Alkynes
.1
Introduction
.2
Nomenclature of Alkynes
.3
Physical Properties of Alkynes
.4
Preparation of Alkynes
.5
Reactions of Alkynes
Structure Of Alkynes
They are unsaturated hydrocarbons – made up of C
and H atoms and contain one or more C ≡ C triple bond
somewhere in their structures.
Their general formula is CnH2n-2.
The alkyne triple bond is composed of one σ and two 2 
covalent bonds, the triple bond can be terminal or internal.
Terminal alkynes have the triple bond at the end of the
carbon chain so that a hydrogen atom is directly bonded to
a carbon atom of the triple bond.
Internal alkynes have a carbon atom bonded to each
carbon atom of the triple bond.
2
Structure of Alkynes
Acetylene (ethyne):
HC
CH
sp Hybrid orbitals
Ground state
2p
Exited state
sp-hybridized
state
2p
2p
2s
2s
1s
Promotion
of electron 1s
sp
Hybridization
1s
The shape of an sp hybrid obital: Each sp hybrid orbital:
50% s character
50% p character
An sp-hybridized carbon atom:
180°
 Each carbon is sp hybridized with a linear
geometry and bond angles of 1800, perpendicular to
the two remaining p orbitals.
 Geometric structure of sp-hybridized C atoms is linear.
 In the mole. of acetylene, the formation of C-Cσbond : spsp overlap;
The formation C-H σ bonds: sp-1s overlap.
The formation of two C-C π bonds: 2py-2py overlap and 2pz-2pz
overlap.
Nomenclature of Alkynes
Nomenclature of Alkenes
1. In the IUPAC system, change the –ane ending of the parent
alkane name to the suffix –yne.
2. Determine the stem name by selecting the longest possible
straight chain containing the C ≡ C triple bond and number
the chain to give the triple bond the lower number.
3. Compounds with two triple bonds are named as diynes, those
with three are named as triynes and so forth.
3. Designate the position of the C ≡ C triple bond by
using the number of the first atom of the double bond
Nomenclature of Alkynes
Examples:
CH3
CH3
C CH
CH3
CH CH2
Br
CH3
C C CH CH3
IUPAC :2,6-Dimethyl-3-heptyne
Common :Isobutylisopropylacetylene
HC
CH2
5-bromo-2-pentyne
Propyne
CH3
C C CH2
Pentyne
CCH2CH3
1-Butyne
6-Ethyl-4-nonyne
7
Nomenclature of Alkynes
4. 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.
 5. In case where double and triple bonds would have the same
position number, the double bond takes the lower number.
6. Compounds with both a double and triple bond are named as
enynes ( ‘’ene’’ comes before ‘’yne’’).
 7. The simplest alkyne, H-CC-H, named in the IUPAC system as
ethyne, is more often called acetylene, its common name.
8. The two-carbon alkyl group derived from acetylene is called an
ethynyl group.
CH3CH CH C
3-Penten-1-yne
CH
HC
CCH2CH
CH2
1-penten-4-yne
1 2
F
4 CH
F
3
HC
1
2
C
4
7
CH
3 4 5
9
6
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
1
CH CH2
Pent-1-en-4-yne
double and triple bonds have have the same position number
thus ene take lower number
Hept-2-en-5-yne
or
2-Hepten-5-yne
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
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:
CH3
C
CH3
CH
CH3
Common : Methyl acetylene
H3C
C
CH CH2
CH3
C C CH CH3
Common : Isobutyisopropylacetylene
C
Common: Isopropylmethylacetylene
Exercise
1. Give the IUPAC and common names of the
following compounds:
b)
a)
c)
C
C
H2
C
H2
C
d) HC
C
CH
C- C(CH3)3
11
Physical Properties of Alkynes
Alkynes are non polar compounds.
Insoluble in water.
Soluble in non polar organic solvents.
They are less dense than water.
Alkynes have low melting points and boiling points.
Melting point and boiling point increase as the
number of carbons increases.
 Terminal alkynes, R-CC-H, are more acidic than
other hydrocarbons.
12
Preparation of Alkynes
Elimination Reactions
1. Dehydrohalogenation of dihaloalkanes (-2HX)
• Dehydrohalogenation is the elimination of (-2HX) from a
dihaloalkane in presence strong base
Br
H
H
H
Br
KOH / alcohol
Heat
Br
NaNH2 / NH3
Heat
Preparation of Alkynes
2. From reaction of sodium Acetylide with Primary Alkyl
Halides
R
C
+
CH
liq NH 3
Na
R C
-
C Na
+
+
1/2 H2
R'
+
Sodium acetylide
R C
-
C Na
+
+
R'
X
R
C
C
NaX
Reactions of Alkenes
Addition Reactions
Like alkenes, alkynes undergo addition reactions because they
contain relatively weak  bonds.
Two sequential reactions can take place: addition of one equivalent
of reagent forms an alkene, which can then add a second equivalent
of reagent to yield a product having four new bonds.
 Reactivity: Alkynes < Alkenes
15
Reactions of Alkenes
1. Addition of hydrogen ( Hydrogenation)
 Alkynes can be partially reduced to cis-alkenes with H2 in the
presence of poisoned catalysts.
Lindlar catalyst: Pd/ CaCO3, Pb(Ac)2-quinoline
C
C
+ H2
Pd(BASO4) or (NiB)
H
H
Cis- alkene
CH3(CH2)3C
CH3(CH2)3
H2
C
C(CH2)3CH3 Lindlar
H
catalyst
16
(CH2)3CH3
C
H
 Alkynes can be reduced to trans-alkenes using Na or Li in liquid
NH3 (meaning Metal-Ammonia )
H
C
C
+ H2
Na or Li / liq. NH3
H
Trans -alkene
CH3(CH2)3C
17
CH3(CH2)3
H
Li or Na
C C
C(CH2)3CH3 Liq. NH
3
H
78% (CH2)3CH3
Reactions of Alkenes
2. Addition of halogen
3. Addition of hydrogen halide
Markovnikov’s rule applies to the addition of HX to vinyl halides
X
R'C
CH
HX
R'C
CH2
X
Excess HX
Alkenyl halide
HX
R'C
CH3
X
Alkyl dihalide
18
4. Addition of water: Hydration
Keto-enol tautomerism
Transformation of Functional groups:
O
C C
C
H(R)
Hydroboration—Oxidation
 Hydroboration—oxidation is a two step reaction sequence that
converts an alkyne to a carbonyl compound.
21
Hydroboration—oxidation of an internal alkyne forms a ketone.
Hydroboration –oxidation of a terminal alkyne form an aldehyde.
22
Reactions of Alkenes
Examples:
Four addition reactions of 1-butyne
23
Exercise
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) 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