Date: Sun Mar 06 2005 - 09:36:46 EST
I recently did my first Grignard reaction in the lab (1-bromobutane, magnesium
turnings and 4-methylpentan-2-one; the expected product is
2,4-dimethyloctan-4-ol) and have got product(s) which change(s) colour on
heating. The product is an oily liquid, b.p. 68-70 C/16mmHg (in the expected
range), nD20=1.441, and is dark green at room temperature, orange between 40-90
degrees (approx) and red above 90 degrees. (The colourant is not stable; in
the last week the green colour has faded to yellow.) The product(s) show a
significant O-H absorption in the IR spectrum, but it/they also decolourise
bromine water despite not showing any definite C=C stretches IR.
Unfortunately, it is not practical to carry out a GC of the product(s), as 2+
hours of retention time are required.
The reaction was carried out under reflux in rigorously dry glassware with
sodium-dried diethyl ether as the solvent and iodine as the oxidising agent for
the magnesium. The reaction mixture was given an aqueous work-up with
hydrochloric acid, extracted into diethyl ether and dried (with the ether being
removed by rotary evaporation) and distilled under reduced pressure using a
Claisen flask, with three RB flasks being used to receive the distillation
products (the first one for products below the expected b.p., the second one
for products at the expected b.p., the third for products above the expected
b.p.). The flasks had to be changed manually, which is probably the greatest
source of user error in the whole experiment.
I admit I did not put any alundum grains (anti-bumping granules) in the flask
while attempting to distil the product, so it boiled rather violently and some
of the distillate mixed with the products.
The question I want to ask is: does anyone have any idea what's causing the
colour changes? I'm really asking the question out of curiosity, as I have
never come across an organic compound which changes colour on heating.
1st year undergraduate
School of Chemistry
University of Leeds
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