Chemistry: tests for functional groups?

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jediknight007
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#1
Report Thread starter 18 years ago
#1
Hi, I currently have Chemistry coursework to do. I'm with the OCR exam board and I have to do the Planning part again since I only got 5/8 in my previous one. I got it before half-term and I have to give it in by the Friday coming up. However, I have had to work every single evening during the half-term, from 5pm-midnight! Well, at least I'm getting paid...

Anyways, I haven't made a single start on it. I kinda have an idea of how to do it since my teacher said most of the marks will be on a flowchart which I have to create to allow people to find out what an unknown compound is. The thing is that I forgot what the tests are for some of the easier functional groups because I'm doing A2 now and I don't have my Chemistry 1 book. My current Chemistry 2 book isn't that helpful either. I know I should have learnt the tests by now but I only got C in my AS exam!

Anyways, the brief I was given says that the unknown compound will contain one of the following functional groups:

alkene, primary alcohol, secondary alcohol, tertiary alcohol, aldehyde, ketone, carboxylic acid, ester or a phenol.

I know the tests for the following functional groups but if anyone knows a better or easier test, please tell me:

(1). carbonyl group - use 2,4 DNP. Produces orange precipitate with a aldehyde or ketone

(1a). aldehyde - use Tollen's Reagent. If a silver mirror forms, it's a aldehyde
(1b). ketone - use Tollen's Reagent. If a silver mirror doesn't form, it's a ketone

(2). phenol or alcohol - use FeCl3. If a violet colour is produced, it's a phenol, if not, it's an alcohol. Or use Diazonium salt, if a yellow colour is produced, it's a phenol, if not, it's an alcohol.

As you can see, I don't know much about tests for functional groups. It seems that I only know tests to distinguish BETWEEN 2 different functional groups but not individual tests for each functional group!

I also need to explain on each step, for example, why does this test produce blah blah blah with a primary alcohol but not with a tertiary alcohol and I also have to write any necessary equations between the reactants and the reagents.

I know it's a lot to ask and I do think it's annoying myself when people don't do their coursework, leave it until the night before and then ask for help but I never got the chance myself to start on mine. I think I'm really lucky to still have a week for this though! It's also the first time I have asked this forum for help on coursework as well but I hope I can get all the help I need here! Thanks!
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rae
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#2
Report 18 years ago
#2
look at the onidation of primary, secondary and tertiary alcohols i.e. tertiarys can't be oxidised
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emporium
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#3
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#3
yeh start on a theme rae introduced:
Oxidise the unknown compound, by putting it in a solution with Potassium Dichromate Solution [an oxidising agent] (K2Cr2O7) and Sulphuric Acid [so to create acidic conditions].
The orange solution will turn green if oxidation of the compound has taken place, if oxidation occurs, it must be one of these:
>Primary Alc
>Secondary Alc
>Aldehyde

Obviously if the result is negative (solution stays orange) then it is one of the others.

That will give you a start, as i can't be bothered to go dig out my organic notes I'll end on that .
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jediknight007
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#4
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#4
(Original post by emporium)
yeh start on a theme rae introduced:
Oxidise the unknown compound, by putting it in a solution with Potassium Dichromate Solution [an oxidising agent] (K2Cr2O7) and Sulphuric Acid [so to create acidic conditions].
The orange solution will turn green if oxidation of the compound has taken place, if oxidation occurs, it must be one of these:
>Primary Alc
>Secondary Alc
>Aldehyde

Obviously if the result is negative (solution stays orange) then it is one of the others.

That will give you a start, as i can't be bothered to go dig out my organic notes I'll end on that .
Well, I already knew that tertiary alcohols can't oxidise but I needed like a specific test.

So if I used the Potassium Dichromate test above and it was positive, could I then use the 2,4 DNP to test for the aldehyde since it's used to test for a carbonyl group?

