i did read, after a google search, that ethanol can be acidified with potassium permaganate to yield acetaldehyde,
Hmm.. That doesn't sound quite right. Where did you read that? KMnO4 will oxidize ethanol mostly to acetic acid... You need a weaker oxidizing agent like PCC, or otherwise special conditions to stop at the aldehyde point. KMnO4 will indeed oxidize ethanol to acetaldehyde, but unless you can remove the aldehyde as it is formed or otherwise stop the oxidizing agent from working, it will easily oxidize the aldehyde again to the carboxylic acid.
For all the info you ever wanted about permanganate oxidations, see for example this excellent review:
Fatiadi, Alexander J. "The Classical Permanganate Ion: Still a Novel Oxidant in Organic Chemistry" Synthesis 1987; 85-127 DOI:
10.1055/s-1987-27859
... And since I'm bored right now, how about some more info about the industrial preparation of acetaldehyde...
From
Ullman's Encyclopedia of Industrial Chemistry, 6th ed:
Dehydrogenation of Ethanol. In the first work on ethanol dehydrogenation, published in 1886, ethanol was passed through glass tubes at 260 °C.
CH3CH2OH (l) ® CH3CHO (l) + H2 (g) DH = + 82.5 kJ/mol
Improved yields are obtained in the presence of catalysts such as platinum, copper, or oxides of zinc, nickel, or cobalt. In later patents, zinc and chromium catalysts [34], oxides of rare earth metals [35], and mixtures of copper and chromium oxides [36] have been reported. The lowest amounts of decomposition products are obtained using copper catalysts. Frequent regeneration of the catalysts is required, however.
Process Description. Ethanol vapor is passed at 260 – 290 °C over a catalyst consisting of copper sponge or copper activated with chromium oxide in a tubular reactor [37]. A conversion of 25 – 50 % per run is obtained. By washing with alcohol and water, acetaldehyde and ethanol are separated from the exhaust gas, which is mainly hydrogen. Pure acetaldehyde is obtained by distillation; the ethanol is separated from water and higher-boiling products by distillation and flows back to the reactor. The final acetaldehyde yield is ca. 90 %. Byproducts include butyric acid, crotonaldehyde, and ethyl acetate.
Oxidation of Ethanol. Oxidation of ethanol is the oldest and the best laboratory method for preparing acetaldehyde. In the commercial process, ethanol is oxidized catalytically with oxygen (or air) in the vapor phase.
CH3CH2OH (g) + 1/2 O2 (g) ® CH3CHO(l) + H2O (l) DH = - 242.0 kJ/mol
Copper, silver, and their oxides or alloys are the most frequently used catalysts [38].
For an example of a simultaneous oxidation – dehydrogenation process, see [39].
Veba-Chemie Process (Fig. (1)). Ethanol is mixed with air and passed over a silver catalyst at 500 – 650 °C (c). The temperature depends on the ratio of alcohol to air and the flow rate of the gas through the catalyst. Alcohol conversion varies between 50 and 70 % and the yield is between 97 and 99 % depending on the reaction conditions. Acetaldehyde and unreacted alcohol are removed from the waste gas by washing with cold alcohol (e) and separated by fractional distillation (h); after concentration the alcohol returns to the reactor. Heat formed in the reaction is utilized for steam production using a waste-heat recovery system immediately after the reaction zone.
The waste gas consists mainly of nitrogen, hydrogen, methane, carbon monoxide and carbon dioxide; it is burned as lean gas with low calorific value in steam generators. Small amounts of acetic acid are obtained as a byproduct.
[34] SU 287 919, 1970.
[35] Heavy Minerals Co., US 2 884 460, 1955 (V.
I.Komarevsky).
[36] Knapsack-Griesheim, DE 1 097 969, 1954 (W. Opitz, W.
Urbanski); DE 1 108 200, 1955 (W. Opitz, W. Urbanski).
[37] W. L. Faith, D. B. Keyes, R. L. Clarks: Industrial
Chemicals, 3rd ed., J. Wiley & Sons, New York 1965, p. 2.
[38] Shell Development Co., US 2 883 426, 1957 (W.
Brackman).Eastman Kodak Co., US 3 106 581, 1963 (S.
D.Neely).Veba-Chemie, DE 1 913 311, 1969 (W. Ester, W.
Hoitmann).
[39] Petroleum Refiner 36 (1957) 249.
-SpicyBrown