Saturday, September 1, 2007

Study Guide TMH JEE Ch.23 Alkynes

JEE 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.

Main Topics Covered in the TMH Book



From X class book

General formula: C(n)H(2n-2)
C two H two ---------Acetylene------------------Ethyne
C three H four------ Methyl acetylene-----------Propyne
C four H six *------ ethyl acetylene------------1-Butyne
--------------------------------------(Triple bond is the
--------------------------------------first carbon bond)
C four H six*--------Dimethyl acetylene---2-Butyne
---------------------------------------(Triple bond is the
----------------------------------------2nd carbon bond)

Preparation of Ethyne:

1. Calcium Carbide + Water = Ethyne + Calcium Hydroxide

(Cold water is added dropwise through a thistle funnel into conical flask containing calcium carbide at room temperature)

2. By dehydrohalogenation reaction

1,2 dibromoethane + KOH(alcoholic) = Ethyne + KBR + water (equation not balanced)

3. Methane on thermal decomposition by pyrolysis gives Ethyne + Hydrogen at around 1500 degree Celsius in an electric arc.
Methods of Preparation of Alkynes; From TMH Book

1. Dehydrohalogenation of vicinal dihalides (came in X class book).

What are vicinal dihalides?

term used to describe the location of two identical chemical groups or atoms which are bonded to adjacent carbon atoms.

The process takes place in two stages.

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

what is vinylic halide?

A vinyl halide in chemistry is any alkene with at least one halide substituent bonded directly on one of the unsaturated carbons.

This first stage requires mild conditions.

second stage: Vinyl halide is unreactive and hence vigorous conditions are required and a strong base NaNH two 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

what is a dicarboxylic acid?

Dicarboxylic acids are organic compounds that are substituted with two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R is usually an alkyl, alkenyl, or akynyl group. Dicarboxylic acids can be used to prepare copolymers such as nylon and polyethylene terephthalate.

In general, dicarboxylic acids show the same chemical behaviour and reactivity as monocarboxylic acids. The ionization of the second carboxyl group occurs less readily than the first one. This is because more energy is required to separate a positive hydrogen ion from the doubly charged anion than from the single charged anion.

A mnemonic to aid in remembering the order of the common nomenclature for the first six dicarboxylic acids is "Oh my, such great apple pie!" (oxalic, malonic, succinic, glutaric, adipic, pimelic).

When one of the carboxy groups is replaced with an aldehyde group, the resulting structure is called a "aldehydic acid".

5. Dehalogenation of vic-tetrahalogen compounds

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

From the Book by Madan and Bisht: 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


From Jauhar's book

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.
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.

M.P. and B.P.

Simple questions on methods of preparing alkynes are given in practice questions for review of concepts of this topic.

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