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A-Level chemistry

what result do carboxylic acids give in tollens reagent test
Reply 1
Tollen's reagent is an oxidising agent. Do carboxylic acids readily oxidise?
Reply 2
Original post by Methene
Tollen's reagent is an oxidising agent. Do carboxylic acids readily oxidise?
No, so they give a negative result?
Reply 3
Original post by zushi1248
No, so they give a negative result?
If So, how else and I differentiate between a primary and secondary alcohol is tollen's reagent gives a negative result for carboxylic acids?
Reply 4
Original post by zushi1248
If So, how else and I differentiate between a primary and secondary alcohol is tollen's reagent gives a negative result for carboxylic acids?

You're on the right lines of thinking... what happens exactly when you oxidise a primary alcohol?

Hint: tollens reagent is used to differentiate between aldehydes and ketones
Reply 5
Original post by Methene
You're on the right lines of thinking... what happens exactly when you oxidise a primary alcohol?
Hint: tollens reagent is used to differentiate between aldehydes and ketones
Primary alcohol are initially oxidised to aldehydes then carboxylic acids, carboxylic acids are fully oxidised, therefore shouldn't give a positive result when tollens is added
Reply 6
Original post by zushi1248
Primary alcohol are initially oxidised to aldehydes then carboxylic acids, carboxylic acids are fully oxidised, therefore shouldn't give a positive result when tollens is added

Correct, but neither will ketones that derived from a secondary alcohol. Only aldehydes will give a positive result with tollen's reagent. How can you obtain an aldehyde?
Reply 7
Original post by Methene
Correct, but neither will ketones that derived from a secondary alcohol. Only aldehydes will give a positive result with tollen's reagent. How can you obtain an aldehyde?
Distillation
Reply 8
Original post by zushi1248
Distillation

Yep, so put it all together...
Reply 9
Original post by Methene
Yep, so put it all together...
No, carboxylic acids give a negative result when tested with tollens, aldehydes give a positive result. This is because aldehydes are readily oxidsed and tollen's reagent acts as an oxidising agent, oxidising the aldehyde into its equivalent carboxylic acid. Aldehydes can be obtained through distillation as it prevents them for being further oxidised by the oxidising agent into a carboxylic acid, which is fully oxidised.

You see, I already knew this, but for a chem practical I have to find a chemical test to differentiate between a primary and secondary alcohol and then an aldehyde, I was just hoping that tollen's would somehow give a positive result for a carboxylic acid, so I could use tollens as a way of differentiating between a primary and secondary, and then Fehling's to differentiate between an aldehyde and a carboxylic acid (and ketone). SO, how can i differentiate between the primary and secondary alcohol?
Reply 10
Original post by zushi1248
No, carboxylic acids give a negative result when tested with tollens, aldehydes give a positive result. This is because aldehydes are readily oxidsed and tollen's reagent acts as an oxidising agent, oxidising the aldehyde into its equivalent carboxylic acid. Aldehydes can be obtained through distillation as it prevents them for being further oxidised by the oxidising agent into a carboxylic acid, which is fully oxidised.
You see, I already knew this, but for a chem practical I have to find a chemical test to differentiate between a primary and secondary alcohol and then an aldehyde, I was just hoping that tollen's would somehow give a positive result for a carboxylic acid, so I could use tollens as a way of differentiating between a primary and secondary, and then Fehling's to differentiate between an aldehyde and a carboxylic acid (and ketone). SO, how can i differentiate between the primary and secondary alcohol?
Heat and distill both solutions with acidified potassium dichromate. Primary alcohol will be oxidised to aldehyde, secondary to ketone. Then you can test with tollen's.
Not sure I fully understand what you mean, but that's my take.
Reply 11
Original post by Methene
Heat and distill both solutions with acidified potassium dichromate. Primary alcohol will be oxidised to aldehyde, secondary to ketone. Then you can test with tollen's.
Not sure I fully understand what you mean, but that's my take.
Ok, thank you
Reply 12
Original post by zushi1248
If So, how else and I differentiate between a primary and secondary alcohol is tollen's reagent gives a negative result for carboxylic acids?
Have you been told you have to use Tollens'?

You could oxidise with reflux both, purify and then test the carboxylic acid (from the primary alcohol) with sodium carbonate / pH indicator - the ketone will be pH neutral.
Reply 13
Original post by Pigster
Have you been told you have to use Tollens'?
You could oxidise with reflux both, purify and then test the carboxylic acid (from the primary alcohol) with sodium carbonate / pH indicator - the ketone will be pH neutral.
I didn't think of this, ok I have a scenario, tell me if this would be correct:
I have 3 unknown, organic compounds which i know aren't halogenoalkanes, amines, nitriles... basically limited to the functional groups of an aldehyde, carboxylic acid, ketone and alcohol.
If I test with tollens and one gives a positive result, I can safely assume its an aldehyde and therefore also a primary alcohol. However if I get a negative result, it could be a ketone or carboxylic acid
Then, if I check the pH of the other two, unknowns using pH indicator and the colour is red, I can assume one of them is a carboxylic acid therefore also a primary alcohol and the other unknown is a ketone and therefore secondary.

Will this be enough to differenciate between the three? Or are the results to vague?

I want to avoid reacting with K2Cr2O7 because i won't have time to do this in my practical
Reply 14
Original post by zushi1248
I didn't think of this, ok I have a scenario, tell me if this would be correct:
I have 3 unknown, organic compounds which i know aren't halogenoalkanes, amines, nitriles... basically limited to the functional groups of an aldehyde, carboxylic acid, ketone and alcohol.
If I test with tollens and one gives a positive result, I can safely assume its an aldehyde and therefore also a primary alcohol. However if I get a negative result, it could be a ketone or carboxylic acid
Then, if I check the pH of the other two, unknowns using pH indicator and the colour is red, I can assume one of them is a carboxylic acid therefore also a primary alcohol and the other unknown is a ketone and therefore secondary.
Will this be enough to differenciate between the three? Or are the results to vague?
I want to avoid reacting with K2Cr2O7 because i won't have time to do this in my practical
Adding H+/Cr2O72- takes next to no time. You add a few drops of your unknown to a test tube add some sulfuric acid and dichromate, perhaps warm in beaker of hot water and look for a colour change. Seconds. You don't need to distil/reflux to see the colour change.

Only the aldehyde will react with Tollens' (named after Mr Tollens) the other will not.

2,4-DNP is an obvious reactant.

Telling the difference between a primary and secondary will be difficult without using H+/Cr2O72-.
Reply 15
Original post by Pigster
Adding H+/Cr2O72- takes next to no time. You add a few drops of your unknown to a test tube add some sulfuric acid and dichromate, perhaps warm in beaker of hot water and look for a colour change. Seconds. You don't need to distil/reflux to see the colour change.
Only the aldehyde will react with Tollens' (named after Mr Tollens) the other will not.
2,4-DNP is an obvious reactant.
Telling the difference between a primary and secondary will be difficult without using H+/Cr2O72-.
I see, okay, thank you so much :grin::grin::grin:
Reply 16
Original post by zushi1248
what result do carboxylic acids give in tollens reagent test
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Reply 17
Original post by aira.a
IMG_1574.jpeg
Dodgy carboxylic acid testing.

Adding sodium to a phenol or an alcohol will fizz and produce H2(g). Stick with the carbonate.

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