Figure 2
Switch on the stirrer, cool the flask with an ice-salt bath until the internal temperature is -10ºC
(pentane or alcohol thermometer) and add the solution/suspension of lithium aluminium hydride in
ether during [!]30 min so that the temperature remains below +10ºC. Follow the reaction by tlc on
silica plates developed in dichloromethane. If necessary add more lithium aluminium hydride in
diethyl ether (carefully weighed out in the dry fume cupboard).
When the reaction is complete, add water (3 ml) to decompose the excess of the reagent. It is
important to ensure that all of the excess reagent is destroyed before continuing (ask a
demonstrator if in doubt). Add 10% (v/v) sulfuric acid (25 ml), then separate the ether layer and
extract the aqueous layer with ether (2 x 50 ml). The combined layers are dried over anhydrous
sodium sulfate. Evaporate the ether (Rotavapor) and by distil the residue under reduced pressure to
isolate the cinnamyl alcohol (b.p. 139ºC/14 mmHg). It should crystallise when cooled, m.p.
33ºC.
Record the yield, refractive index, and nmr and infra-red spectra and interpret the spectra. Transfer
the material to a labelled tube.
If a sample fails to crystallise, purify it as follows: transfer the crude alcohol to a 250 ml conical
flask, and dissolve it in dry ether (35 ml for each 5 g). Add powdered calcium chloride (10 g) and
leave the mixture overnight under N2.
Next day collect the calcium chloride-cinnamyl alcohol solvate under N2, wash it with dry ether (20
ml) and then add it to water (25 ml). Extract the mixture with ether (3 x 25 ml) dry the extract over
solid sulfate, filter the solution and evaporate the ether (Rotavapor) to obtain crystalline cinnamyl
alcohol.
3. Disposal of Lithium Aluminium Hydride
Do this away from flames or other sources of ignition, in the fume-cupboard, and WEAR
GOGGLES OR A VISOR. Dispose of the reagent, both solid and solution, by cautiously adding
it to a solution of ethyl acetate in dry THF (1:2) under an atmosphere of nitrogen. NEVERadd
reagent residues to water or alcohols: there might be a violent explosion. Treat contaminated
apparatus with ethyl acetate. Later, after at least 1 hour, wash the apparatus with water and dilute
hydrochloric acid, and then re-rinse it with water.
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