Cerium(III) chloride

Cerium(III) chloride (CeCl3), also known as cerous chloride or cerium trichloride, is a compound of cerium and chlorine. It is a white hygroscopic solid; It rapidly absorbs water on exposure to moist air to form a hydrate which appears to be of variable composition[2], though the heptahydrate CeCl3.7 H2O is known. It is highly soluble in water, and (when anhydrous) it is soluble in ethanol and acetone[4].

Preparation of anhydrous CeCl3
Simple rapid heating of the hydrate alone may cause small amounts of hydrolysis[1]. A useful form of anhydrous CeCl3 can be prepared if care is taken to heat the heptahydrate gradually to 140 °C over many hours under vacuum[4],[5],[9]. This may or may not contain a little CeOCl from hydrolysis), but it is suitable for use with organolithium and Grignard reagents. Pure anhydrous CeCl3 can be made by dehydration of the hydrate either by slowly heating to 400 °C with 4-6 equivalents of ammonium chloride under high vacuum[1],[6],[12], or by heating with an excess of thionyl chloride for three hours[1],[7]. The anhydrous halide may alternatively be prepared from cerium metal and hydrogen chloride[8].  It is usually purified by high temperature sublimation under high vacuum.

Uses
Cerium(III) chloride can be used as a starting point for the preparation of other cerium salts, such as the Lewis acid, cerium(III) trifluoromethanesulfonate, used for Friedel-Crafts acylations. It is also used itself as a Lewis acid, for example as a catalyst in Friedel-Crafts alkylation reactions.[10]

Luche reduction[11] of alpha, beta-unsaturated carbonyl compounds has become a popular method in organic synthesis, where CeCl3.6H2O is used in conjunction with sodium borohydride. For example carvone gives only the allylic alcohol 1 and none of the saturated alcohol 2. Without CeCl3, a mixture of 1 and 2 is formed.



Another important use in organic synthesis is for alkylation of ketones which would otherwise form enolates if simple organolithium reagents or Grignard reagents were to be used. For example, compound 3 would be expected to simply form an enolate without CeCl3 being present, but in the presence of CeCl3 smooth alkylation occurs[5]:



It is reported[5] that organolithiums work more effectively in this reaction than do Grignard reagents.

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