Acetaldehyde

Acetaldehyde

source : PrepChem.com

source : byju’s

Lab preparation of Acetaldehyde :

Acetaldehyde  is prepared in the laboratory  by oxidation of ethyl alcohol with acidified  potassium dichromate.

K₂Cr₂O₇ + 4H₂SO₄ —–> K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3(O)

[CH₃CH₂OH + O ——–> CH₃CHO + H₂O ] x3

Equation as a whole-

CH₃CH₂OH +K₂Cr₂O₇ + 4H₂SO₄ ——–> K₂SO₄ +Cr₂(SO₄)₃ + 3CH₃CHO + 7H₂O

Method :

A mixture of K₂Cr₂O₇  & H₂O is taken in a round bottom flask with a dropping funnel and a water condenser. The condenser kept in freezing mixture and partly filled with ether.

A mixture of C₂H₅OH and H₂SO₄ is taken in dropping funnel. The flask is heated on water bath and the mixture from the dropping funnel is added drop by drop. The heating is stopped after the start of reaction. Hot water (30-35 ⁰C) is circulated through the condenser. The vapors of C₂H₅OH condense and return to the flask while the vapors of CH₃CHO pass to the receiver. The distillate obtained contains CH₃CHO, C₂H₅OH, water and ether. CH₃CHO is not recovered by fractional distillation.

To recover acetaldehyde the distillate is treated with dry NH₃ when the crystallized product  acetaldehyde ammonia is formed. It is filtered and washed with dry ether. The dried crystals are then distilled with dilute H₂SO₄ when pure acetaldehyde is collected.

Manufacture of Acetaldehyde :

1. By oxidation of C₂H₅OH :

Ethyl alcohol vapors and limited amount of air are passed over heated Ag- catalyst at 300^o C.

2C₂H₅OH + O₂ —–> 2CH₃CHO + 2H₂O

2. By dehydrogenation of Alcohol :

Vapors of C₂H₅OH are passed over heated Cu at 300^o C.

CH₃CH₂OH    ——–> CH₃CHO +H₂

3. By hydration of C₂H₂ :

Acetylene  is passed through 40% H₂SO₄ and 1%  HgSO₄ at 60 ⁰C then acetaldehyde is formed.

Physical properties:

1. It is a colorless volatile liquid. It boils at 21 ⁰C.

2. It has a characteristic pungent smell.

3. It is soluble in H₂O, CHCl₃, C₂H₅OH and ether.