
Alkynes
 
6
 
7
 
Alkynes
 
 
Summary
Summary
 
 sp
 sp
 hybridization occurs when a C has 2 sigma bonds only
 hybridization occurs when a C has 2 sigma bonds only
 sp
 sp
 hybridized orbital has 50% s and 50% p character
 hybridized orbital has 50% s and 50% p character
 The 2 
 The 2 
sp
sp
 hybrids point in opposite directions at 180
 hybrids point in opposite directions at 180
o
o
 to each other
 to each other
 Each 
 Each 
sp
sp
 
 
hybrid is involved in a(
hybrid is involved in a(
σ
σ
)sigma bond
)sigma bond
 The remaining
 The remaining
 p
 p
 orbitals form the 2pi bonds
 orbitals form the 2pi bonds
 The triple bond is one (
 The triple bond is one (
σ
σ
)bond and two pi (∏) bonds
)bond and two pi (∏) bonds
.
 
 
8
 
Alkynes
 
Common  Nomenclature Of Alkynes
Common  Nomenclature Of Alkynes
 
The simplest alkyne its common name is 
The simplest alkyne its common name is 
acetylene
acetylene
Therefore the common names  of alkynes are derived from acetylene
Therefore the common names  of alkynes are derived from acetylene
 ( e.g. Methyl acetylene
 ( e.g. Methyl acetylene
)
Examples:
 
Common
 : Isobutyisopropylacetylene
 
Common
 : Methyl acetylene
 
Common: 
Isopropylmethylacetylene
 
HC 
CH
 
Alkynes
 
9
 
Find the longest chain containing both atoms of the triple bond; this gives the root
Find the longest chain containing both atoms of the triple bond; this gives the root
name.
name.
Add the ending 
Add the ending 
yne
yne
 
 
to the root name
to the root name
.
.
Number the chain, starting at the end closest to the triple bond.
Number the chain, starting at the end closest to the triple bond.
 Give branches or other substituents names and  numbers to locate their positions.
 Give branches or other substituents names and  numbers to locate their positions.
Indicate the number of identical groups by prefixes di, tri, tetra, etc.
Indicate the number of identical groups by prefixes di, tri, tetra, etc.
Place the position numbers and names of the substituent  groups in alphabetical order,
Place the position numbers and names of the substituent  groups in alphabetical order,
before the root name. In alphabetizing ignore prefixes like 
before the root name. In alphabetizing ignore prefixes like 
tert
tert
., di, tri, etc. but
., di, tri, etc. but
include iso and cyclo.
include iso and cyclo.
 
IUPAC Nomenclature of Alkynes
IUPAC Nomenclature of Alkynes
 
10
10
 
Alkynes
 
Exercise
Exercise
 
1)
 
Give the IUPAC and common names of the following
compounds
 
11
11
 
Alkynes
 
Physical Properties
Physical Properties
 
Nonpolar, insoluble in water.
Nonpolar, insoluble in water.
Soluble in most organic solvents ( hexane, benzene…).
Soluble in most organic solvents ( hexane, benzene…).
The boiling points and melting points of alkynes increase with molecular
The boiling points and melting points of alkynes increase with molecular
weight.
weight.
 
 
Branching
Branching
 
 
reduces
reduces
 the boiling point of alkynes
 the boiling point of alkynes
Acidity of alkynes
Acidity of alkynes
Terminal alkynes
Terminal alkynes
, R-C
, R-C
C-H,
C-H,
 are more 
 are more 
acidic
acidic
 than other  hydrocarbons.
 than other  hydrocarbons.
For example, compounds b & d are more acidic than a & c.
For example, compounds b & d are more acidic than a & c.
 
