We do fatty acid methyl esters, and use a procedure adopted from the AOCS method which uses BF3 in NaOH/Methanol.

I need to conduct some training on the procedure, and would like to explain the esterification mechanism. Dusting off my old organic chemistry textbook, I see discussion related to esterification in an acidic medium - basically heating alcohol and acid under reflux with acid.

The AOCS method involves heating in alkaline conditions with alcohol in the presence of BF3 catalyst.

Can anyone describe the mechanism of this esterification reaction?

Thanks,

Mike

FA analysis and GC had to be phased out here, but something of this sort still kindles strong interest. Are you really sure this is not NaOH/MeOH (to solvolize FA ester lipids) followed by BF3/MeOH (to reesterify to FAME)??
Base can only give rise to ester dissociation. Acid does both, ester dissociation and esterification, so here you get an equilibrium between ester and the acid + alcohol. One would think that under basic conditionds one would have too little BF3 (A "Lewis" acid). BF3X- (and possibly breakdown products) can not do the necessary acid catalysis.

The first step is heating the fatty acid in 0.5N NaOH in Methanol. I imagine that this step deprotonates the acid.

The next step is to heat with 15% BF3 in methanol. Somehow the BF3 catalyzes the reaction - I imagine acting as a lewis acid.

The FAMEs are then separated into isooctane and are now ready for analysis.

I just want to be able to explaint this mechanism clearly during my training.

I appreciate the help,

Mike

OK, the world is still normal, steps are involved. The quoted method is then, obviously, for esterified lipids.

If you have FA only, no esterified lipids, the first step (NaOH/MeOH) is absolutely unnecessary, rather counter-productive.

H. W. is correct: for soaps or fatty acids, go directly to BF3-methanol (or H2SO4-methanol) step. For stuff like triglycerides, first saponify with 0.5N NaOH-methanol or 0.5N KOH-methanol on steam bath half-hour, then acidify with BF3-methanol (or H2SO4-methanol) and continue. I work for a large soap manufacturer.

If you are evaluating the cis/trans ratios of your fatty acids or triglycerides then minimize the amount of heating in acid or base or you will have isomer tranformations occurring.

One interesting method of creating methyl esters of the Fatty Acids in triglyceride linkages, is to use the organic base method. Tetramethylammonium hydroxide in methanol will transesterify the lipids into the methyl esters at unelevated temperatures in a short time.

I have used this method to analyse trierucin which I found had 13% of the trans isomer present in what had been sold as 99% cis.

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com

A long time back we came to the conclusion that the best method to convert FA to FAME was with diazomethane, all claims that double bond isomerization occurs turned out to be a myth, apparently based on the wrong interpretation of carbene reactions. I asked questions about Tetramethylammonium hydroxide in several discussions, because we were unhappy with its performance. It turned out that we were not the only ones. So, Rodney, are you sure you actually were not the one that inadvertendly created trans FA with TMAH? If I recall correctly than most, if not all, of the TMAH reaction takes place in the injector at ~300°.

Hans

Hans,

Thanks for your comments.

FYI Free Acids will NOT be esterified by TMAH in methanol at ambient or low temperature conditions.

Only glyceride linkages will be TRANS-esterified by the TMAH.

This procedure was used when the free acid content of a glyceride product was not to be included in the FA content of the glyceride.

Using BF3 in Methanol we found that extended heating (>15 minutes) would begin to see a tiny amount of conversion of the cis isomers to the trans isomers in high purity fatty acid mixes.
Polyunsaturated FAs in fish oils (ie, linolenic, arachidonic)would suffer excessive oxidation reactions if heated @ 100°C for more than a necessary time for esterification.

This BF3 procedure will transesterify esters to their methyl ester form as well as esterify free acids.

It is not so effective with benzoic acids, if I recall correctly.

I agree diazomethane is the superior derivitization procedure. However, allowing unexperienced chemists to use this possibly dangerous procedure in a high pressure business quick production environment was not the best for everyone involved at the time.

In my limited understanding of the process of isomer conversion, the primary cause is temperature and the time of the heating. While the cis is more stable form the equilibration can be pushed to the trans side by temperature in the short term.

