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:
- 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)
Read more about this topic: Lucas' Reagent
Famous quotes containing the words lucas and/or test:
“The beast exists because it is stronger than the thing that you call evolution. In it is some force of life, a demon, driving it through millions of centuries. It does not surrender so easily to weaklings like you and me.”
—Martin Berkeley, and Jack Arnold. Lucas (Nestor Paiva)
“Preoccupation with money is the great test of small natures, but only a small test of great ones.”
—Sébastien-Roch Nicolas De Chamfort (1741–1794)