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Alkynes
C ≡ C
Alkynes are……..•Aliphatic hydrocarbons with one
triple bond between carbons•unsaturated hydrocarbons and
are generally very reactive. Typical reactions involve the addition of hydrogens or halogens.
• traditionally known as acetylenes or the acetylene series
General Formula
CnH2n-2
Alkynes PreparationThe carbon-carbon triple bond of the alkynes is formed in the same
way as a double bond of the alkenes, by the elimination of
atoms or groups from two adjacent carbons. CH3 — CH3 -------→ CH2 ═ CH2 -------→ CH ≡ CH
Alkane Alkene Alkyne
Alkynes Physical Properties• They are insoluble in water. • They are quite soluble in the usual organic
solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.).
• They are less dense than water. • Their boiling points show the usual increase
with increasing carbon number. • They are very nearly the same as the boiling
points of alkanes or alkenes with the same carbon skeletons.
Structure:The carbon atoms in an
ALKYNE bond are “sp”
hybridized.
Example:
HC ≡ CHEthyne
Example:
CH3—C ≡ CH1 – Propyne
Example:
CH3 – C ≡ C – CH3
2 - Butyne
NomenclatureWhat is IUPAC Nomenclature?
IUPAC nomenclature is a system of naming chemical compounds and of describing the science of
chemistry in general. It is developed and kept up to date
under the auspices of the International Union of Pure and
Applied Chemistry (IUPAC).
How to name an alkyneStep # 1
Determine the “ longest continuous chain “of carbons that have the triple bond between two of its carbons. The "longest continuous chain“ does not necessarily have to be straight.
CH2 — C ≡ C — CH2
│ │
CH3CH2CH3
3 - heptyne
How to name an alkyne
Step # 2
Number the carbons in the chain so that the triple bond would be between the carbons with the lowest designated number. If it makes no difference to the triple bond then shift attention to the branched groups.
Example :
CH3 — CH — C ≡ C — C — CH3
│ │
│
CH3 CH3
CH3
2,2,5 – trimethyl – 3 – hexyne
How to name an alkyne
Step # 3Identify the various branching
groups attached to this continuous chain of carbons by name. If two or more alkyl group
are attached to the chain alphabetize, then drop the “ane”
suffix add “yne” to the name.
Example :
CH3 — CH2 — C — CH2 — C ≡ C — C — CH2 — CH3
│
│
│
│
CH3
CH2 — CH3
CH3 — CH2
CH3
3,7 – diethyl – 3,7 – dimethyl – 4 - nonyne
Chemical PropertiesUnlike alkanes, alkynes are unstable and
very reactive. This gives rise to the intense heat (>3000 °C) of the acetylene flame used in welding.
The chemical behavior of the alkynes is characterized by their capacity of giving addition reactions, polymerization reactions and metallic derivates forming reactions. A great part of reactions are known as belonging to acetylene.
Addition Reaction Is a type of chemical reaction in which a compound adds to a multiple bond. In the case of addition reaction take place in ALKYNES, one bond in the triple bond is broken in order to form new bonds.
Alkyne Addition Reaction
Combustion Halogenation Hydrogenation Hydrohalogenation Hydration Polymerization
Combustion ReactionHydrocarbon + O2 ---------------→ CO2 + H2O
Examples :2 C2H2 + 5 02 -------→ 4 CO2 + 2 H2O
C3H4 + 4O2 -------→ 3CO2 + 2H2O
2C4H6 + 11O2 -------→ 8CO2 + 6H2O
Halogenation Reaction
Alkyne + Halogens -------→ Haloalkene -------→ Haloalkane
Example:
CH ≡ CH + Cl2 ------→ CHCl ═ CHCl + Cl2 ------→CHCl2 — CHCl2
Ehyne 1,2 - dichloroethene 1,1,2,2 - tetrachloroethane
Example:
+ 2 Br2--------→
Br Br
Br
Br
1,1,2,2 – tetrabromocyclopentane
CH ≡ C — C — CH3 + 2F2 -------→ HC — C — C —CH3
│
│
CH3
CH3
│
│
│
│
F CH3
F CH3
│
│
F
F
1,1,2,2 – tetraflouro – 3,3 – dimethylbutane
Hydrogenation ReactionAlkyne + Hydrogen --------→ Alkene -------→ Alkane
Example :
CH ≡ CH + 2H2 -------→ CH2 ═ CH2 -------→ CH3 — CH3
Ethyne Ethene Ethane
Example: :
+ 2H2--------→
Cyclobutyne Cyclobutane
HydrohalogenationAlkyne + Hydrogen halides ------→ Haloalkene -------→ Haloalkane
Example:
CH3 — C ≡ CH + 2HCl --------→ CH3 — C — CH3
│
│
Cl
Cl
2, 2 - dichloropropane
HydrationAlkyne + H2O -------→ Aldehyde or Ketones
Example:
CH ≡ CH + H2 — O -------→ H — C — C — H
│
│
H
H
║
O
Ethanal / (Acetaldehyde)
Example:
CH3 — C ≡ CH + H2 — O -------→ CH3 — C — CH3
║O
2 – Propanone
(dimethylketone)
Example:
+ H2 — O -------→ ║
O
Cyclopentanone
- Fin -
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