 
 
12
12
 
Alkynes
 
Terminal alkynes
Terminal alkynes
 
Terminal alkynes
Terminal alkynes
 
Preparation of alkynes
Preparation of alkynes
 
1.
Dehydrohalogenation of geminal or vicinal dihaloalkanes
Dehydrohalogenation of geminal or vicinal dihaloalkanes
(-2HX):
(-2HX):
 
 
 
 
 
 
 
 
13
13
 
Alkynes
 
14
 
2.
From reaction of sodium Acetylide with Primary Alkyl
From reaction of sodium Acetylide with Primary Alkyl
Halides (Alkylationof acetylide with primary alkyl halide)
Halides (Alkylationof acetylide with primary alkyl halide)
 
Alkynes
 
For Terminal C
C     
(use only
 
Acetylene 
H-
C
C
-H
)
 
                                   
   
(use Monosub.         
R-
C
C-H)
 
Reactions of alkynes
Reactions of alkynes
 
1
1
. Addition of hydrogen ( Hydrogenation
. Addition of hydrogen ( Hydrogenation
)
)
 
15
15
 
 
Alkynes can be reduced to
Alkynes can be reduced to
 
 
trans
trans
-alkenes
-alkenes
 using 
 using 
Na or Li in liquid NH
Na or Li in liquid NH
3
3
 
 
 
Alkynes can be partially reduced to 
Alkynes can be partially reduced to 
cis
cis
-alkenes
-alkenes
 with 
 with 
H
H
2
2
 in the presence of
 in the presence of
poisoned catalysts.
poisoned catalysts.
 
Electrophilic Addition Reaction
Electrophilic Addition Reaction
 
Alkynes
 
or (
Z
)
 
or (
E
)
 
Lindar’s Catlyst
 
 
Alkynes can be reduced to
Alkynes can be reduced to
 
 
alkanes
alkanes
 with 
 with 
H
H
2
2
 in the presence of 
 in the presence of 
catalysts as Ni
catalysts as Ni
 
2. 
Addition of halogen  (Halogenation)
Addition of halogen  (Halogenation)
 
16
16
 
 
HC
CH + 
X
2
          H
X
C=C
X
H  + 
X
2
 
          H 
X
X
 C–C
X
X
H
 
If we used  one mole ……(alkene) two mole …….alkane
 
Alkynes
 
Trans
 
17
 
3
3
.
 
A
A
d
d
d
d
i
i
t
t
i
i
o
o
n
n
 
 
o
o
f
f
 
 
h
h
y
y
d
d
r
r
o
o
g
g
e
e
n
n
 
 
h
h
a
a
l
l
i
i
d
d
e
e
 
HC 
 CR + 
H
X
         
H
HC=C
X 
R  + 
H
X
            
H
H
HC – C
X
X
R
 
Alkynes
 
18
 
4
4
.
.
 
 
A
A
d
d
d
d
i
i
t
t
i
i
o
o
n
n
 
 
o
o
f
f
 
 
w
w
a
a
t
t
e
e
r
r
:
:
 
 
H
H
y
y
d
d
r
r
a
a
t
t
i
i
o
o
n
n
 
Alkynes
 
19
 
1. An alkyne’s name ends with
(a) –ane
    
(b) -ene
(c) –yne
    
(d) diene
2. An alkyne function has …….. pi bond(s).
(a) one
    
(b) two
(c) three
    
(d) four
3. Alkynes react with HCl by a mechanism called
(a) elimination
   
(b) Markovnikov addition
(c) 
   
 
(
d) substitution
4. Alkynes react with water in the presence of a catalyst to give
(a) a dialcohol (diol)
  
(b) an alkane
(c) an enol
   
(d) a dibromide
5. The conversion of alkynes to alkanes is an example of
(a) oxidation
   
(b) reduction
(c) chlorination
  
 
(d) dehydration
 
Alkynes
 
Thank You for your kind attention !
Thank You for your kind attention !
 
