Unsaturated Hydrocarbons
Alkynes
1435-1436
2014-2015
1
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
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 Cn H2n—2
 The alkyne triple bond is composed of one σ 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:
HC
3
CH
Alkynes
sp Hybridization Of Alkynes
 This involves the mixing of one s- and one p-orbital forming two sphybrid 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
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Alkynes
Orbital Overlap IN Ethyne




HC
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 1sorbital 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 Å
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Alkynes
sp HYBRIDISATION OF ORBITALS in ALKYNES
The electronic configuration of a carbon atom is 1s22s22p2
2s12p3
2s22p2
promotion
2 x sp
hybridization
2p
Alkynes
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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 sp hybrid is involved in a(σ)sigma bond
The remaining p orbitals form the 2pi bonds
The triple bond is one (σ)bond and two pi (∏) bonds.
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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.
Double and triple bonds are considered to have equal priority: thus in a molecule with
both a double and triple bond, whichever is close to the end of the chain determines
the direction of numbering.
In case where double and triple bonds would have the same position number, the
double bond takes the lower number.
In writing the final name ‘’ene’’ comes before ‘’yne’’ regardless which takes the
lower number (i.e. alphabetical order).
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Alkynes
Examples
IUPAC Names Of Alkynes
1
F
4 CH
F
3
HC
2
2
C
4
7
1
CH
3
9
6
4
5
1
H3C
Br
3
C
C
5
8
CH3
5-Bromo-2-pentyne
Not 1-Bromo-3-pentyne
6-Ethyl-4-nonyne
4,4-Difluoro-3-methyl-1-butyne
2
H
5
HC
4
C
3
CH2
2
CH
1
CH2
Pent-1-en-4-yne
double and triple bonds have have the same position number
thus ene take lower number
1
2
HC
C
3
C
4
5
6
CH-CH2-CH3
Hex-3-en-1-yne
(triple bond closer to the end of chain)
Note: An''e'' is dropped from ''ene''
due to it is followed by a vowel
Alkynes
Common Nomenclature Of Alkynes
 The simplest alkyne its common name is acetylene HC CH
 Therefore the common names of alkynes are derived from acetylene
( e.g. Methyl acetylene)
Examples:
CH3
CH3
C
CH3
CH
C C CH CH3
Common : Isobutyisopropylacetylene
Common : Methyl acetylene
H3C
CH CH2
CH3
C
C
Common: Isopropylmethylacetylene
Alkynes
Exercise
1)
Give the IUPAC and common names of the following
compounds
b)
a)
c)
12
C
C
H2
C
H2
C
d) HC
C
C- C(CH3)3
CH
Alkynes
Physical Properties
 Nonpolar, insoluble in water.
 Soluble in most organic solvents.
The boiling points and melting points of alkynes increase with
molecular weight.
 Branching reduces the boiling point 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)
a)
C
Terminal alkynes
c)
13
C
C
H2
C
H2
C
d) HC
C - C(CH3)3
Terminal alkynes
CH
Alkynes
Preparation of alkynes
1. By dehydrohalogenation of geminal or vicinal dihaloalkanes
(-2HX):
Br
KOH/ alcohol
Br
NaNH2
Br
heat
Strong base
C
C
- 2 HX
X
X
1,2-dihaloalkane
Excess NaNH2
Br
Br
14
- 2HBr
C
C
heat
Alkynes
2. From reaction of sodium Acetylide with Primary Alkyl
Halides (Alkylationof acetylide with primary alkyl halide)
R
C
+
CH
liq NH 3
Na
R C
-
C Na
+
+
1/2 H2
R'
+
Sodium acetylide
R C
-
C Na
+
+
For Terminal CC
R'
X
R
C
C
NaX
(use only Acetylene H-CC-H)
For Non Terminal CC (use Monosub. R-CC-H)
NaNH2/ NH3
CH3Br
Na
- NaBr
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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.
C
C
+
H2
Pd(BASO4) or (NiB)
Lindar’s Catlyst
H
H
Cis- alkene or (Z)
 Alkynes can be reduced to trans-alkenes using Na or Li in liquid NH3
H
C
C
+
H2
Na or Li / liq. NH3
H
Trans -alkene or (E)
H2/ Pd
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Alkynes
2. Addition of halogen (Halogenation)
HCCH + X2
HXC=CXH + X2
H XX C–CXXH
X
C
C
+
X2
(X= Br or Cl)
C
C
X
+
X
X2
X
C
C
X
X
Tetrahalide
Trans
If we used one mole ……(alkene) two mole …….alkane
Br
Br
Br
+ Br2
+ Br2
Br
Br
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Br
Alkynes
3. Addition of hydrogen halide
HC  CR + HX
HHHC – CXXR
HHC=CX R + HX
H
C
C
+
HX
(X= Br , Cl, I)
C
H
+
C
X
A haloalkene
HX
X
C
C
H
X
A gem dihalide
Br
+ HBr
+ HBr
Br
Br
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Alkynes
4. Addition of water: Hydration
H
C
+ HO-H
C
H
H2SO4 / HgSO4
keto-enol tautomerism
O
H
H
O
carbonyl more stable
an enol unstable
Specific example:
C
H C
H
H-OH/ H2SO4, HgSO4
H
H
C
Acetylene
H
C
H
O
H
H
Vinyl alcohol
(an unstable enol)
C
H
C
O
H
Acetaldehyde
(a stable carbonyl compound)
O
HC
C
CH3
H-OH/ H2SO4, HgSO4
C
H
Propyne
CH3
H
C
H3C
O
C
CH3
H
(an unstable enol)
Acetone
(a stable carbonyl compound)
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Alkynes
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
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Alkynes
Thank You for your kind attention !
Questions?
Comments
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