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Why is the Lucas test not recommended to differentiate higher alcohols?
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Michele Cini
Why is the Lucas test not recommended to differentiate higher alcohols?
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.
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.
I had no specific reference in mind, just general knowledge. The other answer is more complete than anything I had in mind.More
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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
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. RiedfordJ. Chem. Ed.1991 68 (8), 704DOI: 10.1021/ed068p704
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.
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.
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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]:
The paper goes on to properly quantify several things:
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)
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
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]:
The paper goes on to properly quantify several things:
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)
References:
[1] "A study of the Lucas test"R. A. Kjonaas and B. A. RiedfordJ. Chem. Ed. 1991 68 (8), 704DOI: 10.1021/ed068p704
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