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 300o C.
2C2H5OH + O2 —–> 2CH3CHO + 2H2O
2. By dehydrogenation of Alcohol :
Vapors of C2H5OH are passed over heated Cu at 300o 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.
