1436-1437
2015-2016
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 simplest alkyne, ethyne (also known as acetylene), has two
carbon atoms and the molecular formula of C2H2. The structural
formula for ethyne is:
Ethyne or acetylene
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Alkynes
The Hybridization of Alkynes.
For the simplest alkyne, Ethyne,or Actelyne
2s2
1s2
Ground state
2py1 2pz1
2px1 2py1 2pz1
Energy
Energy
2px1 2py1 2pz 0
H C C H
2s1
Energy
Example
sp
sp
1s2
1s2
Excited state
sp-hybridized state4
Alkynes
The hybridization of a 2s orbital and 2p orbital to form two linear sp hybridized
orbitals with bond angle 180° and two unhybridized p orbitals perpendiculy
oriented to each other and to the plan of the hybrid sp orbitals.
2s
2py
2px
2pz
Hybridization
The sp orbitals have
½ s character & ½ p character
180°
2pz
sp
5
2py
sp
Alkynes
a) Bond Formation in Acetylene
σ
pz
pz
C
H
sp
sp
C
Py
H
Py
sp-sp
Py-py, and pz-pz
The C(triple bond)C
bond is composed of
one sigma bond and
two pi bonds.
π
H
C
C
H
π
Head-to-head overlap between two sp-hybrid orbitals to form σ bond between
2 carbons of acetylene and side-side overlap of the py orbitals and pz to form two
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π bonds.
Alkynes
7
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 (π) bonds.
Alkynes
The comparative chart of bond length of alkane, alkene and alkyne is given below:
Class
Alkane
Bond
Hybridized bond
orbital
Bond
Length
C C
sp3 sp3
1.54 Å
C H
sp3 1s
1.112 Å
Structure
Tetrahydral
Alkene
C C
sp2 sp2
C H
sp2 1s
1.34 Å
1.108 Å
Trigonal
planar
Alkyne
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C C
sp sp
C H
sp 1s
1. 20 Å
1. 08 Å
Linear
Alkynes
Alkyne
Nomenclature
A. Common
Nomenclature
B. IUPAC Naming
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Alkynes
Common Nomenclature of Alkynes
 The simplest alkyne its common name is acetylene. HC CH
 The alkynes are sometimes referred to as the acetylene series or
derived from acetylene.
H C C H
Acetylene or Ethyne
Methylacetylene
propyne
Ethylacetylene
1-butyne
Dimethylacetylene
2-butyne
Ethylmethylacetylene
2-pentyne
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Alkynes
Give the Common Nome for the following Alkynes.
CH3
C
CH3
CH
Methyl acetylene
CH3
CH CH2
CH3
C C CH CH3
Isobutylisopropylacetylene
H3C C C
HC C C(CH3)3
Isopropylmethylacetylene
H2 H2
C C C C
C CH
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Alkynes
IUPAC Nomenclature of Alkynes
 Find the longest chain that contains the triple bond, then apply with “-yne”
as a suffix indicating an alkyne.
 Number the chain, starting at the end closest to the triple bond.
 Give branches or other substituent names and numbers to locate their positions.
1
2
7
3
9
6
4
5
8
6-Ethyl-4-nonyne
1
H3C
2
C
3
C
4
Br
5
5-Bromo-2-pentyne
Not
1-Bromo-3-pentyne
 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.
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Alkynes
Remember That
In alphabetizing ignore prefixes like
tert., di, tri, etc. BUT iso and cyclo
are considered.
F
3 2 1
4CH HC C CH
F
CH3
4,4-Difluoro-3-methyl-1-butyne
Q: If double and triple are exist in the same molecule ?
If both a double bond and a triple bond are present, the compound is
an alkenyne .
Note that,
An “e” is dropped from “ene” as resion of being followed by vowel “y” in “ yne”.
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.
H
(triple bond closer to the end of chain)
6
1
2 3 4 5
Note: An''e'' is dropped from ''ene'' due to
HC C C CH-CH2-CH3
Hex-3-en-1-yne
it is followed by a vowel
Alkynes
IUPAC Nomenclature of Alkynes
 In case where double and triple bonds would have the same position
number, the double bond takes the lower number.
