Sunday, January 27, 2008

IIt JEE Revision - Alkynes - Preparation

1. Synthesis of carbon and hydrogen
A stream of hydrogen is passed through electric arc struck between carbon electrodes at 3270 K. Ethyne (acytylene) is obtained.

2. From acetylene (ethyne)
Higher alkynes are prepared from ethyne (acytylene) by treating its sodium salt with alkyl halide. Sodium salt of ethyne is prepared from the reaction of ethyne and sodamide (NaNH2).
Sodium salt obtained from ethyne and sodamide is Sodium acetylide (HC≡CNa).
If methyl bromide is added to it, propyne is obtained.
If ethyl bromde is added to it, But-1-yne is obtained.

As higher and higher alkyl bromide are added higher alkynes are obtained in this process.

3. Action of zinc on tetrahalogen derivatives of alkanes
On treatment with zinc, tetrahalides get dehalogated (eliminated from the molecule) and triple bonds forms and alkyne is obtained
1,1,2,2-tetrabromoethane + zinc --> ethyne +zinc bromide

4. Dehydrohalogenation of vicinal dihalides
Vicinal dihalides (having halogen atoms on the adjacent carbon atoms) get dehalogenated on treatment with alcoholic solution of potassium hydroxide.

5. By electrolysis of aqueous solution of potassium salt of fumaric acid
Fumaric acid is a dicarboxylic acid.
Electrolysis of acqueous solution Potassium fumerate gives ethyne

6. Action of water on calcium carbide

CaC2 + 2H2O gives ethyne and Calcium hydroxide

Calcium carbide required is obtained by heating calcium oxide (from limestone) and coke in an electric furnace at 2275 K.


1. Dehydrohalogenation of vicinal dihalides

The process takes place in two stages.

In stage I KOH(alc) reacts with vicinal dihalide and vinylic halide is formed.

second stage: Vinyl halide is unreactive and hence vigorous conditions are required and a strong base NaNH2 (sodamide) is used in the reaction to produce the corresponding alkyne.

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

Secondary and tertiary halides cannot be used because eliminatin is the predominant reaction which results in the formation of alkenes.

3. Hydrolysis of CaC two and Mg two C three.

Calcium carbide in reaction with water gives ethyne and calcium hydroxide.(came in X class)
Magnesium carbide in reaction water gives propyne and magnesium hydroxide.

4. Kolbe's Electrolytic Method:
The electrolysis of an acqueous solution of potassium salt of an unsaturated dicarboxylic acid forms alkyne

5. Dehalogenation of vic-tetrahalogen compounds

Tetrahalogenated alkane in reaction with zinc with ethyl alcohol as catalyst gives alkyne and zinc halide.

Preparation of Acetylene or Ethyne

1. By the action of Alcoholic Potash on Ethylene Bromide

BrH two C - CH two Br + 2KOH = C Two H Two + @KBr + 2 H two O

At the end of first state vinyl bromide is formed.

2. By heating tetra-bromoethane with Zinc

C-two H-two Br-four +2Zn = C-two H-two + 2ZnBr-two

3. By the electrolysis of Acqueous solution of Potassium salts of maleic acid (Kolbe's method)

4. By the action of water on calcium carbide (already covered earlier)

5. By heating Iodoform with silver powder

2HCI-three + 6Ag = C-twoH-two + 6AgI

6. By partial oxidation of Methane

2CH-four + 3[O] = C-twoHtwo + 3H-two O

7. By direct synthesis of carbon and hydrogen (Berthelot's synthesis)

2C + H-two = C-twoH-two


1. Action of water on calcium carbide
2. Dehydrohalogenation of vicinal dihalides
3. Action of zinc on tetrahalogen derivatives of alkanes
4. From acetylene
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
6. Synthesis of carbon and hydrogen

Industrial preparation
Ethyne is prepared on an industrial scale by treating calcium carbide with water.

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