I read about the Lucas test for alcohols in which a cloudy precipitate is produced with a tertiary or secondary alcohol. However, on a side note it was mentioned that this test is useful only for compounds having less than six carbon atoms. Why is this so?
$\begingroup$
$\endgroup$
1
-
1$\begingroup$ This "degre" nomenclature for alcohols is terribly outdated. Profs teaching that today are really old and have learned it from teachers which were already oldfashioned then. $\endgroup$– KarlAug 14, 2019 at 5:51
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
0
The problem is that the alcohols themselves might be insoluble. Ethanol is soluble, yes, but as the alkyl chain length increases, the hydrophobic nature due to the chain starts dominating over whatever hydrophilicity the $\ce{-OH}$ group had been providing for lower homologues.
From source [1]:
.. any alcohol that is insoluble in the reagent might appear to be giving a positive 3° test.
The paper goes on to properly quantify several things:
Based on the results of this study, using 1 drop of alcohol added to 10 drops of reagent, it can be said that, at the very least, all saturated acyclic monofunctional alcohols having six or fewer carbons are soluble in the Lucas reagent.
It is clear that the relative amounts of the sample and the reagent become important in such borderline cases (~6 carbon long chains). If too much alcohol is added, it may cause an illusion of positive lucas test.
From source [1]: (emphasis mine)
That statement may not be true when 3 or 4 drops are used per 10 drops of reagent. In fact, sometimes when 10 drops of reagent are added to 3 or 4 drops of some of the alcohols, an interesting event occurs after a delay of a couple of minutes or so. Apparently, some undissolved alcohol that has been clinging to the bottom of the test tube suddenly breaks loose and, by streaming to the surface of the very dense Lucas reagent, gives a momentary illusion of a positive test. The real cloudiness, alkyl chloride formation, appears later—all the more reason to use a high reagent/alcohol ratio.
References:
[1] "A study of the Lucas test" R. A. Kjonaas and B. A. Riedford J. Chem. Ed. 1991 68 (8), 704 DOI: 10.1021/ed068p704
answered Aug 13, 2019 at 16:51
$\begingroup$
$\endgroup$
3
To get a cloudy "precipitate", you need to dissolve the alcohol in the acid solution. Alcohols get less soluble in water as their chain length increases. By the time you pass 6 carbons, the alcohol itself is more or less insoluble. If it reacts along the interface, the chloride product would just mix into the alcohol layer, with no visible change.
-
1$\begingroup$ Please share the reference you've used $\endgroup$– user78585Aug 13, 2019 at 16:33
-
$\begingroup$ I had no specific reference in mind, just general knowledge. The other answer is more complete than anything I had in mind. $\endgroup$– jeffBAug 13, 2019 at 19:34
-
1$\begingroup$ no problem still I appreciate your efforts $\endgroup$– user78585Aug 14, 2019 at 1:14