Reaction of alkyne with HgSO4 and H2SO4

HgSO4 and aqueous dilute H2SO4 is used for alkyne hydration. Alkynes hydration is much harder than alkene hydration.

Reaction of alkyne with HgSO4 and H2SO4

Alkyne hydration

Carbonyl compounds are given as products. Carbonyl compounds are grouped as aldehydes are ketones. Only ethyne hydration will give the aldehyde, ethanal.


HgSO4 + H2SO4

Both HgSO4 and H2SO4 are used as catalysts and they increase the reaction rate.


Why Mercuric sulfate (HgSO4) is used with sulfuric acid?

In alkene hydration, only dilute H2SO4 is used. But due to slow reaction rate of alkynes and H2SO4, we have to use a catalyst. Therefore we use HgSO4 as the catalyst to increase the reaction rate.


Products of alkyne with HgSO4 and H2SO4 reaction

In case of unsymmetrical alkynes, addition of water occurs according to Morkovnikv's rule. Oxygen atom is added to one carbon atom (in the triple bond) and two hydrogen atoms are added to other carbon atom.


Two hydrogen atoms are added to the carbon which has more hydrogen atoms.

Oxygen atom is attached to the other carbon which has less hydrogen atoms forming a double bond.

Morkovnikvs rule to alkyne hydration

Alkyne hydration examples

Examples of hydrations of alkynes with dilute sulfuric acid and mercuric sulphate (HgSO4) are explained below.



Reacting mixture should be heated to 333K (600C).


Ethyne with HgSO4 and dilute H2SO4

Ethyne, dilute H2SO4 and HgSO4 react to give ethanal. This is the only reaction giving an aldehyde by the reaction of alkyne hydration. Ethyne is symmetrical and we do not need the Morkovnikv's rule to find the addition places.

ethyne hydration with HgSO4 and H2SO4

Propyne with HgSO4 and dilute H2SO4

Propyne, dilute H2SO4 and HgSO4 react to give propanone. Propanone is a ketone compound. Propanone is an unsymmetrical molecule and, has to apply Morkovnikv's to decide what are the products given by propyne.

propyne hydration with HgSO4 and H2SO4


2-butyne with HgSO4 and dilute H2SO4

2-butyne, dilute H2SO4 and HgSO4 react to give 2-butanone. 2-butyne hydration with HgSO4 and H2SO4

Alkyne hydration mechanism

First, water molecule is added to the triple bond. The intermediate product of this reaction is very unstable. Then triple bond is converted to double bond. Therefore, as soon as this intermediate compound converts into carbonyl compound (aldehyde or ketone). Hydration of ethyne will give ethanal which is an aldehyde compound. All other hydrations of alkynes will give ketones.



Propyne hydration gives propanone - mechanism

Alkyne hydration mechanism



Questions of hydrolysis and alkyne

Following questions were asked by students in our ask question forum. We only made slight changes to the questions to correct grammer.



Can I identify ethene from ethyne from reacting with HgSO4 and H2SO4?

Ethene is an alkene and ethyne is an alkyne. Both compounds can be hydrolyzed. But difference is, to the hydrolysis of alkyne, HgSO4 is required with H2SO4. To the hydrolysis of alkene, dilute H2SO4 is enough.

Because we use both gSO4 and H2SO4, both ethene and ethyne are hydrolyzed and give different products ethanol and ethanal. So we have to more reactions to identify ethanol and ethanal.


Identify ethanol and ethanal

Ethanol and ethanal are two different organic compounds, an alcohol and aldehyde respectively. They show different reactions and we can use those to identify them.

  • Ethanol and ethanal are liquids at room temperature.
  • With sodium, ethanol reacts and emit hydrogen gas white ethanal does not react. So ethanol is identified from ethanal and also ethene and ethyne.


Will 1-butyne and 2-butyne will give same product with HgSO4 and H2SO4?

Yes. Both 1-butyne and 2-butyne give 2-butanone as the product. 2-butanone is a ketone.



What are the consituents of the reactant mixture?

H2O, HgSO4, H2SO4



What is the reaction of HgSO4, H2SO4 with alkynes?

Alkenes hydration is done by HgSO4, H2SO4 and H2O. HgSO4, H2SO4 are behaved as catalysts. HgSO4 is used because hydrolysis of alkyne is difficult than hydrolysis of alkenes.



What will give RCH double CHR on react with HgSO4 \ H2SO4

RCH double CHR (R1HC=CHR2 R1, R2 = alkyl groups) is an alkene and react with HgSO4 and dilute H2SO4 to give alcohols. Noe that in alkene hydrolysis, we do not need HgSO4 as a catalyst.



Why dilute sulfuric acid is used instead of concentrated sulfuric acid?

Concentrated H2SO4 is used to dehydrate alcohols to produce alkenes. Therfore dilute H2SO4 is used with HgSO4 for hydration.



What happens when propyne reacts with water in the presence of H2SO4/HgSO4

Propyne is hydrolyzed to give propanone. H2SO4/HgSO4 are used to catalyze the hydrolysis becauase alkyne and water reaction occurs very slowly.



Safety precautions you should be aware in this reaction

You should be careful when you are dealing with chemicals used in this reaction. Read MSDS (Material Safety Data Sheets) before you do this experiment.


Sulfuric acid

Sulfuric acid is a strong acid and it is corrosive, a strong dehydrator. Contacting sulfuric acid on skin will cause severe burns and tissue damage. Wear lab court to prevent contact of sulfuric acid.


HgSO4

  • Fatal if swallowed
  • Fatal in contact with skin
  • Fatal if inhaled
  • May cause damage to organs through prolonged or repeated exposure

Alkynes

  • Flammable
  • Not good to inhale


References

  1. Acetylene Material Data Sheets
  2. Mercuric Sulfate Material Data Sheets