From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem
Subject: Re: Transesterification at home
Date: Thu, 7 May 1998 08:24:06 +12

hom@nature.berkeley.edu (Louis Hom) wrote:
>I'd like to try the pineapple scent synthesis mentioned in another
>thread.

As I noted when I suggested it, I've previously posted details,
which should be available at DejaNews, however as you want to
perform it at home, I'll try to expand the detail and choices.

>Do I need to start with clarified butter?  Do I need to hydrolyze the
>triglycerides first (e.g., w/ strong base)?  Do I need to dissolve my
>reagents in anything?  What should I use to catalyze exchange (like,
>do I need to make ethanoate)?

Clarified butter is preferred, and some shops also sell anhydrous
milk fat for confectionary that is very suitable. Note that most
other fats don't contain butyric acid, and so will not produce
pineapple aroma. Basically, you want to turn the triglycerides into
ethyl esters of their component fatty acids without worrying about
the nausea of soap formation, so reducing water content in the
fat and alcohol is highly desirable. I assume the reaction will
work fine with denatured alcohol/methylated spirits - but I've
not tried technical grade solvents.

The easiest way to transesterify triglyceride fats and oils is using
an ethanolic solution of a catalyst  - common ones are sodium
hydroxide, potassium hydroxide, sodium methoxide, and potassioum
carbonate. As the hydroxides are usually readily available in pharmacy
or hardware stores , let's work with them, but the others are also
effective. The main difference is the reaction rate, and the
undesirable tendency to form soaps - with the hydroxides making the
most. Any water present will remove the catalyst as soap, thus slowing
the reaction ( unless additional excess catalyst is added ) and
possibly introducing emulsion problems. Most of the catalysts are
corrosive to people, so should be handled with appropriate
precautions - no skin contact. Safety glasses and labcoats should
be worn when playing with volatile and corrosive chemicals.

The easiest method is to dissolve the catalyst in alcohol ( ethanol
for ethyl esters, methanol for methyl esters, propanol for propyl
esters etc. ) - usually at concentrations of about 8-10 % w/v.
The fastidious amongst you can make up 2 molar solutions for both
the catalyst and the acid ( but potassium carbonate may not be
soluble at that concentration ) so the process can be simplified
and easily replicated. Note that these catalysts will not convert
any free fatty acids to esters using the suggested conditions,
however only rancid butter should have significant levels of
free fatty acids.

Small scale experiments at ambient temperature can be completed
in 10 minutes, and are good screening tests. Place a couple of
drops of the fat in a stoppered test-tube or 4ml screw cap vial,
add about 1-2 mls of a light hydrocarbon to dissolve it ( iso-octane,
pentane or hexane fraction, 40-60C or 60-80C petroleum spirit
- don't use normal hexane - which is much more toxic, and remember
these are all flammable solvents ). If possible, use pentane or
40-60C fraction if you wish to later remove the solvent and obtain
an ester fraction without solvent.

Add a couple of drops of your ethanolic catalyst solution, stopper,
and shake vigorously for about 5-10 minutes to ensure that any
separate phases mix intimately. Remember to vent the stoppered
tube regularly to avoid pressure, don't heat the tube, and open
cautiously to avoid caustic splashes.  Add a drop of pH indicator
solution ( methyl red, methyl orange - usually in 50% aqueous
alcohol ) and carefully add 2N aqueous hydrochloric acid dropwise,
shaking after each addition, until the lower aqueous phase
just turns acid ( red ). The tube should now smell strongly of
pineapple, and the two phases should cleanly separate on standing.

You can decant off the upper hydrocarbon layer, and warm the
hydrocarbon layer carefully with warm water to slowly evaporate the
hydrocarbon off ( ethyl butyrate boils at 120C, and will mostly
be retained with the higher esters ). Alternatively placing the
solution in a beaker or dish will allow most of the solvent to
evaporate at ambient temperature. You will end up with a
clear gold liquid that has a very strong pineapple smell. Note
that the liquid should not be tasted, because there can be some
residual acids, and other nasties, still present.

To make larger quantities, you can scale up the above, remembering
that good mixing of the phases is critical to drive the reactions
- a vortex mixer is a great help. A good scale is 1 gram of fat,
10 mls of hydrocarbon solvent, 0.5 ml of 2 Molar hydroxide in
alcohol. Mix vigorously for 6-8 minutes, add 2-3 drops of
indicator and add 0.5 ml of 2 Molar aqueous hydrochloric acid
and mix. Add any necessary extra acid dropwise with mixing.

Alternatively, you can move to preparing the esters without
additional hydrocarbon solvents by using heat and refluxing
with an excess of alcohol and about 0.5-1% of catalyst,
( eg 70g fat and 30g ethanol containing 0.5-1g of catalyst ).
Whilstt this is good ttechnique for making larger volumes,
it takes longer and usually requires a hot plate and/or
reflux condensor as reactions usually require 60+C
temperatures to proceed at acceptable rates. Certainly it
can also be performed much faster in a heating block at
higher tempearatures ( 80-100C ) in a screw-cap reaction tube
- but appropriate safety precautions are needed, and it
shouldn't be attempted at home.

Dissolve the catalyst in the alcohol first, warm both fat and
catalyst solution to 60+C, add the fat to the alcoholic
catalyst solution and stir well for about 15-30 minutes
whilst still heating under a reflux condensor. Add an
approximately-equal volume of hot water to stop the reaction
and to dissolve the soap, alcohol, and catalyst ( using very gentle
swirling to mix, otherwise emulsions appear ).

Allow the layers to separate, and run off the lower layer to waste
before it cools. If you want a cleaner ester fraction, you can
rinse the upper ester layer with fresh warm water, still using
gentle swirling, Repeat the washing step several times, each time
providing more vigorous agitation.

Should you kill yourself whilst attempting the above, and you end
up in Hell, then ( if there is any justice ), you should find plenty
of ISPs - and you can email me details of safety instructions that
I accidently omitted.

      Bruce Hamilton