O chem questioni

ddsshin · Jul 27, 2009

this is from topscore 2.
It says you can test for aldehyde and ketone using Tollen's, Schiff's Iodoform and bisulfate. How are their reaction prove those reactants are ketone and aldehyde???? why KMO4 is not test for ketone or aldehyde but alkene?

nze82 · Jul 27, 2009

ddsshin said:
this is from topscore 2.
It says you can test for aldehyde and ketone using Tollen's, Schiff's Iodoform and bisulfate. How are their reaction prove those reactants are ketone and aldehyde???? why KMO4 is not test for ketone or aldehyde but alkene?

These tests result in physical changes that are detectable by naked eye, if the compounds of interest are indeed present:

**Tollen test>>If aldehyde or ketone is present, you'll see formation of silver precipitate at the bottom of the reaction vessel. Essentially, your reaction vessel ends up looking like a mirror.

**Iodoform test>>Idoform is pale yellow. Therefore, if aldehyde and ketone is present you'll see your reaction mixture changing color to pale yellow.

Shinpe · Jul 27, 2009

ddsshin said:
this is from topscore 2.
It says you can test for aldehyde and ketone using Tollen's, Schiff's Iodoform and bisulfate. How are their reaction prove those reactants are ketone and aldehyde???? why KMO4 is not test for ketone or aldehyde but alkene?

As to why a certain test is positive for aldehyde or alkene or something else comes down to whether that reagent will react with the questioned functional group, resulting in a visual difference (usually a precipitate) or not. We don't really have much power over that one ;p

But here's what I know:
Tollen's -> + for aldehyde or beta-hydroxy ketone (and thus for reducing sugars)
Benedict's -> + for aldehyde or alpha-hydroxy ketone (and thus for reducing sugars)
Iodoform -> + for any carbonyl or alcohol that has at least one methyl group attached to it (methylketones and ethanal, secondary alchohols and ethanol)
KMnO4 -> + Alkene
Schiff's test -> + for aldehydes
2,4-DNP(DiNitroPhenylhydrazine) -> + for any carbonyl (aldehyde or ketone)

CANgnome · Jul 7, 2010

on KBB it says KMnO4 reacts with aldehydes to produce carboxylic acids.

Is it only a good detector for alkenes, then because the carboxylic acid does not produce any sort of color change or precipitate as opposed to tollens, etc?

KBB 2009-2010: pg 396

gigawatt · Jul 8, 2010

Here's a good link that covers a variety of qualitative tests for functional groups:

http://www.chemistry.ccsu.edu/glagovich/teaching/316/qualanal/tests/tests.html

I haven't taken the DAT yet so I don't know for sure, but I wouldn't suspect that memorizing this level of detail is very worthwhile. Has anyone had a qualitative test/named reagent question on the real thing?

gigawatt · Jul 8, 2010

CANgnome said:
on KBB it says KMnO4 reacts with aldehydes to produce carboxylic acids.

Is it only a good detector for alkenes, then because the carboxylic acid does not produce any sort of color change or precipitate as opposed to tollens, etc?

KBB 2009-2010: pg 396

I think aldehydes do give a positive potassium permanganate test. Since KMnO4 is such a strong oxidizing agent, you usually have to rule out a bunch of other reactive functional groups before this "test" can give you anything useful. For that reason, I think bromine is the more commonly used test specific to alkenes. It's not the best-phrased question in TopScore.

In any case, I doubt it's necessary to study these tests in detail because any question about them that could come up could probably be answered using your general knowledge of reagents/reactions.

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