Kevin B. answered • 09/08/22
Enthusiastic Orgo Professor Can Help You Unlock Organic Chemistry
I just answered your other question about solubility, so I thought I would tackle this one as well! I hope a similar level of detail is appreciated....
First, "acidified permanganate" is not a chemical test I am experienced with. I usually use dilute (1%), neutral permanganate. KMnO4 is a strong oxidizing agent, and oxidizes many different organic functional groups. This is particularly true in acidic solution. I typically use neutral, dilute KMnO4 to identify alkenes -- most alkenes will react to form brown MnO2 precipitate, and most other FG's will not (at least, not quickly -- a slow reaction over several minutes should not be considered a "positive" KMnO4 test). I do not know whether acidified KMnO4 will react with both alcohols and alkenes -- it may. You should look at information provided by your lab instructor about this particular reagent, to see what it should be expected to test positive for, and what a positive test looks like.
The "Bromine" test (usually, a solution of Br2 in CCl4 solvent, though CH2Cl2 is sometimes used, to avoid use of CCl4) is also a test for alkenes. The reddish orange solution of Br2 reacts with alkenes and the color disappears. Most other FGs do not decolorize Br2 solution, so this is considered a positive test for an alkene.
That said, both of these tests (KMnO4 and Br2/CCl4) have false positives and false negatives, so some care must be taken to not be misled. Very nonpolar solid alkenes can give false negative KMnO4 simply because they are so water insoluble (at least a little has to dissolve in order to react well). That said, this is uncommon, and if a negative KMnO4 test (using dilute, neutral KMnO4) is obtained, I assume that it does NOT have an alkene. That said, the KMnO4 test is rather prone to false positives, as a wide variety of other FGs are slowly oxidized by it, and would also (slowly) form brown MnO2 precipitate. This includes some alcohols (primary and secondary), phenols, aryl amines, some other amines, and aldehydes. As such, while a positive KMnO4 test (using dilute, neutral KMnO4) suggests the presence of an alkene, false positives are not uncommon, so it's not a slam dunk.
For the Br2/CCl4 test, alkenes with strongly electron withdrawing substituents attached to the alkene will still react, but the reaction can sometimes be so slow that it looks like non-reaction. This is true for alkenes which are conjugated with a carbonyl of some sort (ester, ketone, aldehyde, acid, etc.). While they officially still test positive, in that they decolorize the Br2 color, it can be really slow. You can compare with a "control" which you know does not contain an alkene to differentiate, but this is a hassle. And since conjugated carbonyls are not that uncommon, this apparent false negative does happen from time to time. (This same effect could also be caused by other electron withdrawing substituents, but in my experience, it most commonly occurs when a carbonyl is conjugated to the alkene.) In contrast, a non-aromatic alkene which is NOT conjugated with a carbonyl will usually decolorize the Br2 solution within a few seconds.
There are also false positives for the Br2/CCl4 test. Aldehydes and ketones which are "enolizable" (that is, attached to the C=O is an sp3 hybridized carbon with at least one hydrogen) will undergo alpha-bromination, which will use up the Br2 and decolorize the solution. Similarly, benzylic or tertiary alkyl hydrogens can undergo radical bromination, which will also decolorize the Br2 solution. These are both substitution reactions, in contrast with the addition of Br2 to an alkene, and these substitutions will form HBr gas as a byproduct. Theoretically, you can detect the formation of HBr gas by placing a moistened strip of blue litmus paper in the mouth of the test tube, but my experience has been this is extremely unreliable...
That said, in combination, the KMnO4 test (using dilute, neutral KMnO4) and the Br2/CCl4 test together make a very reliable determination of the presence of an alkene. Things which give false results for one typically are not the same FGs which give false results for the other... If you get a clear positive test for BOTH tests, you almost certainly have a non-aromatic alkene, and if you get clear negatives for both, it's very hard to imagine that an alkene is present...
The sodium carbonate test is simple -- reaction with a carboxylic acid will neutralize the carbonate and form bubbles of CO2 gas (and the acid will dissolve), This is like the "volcano" reaction of baking soda and vinegar, though the effect is usually much more subtle for larger organic carboxylic acids. Alkenes and alcohols will NOT form bubbles with Na2CO3, but a carboxylic acid will.
My expectation is that an alcohol would give negative results for ALL THREE of these chemical tests (though, as I mentioned above, I do not experientially know how alcohols would be expected to react with acidified KMnO4). As such, the acid would bubble with sodium carbonate, an alkene would give positive tests for BOTH KMnO4 and Br2/CCl4, and the alcohol would give negative results for all three tests.
The last part of the question -- "can all three reactants be used to identify all three compounds?" -- is a little weirdly phrased. I would initially say "no" in that each individual test cannot be used to identify each functional group. But if the meaning is "Can the results of all three of these tests be used to distinguish these three functional groups from one another?" then I would say "Yes" as explained above.
