sara15543
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Can you use potassium dichromate to distinguish aldehydes and ketones? We were taught it oxidises primary alcohols to aldehydes or carboxylic acids, and secondary alcohols to ketones, and then to use tollens or fehlings reagent to distinguish the aldehydes and ketones. I'm just wondering why you can't use potassium dichromate again because it's also an oxidising agent, why would you use 2 different ones.
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Strelzo
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(Original post by saf1554)
Can you use potassium dichromate to distinguish aldehydes and ketones? We were taught it oxidises primary alcohols to aldehydes or carboxylic acids, and secondary alcohols to ketones, and then to use tollens or fehlings reagent to distinguish the aldehydes and ketones. I'm just wondering why you can't use potassium dichromate again because it's also an oxidising agent, why would you use 2 different ones.
It will oxidise primary alcohols to carboxylic acids and secondary alcohols to ketones. The problem with this is that you cannot distinguish between the 2 original alcohols, which is why Tollens Reagent is added. Tollens Reagent will only oxidise primary alcohols; which you can determine when a silver mirror is produced at the end of the reaction.

Potassium dichromate will not distinguish between primary and secondary alcohols.
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flowerscat
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Potassium dichromate turns from orange to green when
- aldehyde is oxidised to carboxylic acid
- primary alcohol is oxidised to aldehyde
- secondary alcohol is oxidised to ketone

Therefore is not of much use when trying to distinguish between the primary and secondary alcohol.

What is it good for, however, is to distinguish between primary/secondary & tertiary alcohols - potassium dichromate will not oxidise tertiary alcohols.
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sara15543
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(Original post by flowerscat)
Potassium dichromate turns from orange to green when
- aldehyde is oxidised to carboxylic acid
- primary alcohol is oxidised to aldehyde
- secondary alcohol is oxidised to ketone

Therefore is not of much use when trying to distinguish between the primary and secondary alcohol.

What is it good for, however, is to distinguish between primary/secondary & tertiary alcohols - potassium dichromate will not oxidise tertiary alcohols.
But you can use it to distinguish aldehydes and ketones after oxidising the primary and secondary alcohols right? If you removed them by distillation they would be pure
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flowerscat
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(Original post by saf1554)
But you can use it to distinguish aldehydes and ketones after oxidising the primary and secondary alcohols right? If you removed them by distillation they would be pure
Yes, if you had a pure solution of aldehyde and a pure solution of ketone. The aldehyde will be oxidised to COOH (Dichromate turns green), ketone will not (Dichromate remain orange).
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sara15543
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(Original post by flowerscat)
Yes, if you had a pure solution of aldehyde and a pure solution of ketone. The aldehyde will be oxidised to COOH (Dichromate turns green), ketone will not (Dichromate remain orange).
thanks!
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