Lucas' reagent

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Lucas' reagent is a solution of saturated and anhydrous zinc chloride in concentrated hydrochloric acid. This solution is used to classify alcohols of low molecular weight. The reaction is a substitution in which the chloride replaces a hydroxyl group. A positive test is indicated by a change from colourless to turbid, signalling formation of a chloroalkane.[1] The test was reported in 1930 and became a standard method in qualitative organic chemistry.[2] 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), an American chemist.

Lucas test

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:[3]

ROH + HCl → RCl + 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.

An equimolar mixture of ZnCl2 and HCl is the reagent. The alcohol is protonated by this mixture, and H2O group attached to carbon is replaced by the nucleophile Cl−, which is present in excess. 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.[3] 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 between the three classes of alcohols:

  • no visible reaction at room temperature and form an oily layer only on heating: primary, such as normal amyl alcohol (1-Pentanol)
  • solution form oily layer in 3–5 minutes: secondary, such as sec-amyl alcohol (2-Pentanol)
  • solution form oily layer immediately in tertiary alcohol , such as tert-amyl alcohol (2-Methyl-2-butanol)

References

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  3. ↑ 3.0 3.1 Lua error in package.lua at line 80: module 'strict' not found.
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