Question on testing between primary and secondary alcohols? Watch

Alpha-Omega
Badges: 12
Rep:
?
#1
Report Thread starter 7 years ago
#1
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...
0
reply
illusionz
Badges: 16
Rep:
?
#2
Report 7 years ago
#2
If you have two chemicals, one is an acid and one is a ketone the simplest test is some litmus paper!!!
0
reply
Alpha-Omega
Badges: 12
Rep:
?
#3
Report Thread starter 7 years ago
#3
(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
0
reply
illusionz
Badges: 16
Rep:
?
#4
Report 7 years ago
#4
(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!!
0
reply
illusionz
Badges: 16
Rep:
?
#5
Report 7 years ago
#5
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
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#6
Report 7 years ago
#6
(Original post by illusionz)
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.
0
reply
Alpha-Omega
Badges: 12
Rep:
?
#7
Report Thread starter 7 years ago
#7
(Original post by illusionz)
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.

I think I'll stick to the litmus paper test.
0
reply
Alpha-Omega
Badges: 12
Rep:
?
#8
Report Thread starter 7 years ago
#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.
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?
0
reply
illusionz
Badges: 16
Rep:
?
#9
Report 7 years ago
#9
(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.
0
reply
illusionz
Badges: 16
Rep:
?
#10
Report 7 years ago
#10
(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.
2
reply
Alpha-Omega
Badges: 12
Rep:
?
#11
Report Thread starter 7 years ago
#11
(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. +rep
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#12
Report 7 years ago
#12
(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.
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#13
Report 7 years ago
#13
(Original post by Alpha-Omega)
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?
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.
0
reply
PWhitt
Badges: 0
Rep:
?
#14
Report 5 years ago
#14
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.
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#15
Report 5 years ago
#15
(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
Far easier to add a pinch of sodium hydrogen carbonate to each. The acid releases carbon dioxide and you see bubbles.
0
reply
PWhitt
Badges: 0
Rep:
?
#16
Report 5 years ago
#16
(Original post by Alpha-Omega)
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.
0
reply
PWhitt
Badges: 0
Rep:
?
#17
Report 5 years ago
#17
(Original post by charco)
Far easier to add a pinch of sodium hydrogen carbonate to each. The acid releases carbon dioxide and you see bubbles.
It would happen in case of ketone as well, because sulphuric acid has already been added along with the potassium dichromate.
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#18
Report 5 years ago
#18
(Original post by PWhitt)
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.
You are testing the distillate !

There is no sulphuric acid ...
0
reply
charco
  • Community Assistant
Badges: 17
Rep:
?
#19
Report 5 years ago
#19
(Original post by PWhitt)
It would happen in case of ketone as well, because sulphuric acid has already been added along with the potassium dichromate.
You are testing the distillate!

There is no sulphuric acid ...
0
reply
PWhitt
Badges: 0
Rep:
?
#20
Report 5 years ago
#20
(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!!!
The potassium dichromate added was acidified when added, so the litmus would turn red even when it comes to the ketone.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Birmingham City University
    Undergraduate Open Day Undergraduate
    Sat, 23 Mar '19
  • University of Bolton
    Undergraduate Open Day Undergraduate
    Sat, 23 Mar '19
  • Harper Adams University
    Animals and Veterinary Sciences Open Day Undergraduate
    Sat, 23 Mar '19

Where do you need more help?

Which Uni should I go to? (116)
17.6%
How successful will I become if I take my planned subjects? (70)
10.62%
How happy will I be if I take this career? (115)
17.45%
How do I achieve my dream Uni placement? (97)
14.72%
What should I study to achieve my dream career? (63)
9.56%
How can I be the best version of myself? (198)
30.05%

Watched Threads

View All