Understanding the Oxidation Process
* Acid Dichromate: Potassium dichromate (K₂Cr₂O₇) in acidic solution is a strong oxidizing agent. It's commonly used to oxidize alcohols.
* Color Change: The dichromate ion (Cr₂O₇²⁻) is orange in color. When it acts as an oxidizer, it gets reduced to the green Cr³⁺ ion. This color change from orange to green is a visual indicator of the oxidation reaction.
Oxidation Ease and Time
* Primary Alcohols (like Butan-1-ol): These are the easiest to oxidize. They can be oxidized to aldehydes and then further to carboxylic acids.
* Time: The solution will change color relatively quickly due to the ease of oxidation.
* Secondary Alcohols (like Butan-2-ol): These can be oxidized to ketones.
* Time: The oxidation is slower than with primary alcohols. It might take a bit longer for the solution to change color.
* Tertiary Alcohols (like 2-Methylpropan-2-ol): Tertiary alcohols cannot be oxidized by acid dichromate. They lack the hydrogen atom attached to the carbon bearing the hydroxyl group that's required for the oxidation process.
* Time: The solution will not change color because no oxidation reaction occurs.
Why the Difference?
The ease of oxidation is determined by the stability of the carbocation intermediate formed during the reaction.
* Primary Alcohols: Form relatively unstable carbocations.
* Secondary Alcohols: Form more stable carbocations.
* Tertiary Alcohols: Form the most stable carbocations, but the process is not possible with acid dichromate due to the lack of a hydrogen atom on the carbon bearing the hydroxyl group.
Summary
The relative ease of oxidation with acid dichromate follows this order:
1. Primary Alcohols > Secondary Alcohols > Tertiary Alcohols (not oxidized)
This is reflected in the time it takes for the solution to change color, with primary alcohols causing the fastest change and tertiary alcohols not causing any change.