If that works, then I'm gonna have to distinguish between a primary and seconday alchohol. Am I right to say that there is no direct test to distingush between the 2 since both oxidise? Should I go for a test where I oxidise both and then test for the functional group produced? I think when you oxidise a primary alcohol, you get an aldehyde and with further oxidation, you get a carboxylic acid. If you oxidise a secondary alcohol, you get an ketone right? So if I oxidise the 2 alcohols, I could carry out a test for a carboxylic acid, ketone and aldehyde and by doing it that way, I should know which alcohol it was originally right?
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king of swords
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#5
Report 18 years ago
#5
(Original post by jediknight007)
I think when you oxidise a primary alcohol, you get an aldehyde and with further oxidation, you get a carboxylic acid. If you oxidise a secondary alcohol, you get an ketone right? So if I oxidise the 2 alcohols, I could carry out a test for a carboxylic acid, ketone and aldehyde and by doing it that way, I should know which alcohol it was originally right?
Correct.
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emporium
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#6
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#6
(Original post by jediknight007)
If that works, then I'm gonna have to distinguish between a primary and seconday alchohol. Am I right to say that there is no direct test to distingush between the 2 since both oxidise? Should I go for a test where I oxidise both and then test for the functional group produced? I think when you oxidise a primary alcohol, you get an aldehyde and with further oxidation, you get a carboxylic acid. If you oxidise a secondary alcohol, you get an ketone right? So if I oxidise the 2 alcohols, I could carry out a test for a carboxylic acid, ketone and aldehyde and by doing it that way, I should know which alcohol it was originally right?
A primary alcohol will 'oxidise all the way through' to a carboxyic acid under reflux, so a test for an aldehyde is not necessary in order to distinguish whether the original compound is a primary or secondary alcohol (if it is an alcohol at all!).
>Use ur 2,4 DNP to test for a Carbonyl Compound (therefore a Ketone)
>To test for a Carboxylic Acid, pipette the substance (that's been oxidised) into a test tube containing Sodium Hydrogen Carbonate, if a Carboxylic Acid is present, a gas will be effevesced that turns lime water cloudly (Carbon Dioxide).
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jediknight007
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#7
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#7
So which test should I carry out first? The 2,4DNP to see whether it contains a carbonyl functional group or use Potassium Dichromate to see if it can be oxidised?
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Chicken
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#8
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#8
(Original post by jediknight007)
So which test should I carry out first? The 2,4DNP to see whether it contains a carbonyl functional group or use Potassium Dichromate to see if it can be oxidised?
definately do the 2,4-DNP first - I did this piece of coursework last year but I cannot for the life of me remember what tests I did apart from the DNP and Tollen's reagent ones. I do know I did the DNP first though, I think possibly I used PCl5 or something for one of the functional groups, possibly alcohols.
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jediknight007
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#9
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#9
(Original post by Chicken)
definately do the 2,4-DNP first - I did this piece of coursework last year but I cannot for the life of me remember what tests I did apart from the DNP and Tollen's reagent ones. I do know I did the DNP first though, I think possibly I used PCl5 or something for one of the functional groups, possibly alcohols.
What mark did you get? The highest I have ever got was 7/8 in one of my Analysis ones.
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Chicken
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#10
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#10
(Original post by jediknight007)
What mark did you get? The highest I have ever got was 7/8 in one of my Analysis ones.
I think I only got 6 for this piece, so I used my other piece of A2 coursework which I had 7 on. I think my overall mark for A2 coursework was a B.
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jediknight007
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#11
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#11
(Original post by Chicken)
I think I only got 6 for this piece, so I used my other piece of A2 coursework which I had 7 on. I think my overall mark for A2 coursework was a B.
I would be really happy with 6/8 because if I get 6/8 for my 3 other pieces, it's an A! Well, according to my teachers anyways...

Strange thing is that I did so much better with my AS Biology coursework than my AS Chemistry coursework last year, despite finding Biology the more difficult subject! I got 6s on all 4 pieces first time around with Biology but I could only manage 6,5,7,4 for my Chemistry, even after 2 attempts on each piece!

So I should start with 2,4DNP to test for a carbonyl, if it's positive, I can then use Tollen's Reagent to find out whether it's an aldehyde or ketone right?

However, the thing is that 2,4DNP doesn't work for a carboxylic acid which is also a carbonyl! What do I do with that?

So after using 2,4DNP, if it's negative, it means that my unknown compound is either an alkene, alcohol, carboxylic acid, ester or phenol right?

Could I then use the bromine water test to see whether it's an alkene or not?

If it's a negative, then it has to be an alcohol, carboxylic acid, ester or a phenol. What do I do then? Do I use the Potassium Dichromate method as suggested above because alcohols and carboxylic acids oxidise?

-if the Pot Dich test is positive, then I would then have to carry out a further test to see whether it's an alcohol or carboxylic acid and if it's an alcohol, another further test to see which one it is (primary or secondary).

