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 Preparation The 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 — CH → CH2 ═ CH → 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 ≡ CH Ethyne

Example : CH 3 —C ≡ CH 1 – Propyne

Example: CH 3 – C ≡ C – CH Butyne

Nomenclature What 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 alkyne Step # 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. CH 2 — C ≡ C — CH 2 │ │ CH 3 CH 2 CH 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 : CH 3 — CH — C ≡ C — C — CH 3 ││ │ CH 3 2,2,5 – trimethyl – 3 – hexyne

How to name an alkyne Step # 3 Identify 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 : CH 3 — CH 2 — C — CH 2 — C ≡ C — C — CH 2 — CH 3 │ │ │ │ CH 3 CH 2 — CH 3 CH 3 — CH 2 CH 3 3,7 – diethyl – 3,7 – dimethyl – 4 - nonyne

Chemical Properties  Unlike 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 Reaction Hydrocarbon + O → CO 2 + H 2 O Examples : 2 C 2 H → 4 CO H 2 O C 3 H 4 + 4O → 3CO 2 + 2H 2 O 2C 4 H O → 8CO 2 + 6H 2 O

Halogenation Reaction Alkyne + Halogens → Haloalkene → Haloalkane Example: CH ≡ CH + Cl → CHCl ═ CHCl + Cl →CHCl 2 — CHCl 2 Ehyne 1,2 - dichloroethene 1,1,2,2 - tetrachloroethane

Example: + 2 Br → Br 1,1,2,2 – tetrabromocyclopentane CH ≡ C — C — CH 3 + 2F → HC — C — C —CH 3 │ │ CH 3 │ │ │ │ F F │ │ F F 1,1,2,2 – tetraflouro – 3,3 – dimethylbutane

Hydrogenation Reaction Alkyne + Hydrogen → Alkene → Alkane Example : CH ≡ CH + 2H → CH 2 ═ CH → CH 3 — CH 3 Ethyne EtheneEthane

Example: : + 2H → Cyclobutyne Cyclobutane

Hydrohalogenation Alkyne + Hydrogen halides → Haloalkene → Haloalkane Example: CH 3 — C ≡ CH + 2HCl → CH 3 — C — CH 3 │ │ Cl 2, 2 - dichloropropane

Hydration Alkyne + H 2 O → Aldehyde or Ketones Example: CH ≡ CH + H 2 — O → H — C — C — H │ │ H H ║ O Ethanal / (Acetaldehyde)

Example: CH 3 — C ≡ CH + H 2 — O → CH 3 — C — CH 3 ║ O 2 – Propanone (dimethylketone)

Example: +H 2 — O → ║ O Cyclopentanone

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