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].

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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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