Questions?
Comments
 
20
 
Alkynes
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Alkynes are unsaturated hydrocarbons with at least one triple bond, following a molecular formula of CnH2n-2. This group of compounds is discussed in Chapter three, covering topics like structure, hybridization, common naming, physical properties, preparation, and reactions. The sp hybridization of alkynes involves linear arrangement of two sp-hybrid orbitals at a 180-degree bond angle, with remaining p orbitals unchanged. The triple bond in alkynes consists of one sigma bond and two pi bonds, with involved orbital overlap forming linear molecule geometry.


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  1. Unsaturated Hydrocarbons Alkynes 1435 1435- -1436 2014 2014- -2015 1436 2015 1

  2. Alkynes Learning Objectives Chapter three discusses the following topics which have to be understood and memorized : The structure, hybridization and Bonding in alkynes Common and IUPAC naming of alkynes Physical properties of alkynes Preparation of alkynes Reactions of alkynes: addition reactions and acidity

  3. Alkynes Alkynes: Molecular And Structural Formulae The alkynes comprise a series of carbon- and hydrogen- based compounds that contain at least one triple bond. This group of compounds is a homologous series with the general molecular formula of CnH2n 2 The alkyne triple bond is composed of one and two 2 covalent bonds, the triple bond can be terminal or internal. The simplest alkyne, ethyne (also known as acetylene), has two carbon atoms and the molecular formula of C2H2. The structural formula for ethyne is: H C CH 3

  4. Alkynes sp Hybridization Of Alkynes This involves the mixing of one s- and one p-orbital forming two sp- hybrid orbitals of equivalent energy. The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle is 180 to minimize the repulsion between them. The remaining two p orbitals (py and Pz) are unaltered 4

  5. Alkynes Orbital Overlap IN Ethyne H C CH Molecular formula of ethyne is C2H2. In ethyne, each carbon atom is sp-hybridized. In this way, four sp-orbital are generated. One sp- orbital of each carbon atom by overlapping forms a sigma bond between carbon atoms. Remaining one sp-orbital of each carbon atom overlap with 1s- orbital of a hydrogen atom to produce two sigma bonds. Py-orbital and Pz-orbitals of each carbon by parallel overlapping form two pi-bonds between the two carbon atoms. Geometry (shape) of ethyne molecule is linear in which bond angles are 180o and C=C bond length is 1.20 5

  6. Alkynes sp HYBRIDISATION OF ORBITALS in ALKYNES The electronic configuration of a carbon atom is 1s22s22p2 2s12p3 2s22p2 2 x sp 2p promotion hybridization 6

  7. Alkynes 7

  8. Alkynes Summary sp hybridization occurs when a C has 2 sigma bonds only sp hybridized orbital has 50% s and 50% p character The 2 sp hybrids point in opposite directions at 180o to each other Each sphybrid is involved in a( )sigma bond The remaining p orbitals form the 2pi bonds The triple bond is one ( )bond and two pi ( ) bonds. 8

  9. Alkynes Common Nomenclature Of Alkynes The simplest alkyne its common name is acetylene Therefore the common names of alkynes are derived from acetylene ( e.g. Methyl acetylene) Examples: HC CH CH3 CH3 CH3 CH CH2 C C CH CH3 CH3 C CH Common : Isobutyisopropylacetylene Common : Methyl acetylene H3C C C Common: Isopropylmethylacetylene 9

  10. Alkynes IUPAC Nomenclature of Alkynes Find the longest chain containing both atoms of the triple bond; this gives the root name. Add the ending yne to the root name. Number the chain, starting at the end closest to the triple bond. Give branches or other substituents names and numbers to locate their positions. Indicate the number of identical groups by prefixes di, tri, tetra, etc. Place the position numbers and names of the substituent groups in alphabetical order, before the root name. In alphabetizing ignore prefixes like tert., di, tri, etc. but include iso and cyclo. F 1 3 4 5 1 2 4 7 Br 1 3 2 2 3 9 6 4 CH HC C CH H3C C C 5 8 F CH3 5-Bromo-2-pentyne Not 1-Bromo-3-pentyne 6-Ethyl-4-nonyne 4,4-Difluoro-3-methyl-1-butyne 10

