Lucas test: which tube clouds first?

Three tubes of 1°, 2° and 3° alcohol in Lucas reagent on a running clock; turbidity appears at rates set by how easily each forms a carbocation.

Identify the unknown alcohol that turns cloudy within a minute at room temperature.

Primary alcohols react fastest because they are least hindered.

• Esterification: an alcohol or a phenol gives an ester with a carboxylic acid, an acid anhydride or an acid chloride. With acids or anhydrides a little concentrated sulphuric acid is added, and water is removed as it forms because the reaction is reversible.
• With an acid chloride the reaction is run in the presence of pyridine, a base that neutralises the HCl formed and shifts the equilibrium forward. Acetylating salicylic acid gives aspirin, an analgesic, anti-inflammatory and antipyretic.
• Reactions that break the C–O bond are shown by alcohols; phenols undergo this only with zinc. Alcohols react with hydrogen halides to give alkyl halides.
• Lucas test: the reagent is concentrated HCl with ZnCl₂. Alcohols dissolve in it, but the alkyl chlorides formed do not, so the mixture turns cloudy. A tertiary alcohol gives turbidity at once; a primary alcohol gives none at room temperature.
• Phosphorus trihalides convert alcohols into alkyl halides; PBr₃ gives alkyl bromides.
• Dehydration: alcohols lose water to form alkenes when heated with a protic acid (concentrated H₂SO₄ or H₃PO₄) or with catalysts such as anhydrous zinc chloride or alumina. Ethanol with concentrated H₂SO₄ at 443 K gives ethene.
• Ease of dehydration is tertiary > secondary > primary, because tertiary carbocations form most easily. Milder conditions suffice for the higher classes: propan-2-ol gives propene with 85% H₃PO₄ at 440 K, while 2-methylpropan-2-ol gives 2-methylpropene with only 20% H₃PO₄ at 358 K.
• Mechanism for ethanol: the alcohol is protonated; the protonated alcohol loses water to give a carbocation, the slow, rate-determining step; the carbocation loses H⁺ to give the alkene.
• The acid used in the first step is given back in the last step, and ethene is removed as it forms so the equilibrium keeps moving to the right.

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