ALKYNES
• Hydrocarbons containing a carbon-carbon
triple bond are called alkynes.
• The simplest alkyne is acetylene (H-C = C-H
as by its common name. The names of
alkynes generally follow the IUPAC system.
• The longest chain containing the C = C bond
is selected as the base structure. It’s name is
derived from the analogous alkane but ‘yne’
replaces ‘ane’.
• The carbons are numbered from the end
nearest to the C = C bond.
• The general formula is Cn
H
.
2
n
2
3. Hydrocarbon, halogen and other groups
are named as substituents with the number
of the carbon to which the substituent is
attached.
INDUSTRIAL USES OF ALKYNES
• (i) Large quantities of ethyne are
consumed every year. Dissolved under
pressure in acetone, it is sold to be used
as fuel in oxyacetylene torch.
• (ii)It is the organic starting material for the
large scale synthesis of important organic
compounds e.g. CH3
C
O
O
H
.
PREPARATION OF ALKYNES
A carbon-carbon triple bond is formed in the same
way as a double bond, i.e. elimination of atoms or
groups from two adjacent carbons.
Examples:
(a) Dehydrohalogenation of alkyl dihalides
The dihalides are readily obtained from
corresponding alkenes by addition of halogen. This
amounts to conversion – by several steps – of a
double bond into a triple bond.
INDUSTRIAL SOURCES OF
Ethyne is obtainedALKYNES
in large quantities from coke,
the solid residue from the distillation of volatile
compounds of coal. Coke is heated with calcium
oxide CaO at 2000 oC to give calcium carbide and
carbon monoxide. Calcium carbide and water react
to give Ethyne and CaO.
(b) Reaction of Sodium acetylides with
primary alkyl halides
This permits the conversion of smaller alkynes
into larger ones. Practically, the reaction is limited
to the use of primary halides because of the great
tendency for secondary and tertiary halides to
undergo a side reaction.
(c) Dehalogenation of Tetrahalides
REACTION OF ALKYNES
• Just like the alkenes, the chemistry of
alkynes is the chemistry of the carbon
carbon triple bond. Like alkenes, alkynes
undergo electrophilic addition.
• The same reagents will add to triple bonds
as to the double bonds with some
variations in conditions. The reactions of
alkynes can usually be stopped after the
first stage of addition to give alkene
products.
(a) Catalytic Hydrogenation
Catalysts for hydrogenation are same metals as
for alkene, Pt or Pd. The two hydrogens add
from the same side of the triple bond to give Z
alkene, whereas with, Na, NH3
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(b) Addition of Hydrogen Halide
The direction of the addition of HCl to alkyne follows
the Markovnikov’s rule. The carbocation
intermediate formed is the more stable one i.e.
(c) Addition of Water
Hydration of alkynes requires catalysis by both H2
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The addition gives first an unstable enol which quickly
rearranges to a more stable aldehyde or ketone. The addition
of water follows the Markovnikov’s rule. e.g.
(d) Addition of Halogens
Bromine and chlorine add to alkynes to
form dihaloalkenes and tetra haloalkanes
ACIDITY OF ALKYNES
However, disubstituted alkynes which have no replaceable
hydrogen atom do not form such salts. Sodium reacts with
ethylene to liberate hydrogen gas.
i.e.
Sodium acetylides are often used in the synthesis of
higher alkynes
SHORT QUIZ ON CHEMISTRY OF ALKYNES