"Lucas' reagent" is a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution is used to classify

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

s of low molecular weight. The reaction is a

substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord for a related one within a chord progression * Substitution (poetry), a variation in poetic scansion * "Substitution" (song), a 2009 song by Silversun Pi ...

in which the chloride replaces a hydroxyl group. A positive test is indicated by a change from clear and colourless to turbid, signalling formation of a

chloroalkane An organochloride, organochlorine compound, chlorocarbon, or chlorinated hydrocarbon is an organic compound containing at least one covalently bonded atom of chlorine. The chloroalkane class (alkanes with one or more hydrogens substituted by chlor ...

. Also, the best results for this test are observed in tertiary alcohols, as they form the respective alkyl halides fastest due to higher stability of the intermediate tertiary carbocation. The test was reported in 1930 and became a standard method in qualitative organic chemistry. The test has since become somewhat obsolete with the availability of various spectroscopic and chromatographic methods of analysis. It was named after Howard Lucas (1885–1963).

Lucas test

The Lucas test in

alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...

is a test to differentiate between primary, secondary, and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...

s via an S_N1 reaction: : tert-BuOH + HCl → tert-BuCl + H₂O The differing reactivity reflects the differing ease of formation of the corresponding

carbocation A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...

s. Tertiary carbocations are far more stable than secondary carbocations, and primary carbocations are the least stable(due to hyperconjugation). An equimolar mixture of ZnCl₂ and concentrated HCl is the reagent. The

is protonated, the H₂O group formed leaves, forming a carbocation, and the nucleophile Cl⁻ (which is present in excess) readily attacks the carbocation, forming the chloroalkane. Tertiary alcohols react immediately with Lucas reagent as evidenced by turbidity owing to the low solubility of the organic chloride in the aqueous mixture. Secondary alcohols react within five or so minutes (depending on their solubility). Primary alcohols do not react appreciably with Lucas reagent at room temperature. Hence, the time taken for turbidity to appear is a measure of the reactivity of the class of alcohol, and this time difference is used to differentiate among the three classes of alcohols: * no visible reaction at room temperature and forming an oily layer only on heating: primary, such as

1-pentanol 1-Pentanol, (or ''n''-pentanol, pentan-1-ol), is an alcohol with five carbon atoms and the molecular formula C5H11OH.CRC Handbook of Chemistry and Physics 65Th Ed. 1-Pentanol is a colourless liquid with a distinctive aroma. It is the straight-ch ...

* solution forms oily layer in 3–5 minutes: secondary, such as

2-pentanol 2-Pentanol (IUPAC name: pentan-2-ol; also called ''sec''-amyl alcohol) is an organic chemical compound. It is used as a solvent and an intermediate in the manufacturing of other chemicals. 2-Pentanol is a component of many mixtures of amyl alcoho ...

* solution forms an oily layer immediately: tertiary, such as 2-methyl-2-butanol

References

{{reflist Chemical tests Analytical reagents