Ch.23 Alkynes - Core Points for Revision

syllabus

Preparation, properties and reactions of alkynes:

Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);

Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
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1. Alkynes are hydrocarbons with triple bonds. General formula C_nH_2n

2. Methods of Preparation of Alkynes;

1. Dehydrohalogenation of vicinal dihalides
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.

3. Physical properties

State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.

Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.

4. Chemical Properties

The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.

They readily take part in addition reactions and can also be easily oxidized.

5. Acidic property of acetylene

Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.


6. Addition of water (hydration of alkynes) (
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.

Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).

7. Reduction of Alkynes

Reaction Type: Addition


Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.

8. Preparation of alkynes by elimination reactions; To be posted

9. Formation of metal acetylides

Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.

Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.