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After you react rotundone ($\ce{C_{15}H_{22}O}$) with water it loses all of the double bonds (except the carboxylic bond, $\ce{C=O}$) and gets two of the functional group $\ce{-OH}$ (alcohol). Two compounds are formed, one (A) where both the alcohol groups are tertiary and another (B) where one is tertiary and the other is primary.

I'm asked to state and explain the colour change of both A and B when they react separately with acidified potassium dichromate(VI) solution.

Now I can see that compound A would have no colour change because the $\ce{-OH}$ groups are tertiary so the potassium dichromate won't react or oxidise them. I can also see that there will be a colour change on compound B because one of the $\ce{-OH}$ groups is primary, so the potassium dichromate will oxidise and react with it.

Here's my problem:

I'm expected to know that when compound B reacts, it changes from orange to green. How would I know that rotundone is orange in the first place? And what makes it go green? I know that I'm expected to use intuition as we definitely aren't required to know the colour of rotundone.

ron
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ODP
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1 Answers1

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Potassium dichromate (VI) is the species that turns from orange and to green when reduced. Most of the organic substances I came across for testing purposes are colourless, so I would assume that compound B would be as well.

It is just like you are expected to know the colour change of potassium manganate (VII) from purple to colourless when it is reduced.

Those two oxidants were used frequently in my labs, and I'm pretty sure you'll be using them often as well.

I hope this answers your question! :)

Jerry
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  • Yes thank you. Helps me now know that I need to learn a bit more. Although it's AS Level so we probably won't be using them much - did you assume I was an undergraduate? – ODP Apr 09 '13 at 11:35
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    @Olly_price I didn't assume anything. I just thought that these things were taught everywhere as I did A level and didn't go further than that ^^; (and that's a trend I've seen with other people I've answered concerning redox reactions). It would really help if you did some labs with those reactants. Seeing and using them leaves an impression which isn't easy to forget :) – Jerry Apr 09 '13 at 17:51