5
4
3
2
1
HC C CH2 CH CH2
1
5
4
3
3
4
5
HC C CH2 CH CH2
Pent-4-en-1-yne
Pent-1-en-4-yne
2
2
Not
1
1
2
4
5
3
Double and triple bonds have the same number for either ends , thus -ene take
lower number
Alkynes
Physical Properties
 Non-polar, 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.
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Alkynes
Terminal and Internal Alkynes
1- Terminal alkynes
Have a hydrogen atom bonded to at least one of the sp hybridized
carbons (those involved in the triple bond. An example would be
methylacetylene (1-propyne using IUPAC nomenclature).
1-propyne
CH3C
CH
2- Internal alkynes
Have something other than hydrogen attached to the sp hybridized
carbons, usually another carbon atom, but could be a heteroatom. A good
example is 2-pentyne, in which there is a methyl group on one side of the
triple bond and an ethyl group on the other side.
CH3C
2-pentyne
CCH2CH3
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Alkynes
Classify of the following compounds as terminal or internal alkynes.
CH3CH2CH2
C
C H
Terminal Alkyne
CH3CH2CH2
C
C CH2CH3
Internal Alkyne
H C C C(CH3)3
Terminal Alkyne
H2 H2
C C C C
Internal Alkyne
C C H
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Alkynes
The Acidity of terminal Alkynes
Acetylene and all terminal alkynes are acidic in nature i.e. H atom
attached on triple bonded C atom behaves as acidic.
The acidity of a terminal alkyne means the ability for simple
hydrocarbon to be deprotonated.
R C C H
B-
R C C- + B H
 The alkyne triple bond is composed of one σ and two 2  covalent bonds,
the triple bond can be terminal or internal.
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Alkynes
Methods of preparing Alkynes
Dehydrohalogenation
From vicinal
dihalide
20
From geminal
dihalide
Alkylation of acetylide
with primary alkyl
halide
Alkynes
1.
By dehydrohalogenation of geminal or vicinal dihaloalkanes (-2HX):
1.1 From Vicinal dihalides
By treating vicinal dihalides with strong base (alcoholic KOH) , followed by
sodium amide (NaNH2), results in the formation of Alkyne.
The General Reaction
H H
C C
H
KOH (Alcoholic)
C C
NaNH2
C C
X
X X
A vinylic halide
Very unreactive
Examples Br
Br
KOH/ alcohol
heat
1,2-Dibromobutane
Br
NaNH2
heat
1-butyne
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Alkynes
1.
By dehydrohalogenation of geminal or vicinal dihaloalkanes (-2HX):
1.2 From Geminal Dihalides
Alkynes are also prepared by double dehydrohalogenation of geminal
dihalides
The General Reaction
X
C C
X
KOH/ alcohol
heat
C C
X
NaNH2
heat
C C
Example
Br
Br
KOH/ alcohol
heat
Br
NaNH2
heat
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Alkynes
2. From reaction of sodium Acetylide with Primary Alkyl
Halides (Alkylation of acetylide with primary alkyl halide)
 The acidic acetylenic hydrogens in acetylene or 1-alkynes can be replaced by
certain metals to form salts Known as; metal acetylides.
The reaction takes place with sodium; Na in liquid ammonia; NH3 or sodium amide
(NaNH2) to form sodium acetylides.
Sodium acetylides react with primary alkyl halides to yield higher alkynes.
The General Reaction
H C C H
R C C H
Example
CH3C
Na
NH3
Na
NH3
CH + Na
H C C- Na+ R' X
H C C R'
R C C- Na+ R' X
R C C R'
liq NH3
CH3C
CH3C
C
Na+ +
CH3CH2Br
CCH2CH3 + NaBr
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Alkynes
Reactions of Alkynes.
Alkynes undergo reactions that are similar to those of alkenes, except
that they are capable of adding two molecules of a reagent for
each triple bond present.
