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what is lucas test? explain in detail with help of example

Lucas Test is a test which is used to distinguish between primary, secondary and tertiary alcohols. This test is carried out with the help of Lucas reagent, which is an equimolar mixture of Zinc Chloride and hydrochloric acid (ZnCl2 + HCl). It is based on the difference between the reactivity of primary, secondary and tertiary alcohols with hydrogen halides.  The general equation for the reaction is
   ROH + HCl + ZnCl2 →  RCl + H2O

When the reagent is added to tertiary alcohol, a turbidity or cloudiness appears in solution instantaneously. This occurs because the alkyl halide so formed is not water soluble. When the reagent is added to secondary alcohol, turbidity appears after 4-5 minutes. However, with primary alcohols, no turbidity appears, suggesting that no reaction occurs with primary alcohol at room temperature.

Example

(CH3)3-OH  + HCl  +  ZnCl2 →  (CH3)3-Cl  +  H2O.................turbidity appears instantaneously
 tertiary butanol

CH3-CH(OH)-CH2-CH3  +  HCl  + ZnCl2 → CH3-CH(Cl)-CH2-CH3.......turbidity appears after 5 minutes
secondary butanol

CH3-CH2-CH2-CH2OH +  HCl  + ZnCl2 →  X........................no turbidity appears
primary butanol

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View Full Answer

The Lucas reagent contains zinc chloride and hydrochloric acid. When the Lucas reagent is allowed to react with an alcohol, an alkyl halide is formed. This alkyl halide will not be soluble in water, thus cloudiness or a second layer of droplets will form if the test is positive. With this reagent, tertiary alcohols react immediately, secondary alcohols in 3 to 10 minutes, and primary alcohols may take an hour or longer, or may never react.

R-OH + HCl R-Cl + H2O

(insoluble)

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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 hydrogenhalides

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. 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: primary, such as normal amyl alcohol (1-Pentanol)
  • solution turns cloudy in 3-5 minutes: secondary, such as sec-amyl alcohol (2-Pentanol)
  • solution turns cloudy immediately, and/or phases separate: tertiary, such as tert-amyl alcohol (2-Methyl-2-butanol
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