CHE143
ORGANIC AND
ALKYNES
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ANALYTICAL CHEMISTRY
Structure and nomenclature
Physical properties
Synthesis
Reactions
P r e p a r e d b y:
Siti Munirah Muda
STRUCTURE AND NOMENCLATURE
• Alkynes contain a carbon—carbon triple bond.
• 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.
• An alkyne has the general molecular formula C n H 2n-2 , giving it
four fewer hydrogens than the maximum possible for the
number of carbons present. Thus, the triple bond introduces
two degrees of unsaturation.
• Recall that the triple bond consists of 2  bonds and 1  bond.
• Each carbon is sp hybridized with a linear geometry and bond angles of
1800.
IUPAC NOMENCLATURE OF ALKYNES
1.
2.
3.
Find the longest chain containing the triple bond and change
the corresponding “-ane” ending to “-yne”
The chain is numbered in direction that gives the triple bond,
the lower number
Give the location and name of each substituent (alphabetical
order) as a prefix to the name of the main chain.
NOMENCLATURE OF ALKYNES
STRUCTURES
HC
Examples
C
CH3
COMMON NAMES
IUPAC
Acetylene
ethyne
methyl acetylene
propyne
dimethyl acetylene
2-butyne
ethyl acetylene
1-butyne
Give the IUPAC name for each of the
following
PROBLEM: IUPAC NAMES?
Draw the structure for each of the
following
a)
b)
c)
d)
e)
f)
g)
1-chloro-3-hexyne
4-bromo-2-pentyne
4,4-dimethyl-1-pentyne
3-chloropropyne
1,6-heptadiyne
6,6-dimethyl-3-octyne
5-cycloheptyne
PHYSICAL PROPERTIES
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Quite similar to those of alkanes and alkenes with similar carbon skeletons.
All are insoluble in water and soluble in nonpolar solvents such as hexane.
Less dense than water
Alkynes are more linear than alkenes, causing alkynes to have stronger van der
Waals interaction. As a result, alkyne has a higher BP than alkene containing the
same number of carbons.
Physical proper ties of some low molecular weight alkynes
SYNTHESIS OF ALKYNES
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ALKYLATION OF ACETYLENE
DOUBLE DEHYDROHALOGENATION
OF ALKYL HALIDE
DEHYDROGENATION
SYNTHESIS OF ALKYNES
1.
ALKYLATION OF ACET YLENE
General
equation
 Alkylation of acetylene involves a sequence of two separate operations.
 An alkyl halide is added to the solution of sodium acetylide and form an
alkynes.
SYNTHESIS OF ALKYNES
Examples:-
PRACTICE
 Outline efficient synthesis of each of the following
alkynes from acetylene and any necessary organic or
inorganic reagents
 a) 1-heptyne
 b) 2-heptyne
 c) 3-heptyne
SYNTHESIS OF ALKYNES
2) DOUBLE DEHYDROHALOGENATION OF ALKYL HALIDE

Prepare an alkyne by elimination reactions.
Double dehydrohalogenation of vicinal dihalide
SYNTHESIS OF ALKYNES
 Examples:-
SYNTHESIS OF ALKYNES
3) DEHYDROGENATION
 Multiple bonds are form by removing a hydrogen chemically.
REACTION OF ALKYNES
1) HYDROHALOGENATION
(ADDITION OF HYDROGEN
HALIDE)
2) HYDROGENATION
(ADDITION OF HYDROGEN)
3) HALOGENATION (ADDITION
OF HALOGEN)
4) HYDRATION (ADDITION OF
WATER)
HYDROHALOGENATION
(ADDITION OF HYDROGEN HALIDE)
General
equation
 Alkynes react with one molar equivalent of hydrogen chloride or
hydrogen bromide to form haloalkenes, and with two molar equivalents
to form geminal dihalides.
 Both addition are regioselective and follow Markovnikov’s rule.
Example:
Mechanism
Major
product
Example
Minor product
Practice
Major product
HYDROHALOGENATION
(ADDITION OF HYDROGEN HALIDE) -- cont
 Anti Markovnikov addition of hydrogen bromide to alkynes
occurs when peroxides are present in the reaction mixture.
These reaction take place through a free -radical mechanism
HYDROGENATION
(ADDITION OF HYDROGEN)
General reaction
 Conditions for hydrogenation of alkynes are similar to those for alkenes.it
also known as reduction reaction of alkynes.
 In the presence of finely divided platinum, palladium, nickel or rhodium, two
molar equivalents of hydrogen add to the triple bond an alkyne to yield an
alkane.
Example:-
HYDROGENATION
(ADDITION OF HYDROGEN) -- cont
 Hydrogenation can be stopped at the alkene if the less active Lindlar
catalyst used.
 The Lindlar catalyst is a finely divided Pd metal that has been
precipitated onto a CaCO 3 support and then deactivated by treatment
with lead acetate and quinoline, an aromatic amine .
 The hydrogenation occurs with syn stereochemistr y, giving a cis alkene
product .
General
equation
Example
Lindlar
catalyst
HYDROGENATION
(ADDITION OF HYDROGEN) -- cont
Examples:
HALOGENATION
(ADDITION OF HALOGEN)
 Alkynes show the same kind of addition reactions with Cl and Br
that alkenes do.
 With alkynes, the addition may occur once or twice, depending on
the number of molar equivalents of halogen we used.
HYDRATION
(ADDITION OF WATER)
 Hydration of alkyne is expected to yield an alcohol.
 The alcohol would be a special kind, which hydroxyl group is a
substituent on a carbon-carbon double bond.
 This type of alcohol is called an enol
 An important property of enol is their rapid isomerization to aldehydes
or ketones under the conditions of their formation.
HYDRATION
(ADDITION OF WATER) -- cont
EXERCISE