"Lucas' reagent" is a solution of anhydrous

zinc chloride Zinc chloride is the name of inorganic chemical compounds with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water. This salt is hygroscopic and e ...

in concentrated

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

. This solution is used to classify alcohols of low molecular weight. The reaction is a substitution in which the chloride replaces a

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...

group. A positive test is indicated by a change from clear and colourless to turbid, signalling formation of a chloroalkane. 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 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 halides via an S_N1 reaction: : tert-BuOH + HCl → tert-BuCl + H₂O The differing reactivity reflects the differing ease of formation of the corresponding carbocations. 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 HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a spe ...

is the reagent. The alcohol 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 Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

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 straigh ...

* 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 alcohol ...

* solution forms an oily layer immediately: tertiary, such as

2-methyl-2-butanol ''tert''-Amyl alcohol (TAA) or 2-methylbutan-2-ol (2M2B), is a branched pentanol. Historically TAA has been used an anesthetic and more recently used as a recreational drug. TAA is mostly a positive allosteric modulator for GABAA receptors in ...

References

{{reflist Chemical tests Analytical reagents