Identifying Esters/Carboxyllic Acid (1 Viewer)

[Pedro123]

1. Is there a chemical test to identify an ester (Non-spectra based)? This was one of the questions on an HSC paper, and they don't indicate any other than smelling it, so I assume not
2. The oxidation of an aldehyde to form a carboxylic acid has to be done with an acidified oxidising agent, so how would adding sodium carbonate be an appropriate confirmation test? Wouldn't it anyway produce CO2 gas, even without the carboxylic acid? (Due to the oxidising agent being acidified) In that case, what chemical test should be used?

Thanks

 

[icycledough]

1) Yh, from what I've read, the only way to go about it is to perform spectra-based tests, which doesn't really help with what you're asking. Definitely don't mention smelling or tasting as a way to identify an ester (it isn't safe for you)

 

[Pedro123]

Also, forgot to put it in OG question, what is the identifying test for alcohols? (Just whether it has formed, not differentiating them)

 

[cassicowfan]

Also, forgot to put it in OG question, what is the identifying test for alcohols? (Just whether it has formed, not differentiating them)

add sodium metal and observe bubbling

 

[someth1ng]

As mentioned, spectroscopy is probably the easiest (and best) way.

If you're curious (this is easily beyond syllabus lol), you can always look up TLC stains and you'll see a shit load of ways to visualise different compounds.

Try this: https://www.epfl.ch/labs/lcso/wp-content/uploads/2018/06/TLC_Stains.pdf

 

[tito981]

2. The oxidation of an aldehyde to form a carboxylic acid has to be done with an acidified oxidising agent, so how would adding sodium carbonate be an appropriate confirmation test? Wouldn't it anyway produce CO2 gas, even without the carboxylic acid? (Due to the oxidising agent being acidified) In that case, what chemical test should be used?

Thanks

i think the oxidising agent doesnt produce CO2, isnt the CO2 produced due to the carbonate group? Also a way to distinguish the acid is litmus. Blue litmus - turns Acid - Red, the acronym BAR can be used to remember more easily.

 

[Pedro123]

i think the oxidising agent doesnt produce CO2, isnt the CO2 produced due to the carbonate group? Also a way to distinguish the acid is litmus. Blue litmus - turns Acid - Red, the acronym BAR can be used to remember more easily.

What I mean is if the oxidising agent that you add is acidified, then irrespective of whether the aldehyde is oxidised to the ketone, the sodium carbonate will still produce CO2 gas. The same applies for a litmus test

 

[tito981]

What I mean is if the oxidising agent that you add is acidified, then irrespective of whether the aldehyde is oxidised to the ketone, the sodium carbonate will still produce CO2 gas. The same applies for a litmus test

i think for HSC questions they normally state if the solutions undergoes a colour change which is indicative of no oxidising agent being left, because the agent is assumed to be always limiting the reaction.

 

[tito981]

i think for HSC questions they normally state if the solutions undergoes a colour change which is indicative of no oxidising agent being left, because the agent is assumed to be always limiting the reaction.

and they would never add carbonate to a coloured solution which defeats the whole point of the confirmatory test for the acid.

 

[CM_Tutor]

What I mean is if the oxidising agent that you add is acidified, then irrespective of whether the aldehyde is oxidised to the ketone, the sodium carbonate will still produce CO2 gas. The same applies for a litmus test

Assume that chemical tests are done on purified products, not with possible unreacted materials (from the oxidation, say) still being around. If there is residual acid present then you are correct, added sodium carbonate would react with it regardless of whether the product formed is an acid or not.

 

[CM_Tutor]

add sodium metal and observe bubbling

Remember, though, that sodium will also produce hydrogen with carboxylic acids.

 

[someth1ng]

Remember, though, that sodium will also produce hydrogen with carboxylic acids.

It would also reduce carbonyls to the alcohol. I would recommend every just...don't use sodium metal lol.

 

[CM_Tutor]

It would also reduce carbonyls to the alcohol. I would recommend every just...don't use sodium metal lol.

No HSC student will be given sodium metal to use. The fact that acids react with sodium is part of the year 11 course, so they should realise that sodium metal plus an organic substance producing CO₂ can be a carboxylic acid or an alcohol. They hardly touch reduction, so reaction of a carbonyl is not one they know or could easily predict. I do agree that tests without sodium metal are desirable, but then the extent of tests used seems to vary between schools.

SGS has asked questions using the ceric ammonium nitrate test which most students won't know. In asking the silver mirror test, I was careful not to ask anything that went beyond either what is known from the syllabus or what can be deduced from the question.