Re: ORGLIST: mechanism_of_reaction?

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From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Thu May 10 2001 - 07:26:27 EDT


Hello Jacek,

I prefer to reply to your comments through ORLIST, where your initial
querry was placed. This may induce others to add their knowledgable
contributions.

1. Let me start where you say: "The intensity of the colour depends only on
the concentration of a triterpene, but the spectra maximum wavelength
doesn`t change and the colour is fairly stable (abs. of water)"

It is not clear to me from this whether for quantitative analysis you (a)
need to develop a calibration curve for every batch of triterpene you
handle or (b) epsilon values are known for specific triterpene (perhaps for
all triterpenes?), provided you follow a fixed analytical procedure. In
case (a) it is even possible that you are geting colors from trace
impurities inherent to the batch, that are being carbonized by H2SO4; you
may even get quite a good repeatability between laboratories if all get
samples from the same stock. Case (b) would be more to the liking of
analytical chemists, as they would be able to repeat a determination of a
specific compound, its origin notwithstanding, without calibration curves,
even if the actual process is not understood. Personally I wouldn't rely on
these analyses.

Wouldn't steroids and other classes of organic compound give similar
"quantitative" results?

2. You say "It seems that nobody has investigated the matter so far".

In fact, pyrolytic carbonization has been amply investigated, but the
process does not belong to the realm of the Daltonides; clean-cut reaction
mechanisms of the type we like to adduce in organic chemistry are not
easily discerned from the mess one usually gets in such processes. Similar
difficulties may attain treatment with concentrated sulfuric acid. Perhaps
somehow following the kinetics of the process with H2SO4 by EPR and C-13
NMR may throw some light on the mechanisms.

3. As for "Terpenoids give similar reaction with SbCl3, so it might be
connected
with 1dehydration of the structures or 2 dehydratation connected with
dedehydration and a rearangement of the inserted double bonds"

This may be correct. IMHO H2SO4 and SbCl3 are quite different stories.

All the best,

Jacob

>Hello Jacob,
>
>Thank you for answering my question.
>I know about yellowing and darkening of organic compounds in the
>presence of conc. sulphuric acid however I still haven`t found any information
>on the mechanism of the reaction. It seems that nobody has investigated the
>matter so far. Besides the reaction of triterpene like structures with the
>acid is
>rather specific.
>
>The intensity of the colour depends only on the concentration of a
>triterpene, but the spectra maximum wavelength doesn`t change and the
>colour is fairly stable (abs. of water). Some people believe that they could
>measure the quantity of a triterpenoid in a spectrophotometric method.
>Personaly I feel that the method may not be very selective but I must
>first check it.
>
>Terpenoids give similar reaction with SbCl3, so it might be connected
>with 1dehydration of the structures or 2 dehydratation connected with
>dedehydration and a rearangement of the inserted double bonds.
>
>With best regards,
>Jacek Achremowicz

At 10:48 10/5/1, Jacek Achremowicz wrote:
>Jacob Zabicky wrote:
>
>> Hello Jacek,
>>
>> The "yellowing" you are observing with H2SO4 is probably a carbonization
>> process, taking place whenever this concentrated reagent is in touch with
>> an organic material consisting of heavy molecules. Such reactions need not
>> be specific to triterpenes or steroidals. Just put a drop of concentrated
>> sulfuric acid on filter paper and you'll notice a gradual yellowing to
>> browning to blackening of the spot.
>>
>> All the best
>>
>> At 10:34 26/4/1, Jacek Achremowicz wrote:
>> >Dear Members of Orglist,
>> >
>> >I am looking for any information about mechanism of reaction of
>> >triterpene or steroid type compounds with conc. sulfuric acid or SbCl3.
>> >
>> >The structures with the reagents give unstable colour compounds yellow,
>> >red or brown depending on the structure of the compound (in CHCl3
>> >solution or in TLC).
>> >
>> >I have been working with the triterpene like compounds for 2 years but I
>> >have not found the information yet, so if anybody would help me I would
>> >be very grateful.


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