Distinction of primary, secondary and tertiary alcohol by Victor Mayer's Method

(a) Oxidation test
(b) Lucas test
(c) Victer meyer test (best method)
(d) Catalytic dehydrogenation by Cu/300°C.


(a) Oxidation with sodium Dichromate (Na2Cr2O7)
Primary and secondary alcohols are easily oxidized by reagents containing chromium (VI). For low-molecular-weight alcohols that are water-soluble, water solutions of sodium dichromate in the presence of acid are used. A primary alcohol such as
n-butyl alcohol is oxidized first to an aldehyde, which is oxidized further by the same reagent to a carboxylic acid.




This process is not of practical use for preparing aldehydes from primary alcohols because it undergo further oxidation (except for a few cases where the aldehyde that is formed has a boiling point low enough that it can be distilled out of the reaction mixture).
A secondary alcohol, for example
sec-butyl alcohol, is oxidized to a ketone, which usually does not undergo further oxidation due to absence of reducing hydrogen. For a ketone to be oxidized, carbon-carbon bonds would have to be broken because there are no more carbon-hydrogen bonds available on the carbonyl carbon atom.



The tertiary alcohol
tert-butyl alcohol is not oxidized by acidic sodium dichromate solution. The carbon atom bearing the hydroxyl group in this alcohol does not have a hydrogen atom on it. Here, too, carbon-carbon bonds would have to be broken to allow oxidation to take place.




The oxidizing solution in the above examples, an acidic solution of sodium dichromate, contains a variety of species in equilibrium. Most important of these is the acid chromate ion, HCrO
4¯.




(b) Lucas test: Alcohols react with concentrated hydrochloric acid in presence of anhydrous zinc chloride to form alkyl halides.



The three types of alcohols undergo this reaction at different rates. The rates of reaction with
Lucas reagent

[conc. HCl + ZnCl
2 (anhydrous) follow the following order:

Tertiary alcohol > Secondary alcohol > Primary alcohol


Lucas test is based on this order because formation of intermediate carbocation takes place. An unknown alcohol (monohydric) is mixed with conc. HCl and ZnCl
2
at room temperature. The alkyl chloride formed is insoluble in the medium; thus the solution becomes cloudy before it separates as a distinct layer.

The following observations are made:

(a) If cloudiness appears immediately, the alcohol is tertiary.

(b) If cloudiness appears within 5–10 minutes, the alcohol is secondary.

(c) If the solution remains clear, i.e., on cloudiness is formed the alcohol is primary.

Note:
Primary alcohols do not react with Lucas reagent at room temperature. It requires high temperature.

(c) Victor Meyer’s test:
In this test, the following steps are involved:

(i) Alcohol is reacted with conc. HI or red phosphorus and iodine to form the corresponding alkyl iodide.

(ii) Alkyl iodide is treated with silver nitrite when corresponding nitroalkane is formed.

(iii) Nitro alkane is treated with nitrous acid (NaNO
2+ H2SO4) and the solution is made alkaline by addition of excess of caustic soda.

1 comment:

ishwar tiwari said...

Oh!, Thank you very much!