Alcohol 3 :

Problem

Solution

When CrO₃ suspended in CH₂Cl₂ (in which it is virtually insoluble) is treated with pyridine 3 (pyr.), a soluble bis-pyridine complex is formed (Collins' reagent). The non-bonding electrons of pyridine coordinate with the electron deficient chromium atom. There is no water present. Propionaldehyde 2 is formed. Oxidation stops at the aldehyde. Oxidation to a carboxylic acid does not occur in the absence of water. See Problem 2 in Alcohols. But there is a complication that may arise. Think about how an alcohol can react like water, then click here.