  11. Alkynes Exercise 1) Give the IUPAC and common names of the following compounds b) C CH a) H2 C H2 C C C(CH3)3 - C C HC d) c) 11

  12. Alkynes Physical Properties Nonpolar, insoluble in water. Soluble in most organic solvents ( hexane, benzene ). The boiling points and melting points of alkynes increase with molecular weight. Branching reduces the boiling point of alkynes Acidity of alkynes Terminal alkynes, R-C C-H, are more acidic than other hydrocarbons. For example, compounds b & d are more acidic than a & c. b) C CH a) H2 C c) Terminal alkynes H2 C C C(CH3)3 - C C HC Terminal alkynes d) 12

  13. Alkynes Preparation of alkynes 1. (-2HX): Dehydrohalogenation of geminal or vicinal dihaloalkanes Br KOH/ alcohol NaNH2 Br Br heat heat Strong base C C C C - 2 HX X X 1,2-dihaloalkane Excess NaNH2 - 2HBr Br Br 13

  14. Alkynes 2. From reaction of sodium Acetylide with Primary Alkyl Halides (Alkylationof acetylide with primary alkyl halide) liq NH3 C- Na+ + + R C R C CH Na H2 1/2 Sodium acetylide + + C- Na+ NaX X R' R' R C R C C For Terminal C C (use only Acetylene H-C C-H) (use Monosub. R-C C-H) NaNH2/ NH3 CH3Br Na - NaBr 14

  15. Alkynes Reactions of alkynes Electrophilic Addition Reaction 1. Addition of hydrogen ( Hydrogenation) Alkynes can be partially reduced to cis-alkenes with H2 in the presence of poisoned catalysts. Pd(BASO4) or (NiB) Lindar s Catlyst C C + H2 H H or (Z) Cis- alkene Alkynes can be reduced totrans-alkenes using Na or Li in liquid NH3 H Na or Li / liq. NH3 C C + H2 H Trans -alkene or (E) Alkynes can be reduced toalkanes with H2 in the presence of catalysts as Ni H2/ Ni 15

  16. Alkynes 2. Addition of halogen (Halogenation) HC CH + X2 HXC=CXH + X2 H XX C CXXH X X X + + X2 C C X2 C C C C X X X (X= Br or Cl) Tetrahalide Trans If we used one mole (alkene) two mole .alkane Br Br Br + Br2 + Br2 Br Br Br 16

  17. Alkynes 3. Addition of hydrogen halide HC CR + HX HHC=CX R + HX HHHC CXXR X H + + HX C C HX C C C C H X H X (X= Br , Cl, I) A haloalkene A gem dihalide Br + HBr + HBr Br Br 17

  18. Alkynes 4. Addition of water: Hydration H H H2SO4 / HgSO4 keto-enol tautomerism HO-H H + C C O H O carbonyl more stable an enol unstable Specific example: H H H H / H2SO4, HgSO4 H-OH C H H C C C H C C Acetylene O H H O H Vinyl alcohol (an unstable enol) Acetaldehyde (a stable carbonyl compound) O CH3 H / H2SO4, HgSO4 H-OH HC C CH3 C C H3C C CH3 O H H Acetone Propyne (an unstable enol) (a stable carbonyl compound) 18

  19. Alkynes 1. An alkyne s name ends with (a) ane (c) yne 2. An alkyne function has .. pi bond(s). (a) one (c) three 3. Alkynes react with HCl by a mechanism called (a) elimination (c) 4. Alkynes react with water in the presence of a catalyst to give (a) a dialcohol (diol) (c) an enol 5. The conversion of alkynes to alkanes is an example of (a) oxidation (c) chlorination (b) -ene (d) diene (b) two (d) four (b) Markovnikov addition (d) substitution (b) an alkane (d) a dibromide (b) reduction (d) dehydration 19

  20. Alkynes Thank You for your kind attention ! Questions? Comments 20

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