Esterification, HX and dehydration

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.