Differentiation between primary, secondary, and tertiary alcohols (Lucas test)
Differentiation between primary, secondary, and tertiary alcohols
(Lucas test).
The test depends upon the different rates of formation of the alkyl chlorides upon treatment with a hydrochloric acid – zinc
chloride reagent * (containing 1 mole of acid to 1 mole of anhydrous zinc chloride) and with hydrochloric acid. It applies only to aliphatic and cycfoaliphatic alcohols.To 1 ml. of the alcohol in a small test-tube, add quickly 6 ml. of Lucas’
reagent at 26-27°, close the tube with a cork, shake, and allow to stand. Observe the mixture during 6 minutes. The following results may be obtained:—
* Lucas* reagent is prepared by dissolving 68 g. (0-5 mole) of anhydrous zinc chloride (fused sticks, powder, etc.) in 62-6 g. (0-6 mole) of concentrated hydrochloric acid with cooling to avoid loss of hydrogen chloride.
(a) Primary alcohols, lower than hexyl, dissolve ; there may be some darkening, but the solution remains clear.
(b) Primary alcohols, hexyl and higher, do not dissolve appreciably ; the aqueous phase remains clear.
(c) Secondary alcohols : the clear solution becomes cloudy owing to the separation of finely-divided drops of the chloride A distinct upper layer is visible after one hour except for iso-propyl alcohol .
(d) Tertiary alcohols : two phases separate almost immediately owinG to the formation of the tertiary chloride . If a turbid solution is obtained, suggesting the presence of a secondary alcohol but not excluding a tertiary alcohol, a further test with concentrated
hydrochloric acid must be made. Mix 1 ml. of the alcohol with 6 ml. of concentrated hydrochloric acid, and observe the result : —
(e) Tertiary alcohols : immediate reaction to form the insoluble chloride which rises to the surface in a few minutes.
CRYSTALLINE DERIVATIVES OF ALIPHATIC ALCOHOLS
The acid chloride is available commercially, but it is more economical to
prepare it from the acid as and when required. Furthermore, 3 : 5-dinitrobenzoyl
chloride tends to undergo hydrolysis if kept for long periods,
particularly if the stock bottle is frequently opened. The substance may,
however, be stored under light petroleum.
Method
. Mix 1-0 g. of 3 : 5-dinitrobenzoic acid with 4 ml. of thionyl chloride in a dry 50 ml. conical flask ; fit a reflux condenser, carrying a plug of cotton wool at the upper end, into the flask and heat on a water bath for 15-30 minutes, Remove the condenser and heat the flask in a boiling water bath (until the excess of thionyl chloride has evaporated. Use the resulting 3 : 5-
dinitrobenzoyl chloride (about 1-0 g.) immediately. Add 0-5-1 ml. of the alcohol, cork the flask loosely, and heat on a water bath for 10 minutes : secondary and tertiary alcohols require longer heating (up to 30 minutes). Cool the mixture, add 10 ml. of 5 per cent, (or saturated) sodium bicarbonate solution, break up the resulting solid ester with a stirring rod (alternatively, stir until crystalline), and filter at the pump ; wash with a little sodium bicarbonate solution,
followed by water, and then suck as dry as possible. Dissolve the crude
In: Name Reactions · Tagged with: ALCOHOLS