IIT JEE Revision Addition reactions of alkynes

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.

I. Addition reactions (specially given in jee syllabus)

1. Addition of Hydrogen

If the triple bond is not present at the end of the chain of the molecule (it is not a terminal alkyne), its reduction (addition of hydrogen) produces either a cis alkene or a trans alkene depending upon the choice of reducing agent.

2. Addition of halogens

chlorine and bromine add on alkali

3. Addition of hydrogen halides

This addition takes place in accordance with Markonikov's rule(do you remember the rule?).
Peroxides have the same effect on addition of the HBr to acetylenes (alkynes) as they have on alkenes (do you remember the effect?).

4. Addition of water (hydration of alkynes) (specially given in jee syllabus)
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).

Examples:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.

In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.

The conversion of enol form into keto form is termed tautomerism



5. Addition of hypohalous acid (HOX)

Alkynes react with two molecules of hypohalous acids in two stages.
For example take ethyne or acytelene and HOCl.
In the first stage HO gets added to one carbon and Cl adds to the other carbon.
In the second stage one more HOCl gets added to the intermediate product which has a double bond. The addition now follows markonikov's rule. OH gets added to HC-OH and Cl gets added to CH-Cl. Two OHs create instability and H2O molecule gets removed.
An aldehyde 2,2-Dichloroethanal (Dichloroacetaldehyde) is formed.

6. Addition of H2SO4
Acetylene adds two molecules of concentrated H2SO4 in two stages and forms ethylidene hydrogen sulphate as the final product.

In the first stage Vinylhydrogen sulphate H2C=CH-OSO3H is formed. (H gets added to one CH and OSO3H gets added to the other CH).
The addition of second molecule follows markownikov's rule. H gets added to CH2 and OSO3 gets added to the other carbon. Thus two functional groups OSO3H gets added to one carbon.
The final product is Ethylidine hydrogen sulphate.