What is Lucas reagent 1

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The Lucas sample (also Lucas test) is a detection reaction in organic chemistry to differentiate between primary, secondary and tertiary alcohols. Depending on the position of the hydroxyl group in the alcohol molecule, alcohols react with nucleophilic substitution at different speeds or not at all. Since the alcohol is completely in the added Lucas reagent must solve, the test is limited to alcohols with fewer than six carbon atoms[1].

Table of Contents

The reagent


The Lucas reagent - named after Howard J. Lucas - is a solution of zinc chloride in concentrated hydrochloric acid; The hydrochloric acid is a reactive component and zinc chloride serves as a catalyst. This mixture was already before the publication of his test by Norris and Taylor[2] has been used for other purposes, as Lucas himself writes[3].

execution


To carry out the Lucas test, a few drops of the alcohol to be tested are placed in the Lucas reagent and shaken. The test is positive if a milky-cloudy liquid forms. This happens at different speeds, depending on the position of the hydroxyl group:

  • Primary alcohols: reaction only when the sample is heated[1]
Examples: ethanol or 1-propanol
  • Secondary alcohols: reaction within a short time (about five minutes)
Examples: 2-propanol or 2-butanol
  • Tertiary alcohols: instantaneous reaction
Example: 2-methyl-2-propanol.

The reaction time is longer, the smaller the ratio of reagent to alcohol and the lower the temperature.

Explanation


Low molecular weight alcohols dissolve very well in water and also in the Lucas reagent. Depending on the position of the hydroxyl group, the alcohols react with the reagent by substituting the OH group with a chlorine atom. The inert primary alcohols do not react noticeably and the solution remains clear. Secondary alcohols react in a few minutes, and the resulting chlorinated hydrocarbon initially clouds the solution and then separates out as a water-insoluble phase. The much more reactive tertiary alcohols instantly show turbidity and phase separation:

Exceptions


An exception is allyl alcohol, in which the reaction - like the tertiary alcohols - occurs immediately. In addition, the resulting allyl chloride is im Lucas reagent soluble, so that the necessary cloudiness does not occur.

Phenols give a negative result because neither an SN2 reaction - this requires a nucleophilic attack from the back, i.e. from the inside of the ring - still SN1 reaction - a phenyl cation is not formed under these conditions - is possible.

Individual evidence


  1. abPaula Y. Bruice and Thomas Lazar, Organic chemistry (2007), p. 502 ISBN 978-3-8273-7190-4
  2. ↑ Norris and Taylor, J. Am. Chem. Soc46, 753 (1924)
  3. ↑ H.J. Lucas,J. Am. Chem. Soc52, 802 (1930)

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Categories:Detection reaction | Name reaction




Status of information: 11/23/2020 6:25:42 AM CET

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