How to test for them the simple way using solutions taught in A-Level?
How about this?
Two test tubes with solution. One is primary alcohol; the other is a secondary alcohol. Which one is which?
1. Oxidise both under reflux using potassium dichromate (VI) and dilute sulfuric acid. 2. Primary alcohol would turn to a carboxylic acid; the secondary alcohol would turn to a ketone.
Here's the one I'm not sure about...
3. Add PCl5 to them; the one that produces steamy fumes is the carboxylic acid. Hence, it was the primary alcohol.
I've read that step 3 only works for "dry substances." The problem is oxidising an alcohol also produces water as a product.
Would this still work?
If I'd have to dry them, I'd have to distill them and this just makes the whole thing too long...
While it is true that a ketone has protons with a pKa of around 20, if this proton is removed then you end up with a molecule which has an oxygen with a negative charge. This will quickly pick up the proton which was lost. For more information see this http://en.wikipedia.org/wiki/Keto-enol_tautomerism
Litmus paper will tell you which is which quite easily. If you were really worried you could always use some indicator paper which would tell you the pH of the solutions.
PS Don't believe everything you read on the internet!!
When I was doing A levels we were taught about Fehling's solution and Tollen's reagent, which tell you whether you have an aldehyde or a ketone. I can't remember what the A level method of partially oxidising a primary alcohol to an aldehyde is though (full oxidation takes you to a carboxylic acid).
When I was doing A levels we were taught about Fehling's solution and Tollen's reagent, which tell you whether you have an aldehyde or a ketone. I can't remember what the A level method of partially oxidising a primary alcohol to an aldehyde is though (full oxidation takes you to a carboxylic acid).
Edit you seem to have deleted your post
A' level method for partial oxidation is to perform the reaction in a distillation vessel above the boiling point of the aldehyde, but below the bps of the other components.
When I was doing A levels we were taught about Fehling's solution and Tollen's reagent, which tell you whether you have an aldehyde or a ketone. I can't remember what the A level method of partially oxidising a primary alcohol to an aldehyde is though (full oxidation takes you to a carboxylic acid).
Edit you seem to have deleted your post
To be honest, I'm a bit skeptic on the litmus test as I haven't heard of that method from my teacher before as a test between carboxylic acid and alcohol.
Tollen's reagent and Fehling's solution are reagents to test between aldehydes and ketones. The problem to this is aldehyde is synthesised by partially oxidising a primary alcohol; while the ketone is by fully oxidising a secondary alcohol. This just makes the whole thing complicated as I don't know which one to partially oxidise and which to fully oxidise.
There's this Lucas' test I found in wikipedia which seems very straightforward. Although, it was never mentioned in Edexcel A-Level books so I doubt it's a good idea to mention it in the exam.
A' level method for partial oxidation is to perform the reaction in a distillation vessel above the boiling point of the aldehyde, but below the bps of the other components.
Could you confirm for me if the litmus paper test would work to test between carboxylic acids and ketones? Thanks
Also, would the phosphorus pentachloride test work to test for the -OH group even though the solution has water?
A' level method for partial oxidation is to perform the reaction in a distillation vessel above the boiling point of the aldehyde, but below the bps of the other components.
Bizarre. Wonder why they wouldn't just use PDC/PCC.
Bizarre. Wonder why they wouldn't just use PDC/PCC.
It's used to illustrate the different intermolecular forces present in alcohols, aldehydes and carboxylic acids.
Use distillation to obtain aldehydes and reflux to obtain carboxylic acids.
It serves as a preparation of aldehydes and allows the students to understand the difference between reflux and distillation, using easy experiments with simple, but important theory, behind them.
GUYS, LITMUS TEST WILL NOT WORK! Remember, you're adding ACIDIFIED potassium dichromate, i.e. potassium dichromate with dilute sulphuric acid. So, in presence of the sulphuric acid, the litmus paper WILL turn red, REGARDLESS of whether the alcohol was oxidized to be a carboxylic acid or a ketone.
To be honest, I'm a bit skeptic on the litmus test as I haven't heard of that method from my teacher before as a test between carboxylic acid and alcohol.
Tollen's reagent and Fehling's solution are reagents to test between aldehydes and ketones. The problem to this is aldehyde is synthesised by partially oxidising a primary alcohol; while the ketone is by fully oxidising a secondary alcohol. This just makes the whole thing complicated as I don't know which one to partially oxidise and which to fully oxidise.
There's this Lucas' test I found in wikipedia which seems very straightforward. Although, it was never mentioned in Edexcel A-Level books so I doubt it's a good idea to mention it in the exam.
I think I'll stick to the litmus paper test.
How would a litmus test be appropriate when you've already added dilute sulphuric acid along with potassium dichromate? Litmus is going to turn color because of the presence of the acid already added, regardless of whether the results are ketones or carboxylic acids.
How would a litmus test be appropriate when you've already added dilute sulphuric acid along with potassium dichromate? Litmus is going to turn color because of the presence of the acid already added, regardless of whether the results are ketones or carboxylic acids.