Alkynes- Definition- Preparation- properties

1. Dr.P.GOVINDARAJ
Associate Professor & Head , Department of Chemistry
SAIVA BHANU KSHATRIYA COLLEGE
ARUPPUKOTTAI - 626101
Virudhunagar District, Tamil Nadu, India
UNSATURATED HYDROCARBONS: ALKYNES

2. UNSATURATED HYDROCARBONS: ALKYNES
Definition
The unsaturated hydrocarbons having the general formula CnH2n-2 and containing
one carbon – carbon triple bond are called alkynes.
Example :
FORMULA COMMON NAME IUPAC NAME
HC≡CH Acetylene Ethyne
CH3−C≡CH Allylene Propyne
CH3- C≡C-CH3 Dimethyl acetylene 2-Butyne

3. Preparation of Alkynes
i. Acetylene from calcium carbide
• Calcium carbide are placed on a layer of sand in a flask fitted with a dropping funnel and a
delivery tube shown in the diagram
UNSATURATED HYDROCARBONS: ALKYNES

4. UNSATURATED HYDROCARBONS: ALKYNES
• Water is dropped from the dropping funnel and acetylene is formed
• The evolved acetylene is purified by pubbling it through acidified copper sulphate and
collected over water
CaC2 + 2H2O Ca(OH)2 + HC≡CH
ii. Dehalogenation of tetra halides
On dehalogenation of tetra halides by zinc leads to give alkynes

5. iii) Dehydrohalogenation of vicinal dihalides
On dehydrohalogenation of vicinal dihalides with sodium amide(NaNH2)
gives alkynes
UNSATURATED HYDROCARBONS: ALKYNES

6. Chemical properties of Alkynes
i) Formation of acetylides
• Acetylene forms metallic derivatives called acetylide by replacement of one or both
hydrogen atoms with sodium metal. This is due to the acidic nature of its hydrogen atoms
UNSATURATED HYDROCARBONS: ALKYNES

7. ii) Ozonolysis
• Ozonolysis of alkynes form diketones which are further oxidized by hydrogen peroxide
(H2O2) leads to give acids
UNSATURATED HYDROCARBONS: ALKYNES
• In general

8. iii) Oxidation reactions
Alkynes are oxidized into acids with oxidizing agents
UNSATURATED HYDROCARBONS: ALKYNES
Examples
b. Acetylene oxidized with alkaline KMnO4 gives oxalic acid
a. Acetylene oxidized with chromic acid(H2CrO4) gives acetic acid

9. iv. Addition of bromine
Liquid bromine adds on to alkynes to give tetra bromide where as bromine water reacts
with alkynes gives dibromides
UNSATURATED HYDROCARBONS: ALKYNES
c. Acetylene oxidized by ozone followed by H2O2 gives formic acid

10. THANK YOU