The process of oxidation of an alcohol to aldehyde requires an oxidizing agent that removes two hydrogen atoms from the molecule and concludes when the carbon and oxygen atoms form a double bond. Only primary alcohols, alcohol molecules with the -OH group at one end of the carbon chain, can form aldehydes. Secondary and tertiary alcohol molecules form ketones when oxidized.
The oxidization of primary alcohols to aldehydes requires specific reaction conditions. With the incorrect conditions or oxidizing agent, the alcohol oxidizes all the way to carboxylic acid rather than stopping at aldehyde. The oxidizing agent of choice for the oxidization of alcohols to aldehydes is typically pyridinium chlorochromate, or PCC. This oxidizing agent is used in a methylene chloride solution. Therefore, PCC is not in a hydrated solution. Without water molecules present in the solution, the reaction from primary alcohol to aldehyde stops and does not run to completion.
The oxidation of ethanol in the human liver occurs after consumption of alcohol. In this case, the oxidizing agent is an enzyme called alcohol dehydrogenase. Inside the liver, ethanol oxidizes to ethanal. However, the inside of the human body is a highly hydrated environment and the aldehyde further oxidizes to acetic acid. Buildup of ethanal in the liver is one of the reasons why people sometimes feel sick after drinking large amounts of alcohol.