That is why although the synthesis of the trierucin used 99.5% erucic acid, after esterification of the glycerol, 13% of the cis FA content had been transformed into the trans isomer. I was shocked and the manufacturer was also. They checked and confirmed my finding and replaced the out of spec material.

I also performed experiments in 1981 using 99.8% oleic acid and found that heating @ 100°C would convert >99% of the FA to the methyl ester using BF3 in Methanol in SIX minutes.

Best wishes,

Rodney George

Hans,

May I add a comment?

The injector derivitization use of TMAH was implemented by most people using too high a concentration of acid. It was also highly variable in its results due possibly to the changes in speed of vaporization and the direction of the spray from the needle. The cleanliness of the injection liner also seemed to bear on the results. I NEVER liked it or used it to esterify free acids if I had a choice.

Thanks again for your comments.

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com

Rodney, thanks for your interesting comments, especially your last post seconds our experience. On erucic acid: If I recall correctly itīs an open chain C22:1, therefore one should expect the trans to be more thermodynamically stable. Unless there are some constraints, the trans is usually the more stable, thus the hydrogenation (hardening) of oils was often accompanied by trans FA formation. (Equilibrium will be almost entirely on the trans side for most unsaturated FA)

Hans,

Tell me if you know the answer to this question, please. (or anyone else, too)

If I remember correctly the trans isomer IS preferred thermodynamically,
but
the cis isomer is the more common isomer found in nature.

I seem to remember an article somewhere that discusses the activitation energy to get to the trans isomer is higher than the activation energy to get to the cis isomer, and that was the reason that a pure trans isomer will slowly turn to the cis with time, but yet the cis isomer can isomerize to the trans with heating.

Wish my memory bank wasn't so bankrupt.

Oh, I did remember that there was a catalyst required in the use of TMAH in methanol to transesterify mono-/di-/triglycerides.

I believe it was ethyl ether.

Thanks.

Rodney George

On cis in nature: OK, thatīs life, it is highly counter-thermodynamic, entropy decreasing.....

On the activation energy: The activation energy of going from trans to the activation state is higher than for the same step for the cis isomer, simply because the cis isomer is at a higher energetic state than the trans (so closer to the activation energy, hence its activation energy is lower). Your memory just played a tiny trick, my memory often hits me harder than that. (This stuff is "in the blood", being an old physical organic chemist)

On the catalyst:
Rodney, you are talking about the room temp. (or so)reaction?

Hans,

Yes you are right, I am thinking of the available energy at room temperature to do the conversion from one to the other. It is easier based on the amount of energy at room temp (what small trace of a mole has the higher kinetic energy) to reach the activation point from the cis form than the trans, since the energy state of the trans is lower.

Thinking out loud using a keyboard can be quite embarrassing using a neural net that has suffered the exposure of thousands of chemicals for so many years. Some of the connections may have become oxidized.

Concerning the reaction temperature:

The temperature used was either 20°C or 37°C.

I think I used both but the 20°C gave better results, and of course, was easier.

After transesterification was complete (and I don't remember if it was an hour or overnight or both) a saturated aqueous NaCl solution was introduced. The resulting ether layer was removed to a clean dry glass tube and a portion of hexane was added to encourage the remaining water to attach itself to the glass and not stay in the ether solution.

This hexane ether solution was then injected into the high resolution capillary GC.

My neural net connections do require repeated access attempts sometimes for complete connectivity to be reached.

Of course, if I had gone out to the attic over the garage and found my old lab notebook, it would have been more accurately presented the first time, but that is another health hazard assessment story in itself.

Rodney George

Thanks for the prep details.
I know too well what you are talking about on those neural connections.

Hans

Hello,

I am trying to strip oleic acid from lithium metal. The oleic acid is used in the dispersion of the metal. I need to convert the oleic acid to methyl oleate after I have recovered residual oleic acid from the metal. GC is the analytical technique I will be using for analysis. Please tell me what is the best, easist and safest procedure to perform this conversion. I am not familiar with this procedure at all so details are greatly appreciated.

Thanks,

Stacie