Distinguishing Primary, Secondary and Tertiary Alcohols: the Lucas Test

Because primary, secondary and tertiary alcohols often look deceptively similar on paper, a

simple, fast chemical test that tells them apart by how quickly they react is extremely useful, and

the standard such test uses the Lucas reagent -- a mixture of concentrated hydrochloric acid

and anhydrous zinc chloride ($\text{ZnCl}_2$), the latter acting as a Lewis-acid catalyst.

The reaction. All three classes of alcohol react with the Lucas reagent to give the

corresponding alkyl chloride, which -- unlike the starting alcohol -- is insoluble in the aqueous

reagent, so its formation is visible as a turbidity or cloudiness, and eventually as a separate

oily layer, in the originally clear solution:

Why the rate differs so sharply. The zinc chloride first coordinates to a lone pair on the

alcohol's oxygen, converting the poor leaving group $-\text{OH}$ into a much better one; the

alcohol then ionises to a carbocation, exactly as in acid-catalysed dehydration, before

$\text{Cl}^-$ captures the carbocation ($S_N1$ mechanism). Because this pathway's rate-determining

step is carbocation formation, it is governed directly by carbocation stability, $3° > 2° > 1°$ --

so a tertiary alcohol, which forms a comparatively very stable tertiary carbocation almost

instantly, reacts with the Lucas reagent immediately at room temperature (turbidity appears within

seconds to a minute or two, with no heating needed). A secondary alcohol forms a less stable

secondary carbocation more slowly, so turbidity typically takes about five to ten minutes to

appear at room temperature. A primary alcohol would have to form the least stable, primary

carbocation, which essentially does not happen readily under these conditions; instead it reacts,

if at all, by a slow, direct $S_N2$ displacement, and gives no visible turbidity at room

temperature at all -- reaction only proceeds, slowly, on heating.

Using the test. In practice, then, a chemist adds the Lucas reagent to an unknown alcohol at

room temperature and simply watches: immediate cloudiness identifies a tertiary alcohol; cloudiness

after several minutes identifies a secondary alcohol; no visible change at room temperature …