-if the Pot Dich test is negative, it means I have a tertiary alchohol, an ester or a phenol right? Now this is where I'm completely stuck since I have only been doing esters and phenols for about 2 weeks. Could I use the Litmus Blue test to see whether it's a phenol because it would turn red if it's positive?

So would this method work? I have to make sure that my flowchart is as simple as possible. The explanations of each step is fine for me since I should find the information in my textbook.
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Chicken
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#12
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#12
That sounds fine by me, its similar I think to what I did. I honestly can't remember what i did though - its all on the computer at home, so i can't even get it to find out what I did for you!!!
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jediknight007
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#13
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#13
Or could I do it this way:

Use Litmus Blue first. If it turns red, it's a Phenol or a Carboxylic Acid. If not, it's one of the others.
-if it's red, I could then use the Potassium Dichromate to see whether it's a Carboxylic Acid or not since it's oxidised.

If it ain't red, I would have either an alkene, alcohol, ketone, ester or aldehyde.
I could then use the 2,4DNP to test for an aldehyde or ketone. Then if it's positive, blah blah blah.

If it's negative, it's now either an alkene, ester or alcohol.
I could use Bromine Water to test for an alkene.

If it's negative, I would then have either an ester or an alcohol. Blah blah blah blah.

Ok, this way seems much longer....but my teacher told me that she would do it this way. Argh!!!!! Does it really matter which way you do it?
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Chicken
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#14
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#14
It doesn't make much difference to be honest, but one way might be slightly more advanced so would get you higher grades etc though i really don't see how it makes much difference!!!
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jediknight007
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#15
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#15
(Original post by Chicken)
It doesn't make much difference to be honest, but one way might be slightly more advanced so would get you higher grades etc though i really don't see how it makes much difference!!!
Oh well, I guess I'm gonna have to draw up 2 rough flow charts and show them to my teacher tomorrow! Hope they can also tell me what tests I can use for each functional group! I mean, they can't be that cruel...
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Chicken
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#16
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#16
(Original post by jediknight007)
Oh well, I guess I'm gonna have to draw up 2 rough flow charts and show them to my teacher tomorrow! Hope they can also tell me what tests I can use for each functional group! I mean, they can't be that cruel...
hehe!!! Sorry I haven't been much help at all!!!
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jediknight007
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#17
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#17
Wait, if I use Potassium Dichromate and it turns green, it's either a primary or secondary alcohol right? (I know it's also an aldehyde but I would have already eliminated that using the 2,4DNP). Sorry for making this mistake before where I said it would also be a carboxylic acid. So therefore, at the end, I would be stuck with an ester and a carboxylic acid. What test can I carry out there? Could I do like a reduction reaction since carboxylic acids can be reduced (to aldehydes) and esters can't (I think!). For a moment, I was thinking of smelling the product but then I remembered that both carboxylic acids and esters produce distinctive fruity smells!
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Chicken
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#18
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Smelling might not count as an advanced enough method of distinction!!! I've just had a look in my Organic chemistry text book and its useless, it doesn't say about distinguishing groups or anything!! Maybe I wasn't looking in the right places though.. Try using an internet search or somehrting, you never know it might turn some useful info out!!!
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jediknight007
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#19
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#19
(Original post by Chicken)
Smelling might not count as an advanced enough method of distinction!!! I've just had a look in my Organic chemistry text book and its useless, it doesn't say about distinguishing groups or anything!! Maybe I wasn't looking in the right places though.. Try using an internet search or somehrting, you never know it might turn some useful info out!!!
Well, I have tried stuff like 'test for primary or secondary alcohol' or 'test for carboxylic acids' but it's mostly crap and not A-level type.i just need to know 2 more tests; distinguishing between a primary and secondary alcohol and distinguishing between a carboxylic acid and an ester.
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Chicken
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#20
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#20
(Original post by jediknight007)
Well, I have tried stuff like 'test for primary or secondary alcohol' or 'test for carboxylic acids' but it's mostly crap and not A-level type.i just need to know 2 more tests; distinguishing between a primary and secondary alcohol and distinguishing between a carboxylic acid and an ester.
Right well I've looked all through my text books (which are uni not A-Level) ones, and they don't have any distinguishing tests in them whatsoever. So I'm afraid i can't help anymore (not that I helped to begin with!), the only thing is to keep on trying interent searches. Sorry!!!
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