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 -