Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    How do you use a test tube reaction to distinguish between an alcohol and and aldehyde? What would be the observations with each?

    I am thinking hydrogenation (reduction) but what would be the observations?

    Thank you.


    Posted from TSR Mobile
    Offline

    17
    ReputationRep:
    Only one will make an ester.
    What you're expected to say is Tollens, 2,4-D.N.P. et.al., though.
    Offline

    14
    ReputationRep:
    One way of doing it would be to oxidise both under distillation such that you get a carboxylic acid in the case of the oxidation of the aldehyde, and in a separate apparatus you would get an aldehyde or ketone depending on the class of alcohol. With each distillate, you could react them separately with sodium carbonate or sodium hydrogen carbonate, you would observe effervescence with the carboxylic acid as CO2 is released. You wouldn't observe anything from the alcohol distillate; the aldehyde. Between distillations, make sure to keep track of which is which. There is another way with two oxidations under distillation, but that would only work in the case of a primary alcohol because you wouldn't distinguish between them upon the second oxidation since the ketone would be resistant to oxidation in these conditions, as would the carboxylic acid.
    Offline

    1
    ReputationRep:
    the question wants a test tube reaction which would distinguish between them - A levels expect tollens reagent of fehlings solution - the alcohol reacts with neither whereas the aldehyde gives a silver mirror with tollens and a brick red precipitate with fehlingd
    Offline

    14
    ReputationRep:
    (Original post by GDN)
    the question wants a test tube reaction which would distinguish between them - A levels expect tollens reagent of fehlings solution - the alcohol reacts with neither whereas the aldehyde gives a silver mirror with tollens and a brick red precipitate with fehlingd
    Tollens and fehlings both can oxidise some alcohols.
    Offline

    20
    ReputationRep:
    (Original post by Protoxylic)
    One way of doing it would be to oxidise both under distillation such that you get a carboxylic acid in the case of the oxidation of the aldehyde, and in a separate apparatus you would get an aldehyde or ketone depending on the class of alcohol. With each distillate, you could react them separately with sodium carbonate or sodium hydrogen carbonate, you would observe effervescence with the carboxylic acid as CO2 is released. You wouldn't observe anything from the alcohol distillate; the aldehyde. Between distillations, make sure to keep track of which is which. There is another way with two oxidations under distillation, but that would only work in the case of a primary alcohol because you wouldn't distinguish between them upon the second oxidation since the ketone would be resistant to oxidation in these conditions, as would the carboxylic acid.
    That won't always work, because you don't know the type of alcohol you have. i.e. how will you oxidise a tertiary alcohol?



    (Original post by Protoxylic)
    Tollens and fehlings both can oxidise some alcohols.
    At A-level specification, Tollens solution or Fehling's reagent are both acceptable
    Offline

    14
    ReputationRep:
    (Original post by R Dragon)
    That won't always work, because you don't know the type of alcohol you have. i.e. how will you oxidise a tertiary alcohol?





    At A-level specification, Tollens solution or Fehling's reagent are both acceptable
    It would work in the case of the tertiary alcohol, because the distillate that gives an effervescence will be the carboxylic acid and hence you have identified what was the aldehyde. Remember, the question asked for any alcohol VS an aldehyde. For any class of alcohol, you won't get a effervescence for the oxidation product of that alcohol (under distillation) when you react it with sodium carbonate.
    • Community Assistant
    Offline

    21
    ReputationRep:
    Community Assistant
    (Original post by Meessh123)
    How do you use a test tube reaction to distinguish between an alcohol and and aldehyde? What would be the observations with each? (...)
    One of the best way is to use the Fehling reaction. Add copper sulfate solution and potassium sodium tartrate solution to a test tube. Heat this test tube on a bunsen burner, after that add the solution to the test tube which is supposed as the aldehyde.

    If the fehling reaction is positive, the solutions in the test tube change to red. That is the proof of the aldehyde. If the fehling reaction is negative, the solutions in the test tube do not change to red. That is the proof that the added solution is not an aldehyde, its an alcohol.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Sorry I'm still confused.

    The alcohol in this question is a secondary alcohol. It should be a test tube reaction.


    Posted from TSR Mobile
    Offline

    14
    ReputationRep:
    (Original post by Meessh123)
    Sorry I'm still confused.

    The alcohol in this question is a secondary alcohol. It should be a test tube reaction.


    Posted from TSR Mobile
    If it is asking for a test tube reaction, then for A-Level the only one taught is fehlings or tollens. The positive result deriving from the presence of the aldehyde. Positive result for tollens is a silver mirror and/or black ppt. The positive result for fehlings is a colour change from blue to red due to the formation of of a ppt.

    The reason why I had gone into the detail of oxidations is because in reality, some alcohols are oxidised by fehlings/tollens. However, at A-Level you assume no alcohols can be oxidised by fehlings or tollens.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by Protoxylic)
    If it is asking for a test tube reaction, then for A-Level the only one taught is fehlings or tollens. The positive result deriving from the presence of the aldehyde. Positive result for tollens is a silver mirror and/or black ppt. The positive result for fehlings is a colour change from blue to red due to the formation of of a ppt.

    The reason why I had gone into the detail of oxidations is because in reality, some alcohols are oxidised by fehlings/tollens. However, at A-Level you assume no alcohols can be oxidised by fehlings or tollens.
    Ok thank you.



    Posted from TSR Mobile
    Offline

    20
    ReputationRep:
    (Original post by Protoxylic)
    It would work in the case of the tertiary alcohol, because the distillate that gives an effervescence will be the carboxylic acid and hence you have identified what was the aldehyde. Remember, the question asked for any alcohol VS an aldehyde. For any class of alcohol, you won't get a effervescence for the oxidation product of that alcohol (under distillation) when you react it with sodium carbonate.
    But that's essentially 2 reactions to distinguish between them >.>

    Just go with the easy option, eh? :P
    Offline

    14
    ReputationRep:
    (Original post by R Dragon)
    But that's essentially 2 reactions to distinguish between them >.>

    Just go with the easy option, eh? :P
    Better than oxidising the alcohol and getting two positive tests. I misread that it was a test tube reaction and so deduced it was an A Level question in which assumption that the alcohol is not oxidised by fehlings or tollens is needed
    Offline

    20
    ReputationRep:
    (Original post by Protoxylic)
    Better than oxidising the alcohol and getting two positive tests. I misread that it was a test tube reaction and so deduced it was an A Level question in which assumption that the alcohol is not oxidised by fehlings or tollens is needed
    You can't oxidise the alcohol without acidifying it first...
    Offline

    14
    ReputationRep:
    Ignore this post.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: December 4, 2014

3,426

students online now

800,000+

Exam discussions

Find your exam discussion here

Poll
Should universities take a stronger line on drugs?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.