Question on testing between primary and secondary alcohols?

If you have two chemicals, one is an acid and one is a ketone the simplest test is some litmus paper!!!

Reply 2

Original post by illusionz

If you have two chemicals, one is an acid and one is a ketone the simplest test is some litmus paper!!!

I thought of that too but I read somewhere that ketone is also acidic.

http://diabetes.wikia.com/wiki/Ketones

(edited 12 years ago)

Reply 3

Original post by Alpha-Omega

I thought of that too but I read somewhere that ketone is also acidic.

http://diabetes.wikia.com/wiki/Ketones

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!!

Reply 4

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

Reply 5

Original post by illusionz

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.

Reply 6

Original post by illusionz

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.

Reply 7

Original post by charco

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?

(edited 12 years ago)

Reply 8

Original post by charco

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.

Reply 9

Original post by Alpha-Omega

Also, would the phosphorus pentachloride test work to test for the -OH group even though the solution has water?

You would not want to be standing anywhere near a beaker with a substantial amount of water which had just had a load of PCl5 added to it.

In other words, no.

Reply 10

Original post by illusionz

You would not want to be standing anywhere near a beaker with a substantial amount of water which had just had a load of PCl5 added to it.

In other words, no.

Thanks. Litmus, it is then. :smile:

+rep

Reply 11

Original post by illusionz

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.

Reply 12

Original post by Alpha-Omega

Litmus paper would certainly show the difference.

Another simple test that appears on A level courses is the use of a small chunk (1-2 mm³) of sodium.

With alcohols there is a steady evolution of hydrogen (effervescence), while there is no effervescence with aldehydes and ketones.

Once again, the test is interfered with if there are substantial amounts of water impurity.

Reply 13

PWhitt

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.

Reply 14