Chemistry · Ch 10 — Halogen Derivatives
From alcohol
From alcohol
The hydroxyl group of an alcohol can be replaced by a halogen atom in three ways. (a) Using a halogen acid (HX): the conditions needed depend on both the alcohol's structure and the acid used, and the order of reactivity of alcohols towards a given haloacid is tertiary > secondary > primary (R-OH + HX, under suitable conditions, gives R-X + H2O). Hydrogen chloride is used together with zinc chloride (the Grooves' process, using Lucas reagent) for primary and secondary alcohols, while tertiary alcohols react readily with concentrated HCl alone, without needing zinc chloride. Constant-boiling (48%) hydrobromic acid is used to make alkyl bromides directly; primary alkyl bromides can also be made from NaBr + H2SO4, which generates HBr in situ. Alkyl iodides are best made by heating the alcohol with sodium or potassium iodide in 95% phosphoric acid (NaI/H3PO4 generates HI in situ) rather than with HI itself, partly because phosphoric acid does not oxidise the iodide ion the way concentrated sulphuric acid would. (b) Using a phosphorus halide: 3R-OH + PX3 -> 3R-X + H3PO3 (the trihalide is usually generated in situ from red phosphorus and Br2 or I2), while phosphorus pentachloride reacts as R-OH + PCl5 -> R-Cl + HCl + POCl3. (c) Using thionyl chloride: straight-chain primary alcohols react as R-OH + SOCl2 (he …
Worked out. During replacement of -OH by -X, alcohols -- especially secondary and tertiary ones that can form a relatively stable carbocation -- sometimes undergo skeletal rearrangement partway through the reaction, giving an alkyl halide with a different carbon skeleton than expected. This tendency is minimised by using a phosphorus halide instead of the corresponding halogen acid: straight-chain primary alcohols treated with a phosphorus trihalide reliably give the unrearranged alkyl halide, because the mechanism does not pass through a free carbocation intermedi …