X Y
General Reaction
X Y
C C
X Y
C C
C C
X
Y
X Y
Alkene (Cis or Trans)
Alkane
Addition Reaction for Alkynes
1
Addition of Hydrogen
2
Addition of Halogen
3
Addition of Hydrogen halide
4
Addition of water: Hydration
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Alkynes
Reactions of Alkynes: Electrophilic Addition
Hydrogenation of alkynes can be partially or complete reduction depending on
the reagent used as follow:
1
1.Complete Addition
Addition of Hydrogen
Controlled Addition
2.Cis-Alkene
Case 1: complete Addition
Alkyne may add hydrogen in the
presence of suitable catalyst (pd, Pt,or
Ni), and the final product is always an
alkane.
Controlled Addition
3.Trans-Alkene
Pt
Alkyne
Alkane25
Alkynes
Reactions of Alkynes: Electrophilic Addition
Addition of Hydrogen
1
Case 2-1: partially or controlled Addition (cis-product)
Hydrogenation can be stopped at the intermediate alkene stage by the
use of modified catalyst such as Lindlar Catalyst. As a result of this
method cis alkene is affords.
C C
+
H2
Lindlar's Catalyst
Pd(BaSO4)
R
or NiB
H
R
C
C
H
Cis-alkene
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Alkynes
Reactions of Alkynes: Electrophilic Addition
Addition of Hydrogen
1
Case 2-2: partially or controlled Addition (trans-product)
When we use Na; Sodium or Li; Lithium metal dissolved in NH3;
liquid ammonia , we obtain trans alkene as product.
C C
+
H2
Na or Li
in Liquid NH3
R
H
C
H
C
R
Trans-alkene
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Alkynes
Reactions of Alkynes: Electrophilic Addition
Examples
Uncontrolled addition
2H2 / Pt
n-Butane
Lindlar’s Catalyst
Pd (BaSO4)
2-Butyne
Or NiB
Cis-2-butene
Controlled Addition
Na or Li
in NH3
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Trans-2-butene
Alkynes
Reactions of Alkynes: Electrophilic Addition
Addition of Halogen
2
Alkynes react with halogens to yield tetrahaloalkanes. Two molecules of
the halogen (chlorine or bromine) add to the triple bond as follows:
The General Reaction
X
X X
X2
X2
C C
C C
C C
CH2Cl2 or CCl4
CH2Cl2 or CCl4
X
Example
X X
tetrahaloalkane
dihaloalkene
Cl
Cl Cl
Cl2
Cl2
CH2Cl2 or CCl4
CH2Cl2 or CCl4
Cl
!
Cl Cl
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Initial addition of halogen X2 usually gives trans dihalide intermediate.
Alkynes
Reactions of Alkynes: Electrophilic Addition
3
Addition of Hydrogen halide
The addition of hydrogen chloride HCl, bromide HBr, or iodide HI to
alkynes follows Markovnikov’s rule.
If two moles of HX is added:
 First halogen follows alkene hydrohalogenation regioselectivity rules,
i.e; obeys Markovnikov’s rule.
 Second halogen goes on same carbon as first halogen; geminal dihalide.
The General Reaction
H Br
H
C C
HBr
HBr
C C
Br
Halo alkene
C C
H Br
geminal dihalide
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Alkynes
Reactions of Alkynes: Electrophilic Addition
Addition of Water : Hydration
4
Water adds to alkynes in the presence of dilute sulfuric acid
and murcuric sulfate catalyst. to yield an enol. However the
initially formed enol reacts further to produce a ketone.
General equation
R
C
C
R + H2O
Enol form
Less stable
HgSO4/ H2SO4
Keto form
More stable
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Alkynes
Reactions of Alkynes: Electrophilic Addition
Specific example:
H
C
C
H
Acetylene
H
H
H-OH
/ H2SO4, HgSO4
C
H
C
H
O
H
H
Vinyl alcohol
(an unstable enol)
H
C
H
C
O
Acetaldehyde
(a stable carbonyl compound)
O
C
HC
CH3
H-OH
/ H2SO4, HgSO4
C
H
Propyne
!
CH3
H
C
H3C
O
(an unstable enol)
C
CH3
H
Acetone
(a stable carbonyl compound)
To hydrate an alkyne a mercury catalyst is added
(in contrast to alkene reactions when acidic water alone is sufficient)
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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) Free radical substitution
(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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