IChO 1990, Теория — Задача 6. Problem 6 — Bio-organic Chemistry
Twenty-second International Chemistry Olympiad — 1990 — Theoretical problems (Volume 2, 2009)
Внимание: Бележка към темата
Attributed archive/compiler teaching source. Retain the Volume 2 attribution and native copying conditions; the printed teaching grant does not establish a broader commercial licence. This 2009 collection is not an unchanged complete jury packet: the editorial preface describes recreated presentation and schemes, numerical corrections, brief keys and omitted practical material. Printed errors, absent measurements, incomplete methods, missing forms and missing answers remain disclosed.
Compilation edition and teaching attribution
Anton Sirota (editor), The Competition Problems from the International Chemistry Olympiads, Volume 2, IChO International Information Centre, Bratislava, IUVENTA, 2009.
Attributed archive/compiler teaching source. Retain the Volume 2 attribution and native copying conditions; the printed teaching grant does not establish a broader commercial licence. This 2009 collection is not an unchanged complete jury packet: the editorial preface describes recreated presentation and schemes, numerical corrections, brief keys and omitted practical material. Printed errors, absent measurements, incomplete methods, missing forms and missing answers remain disclosed.
Native copyright and teaching-copy conditions
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS,
Volume 2
21st – 40th IChO (1989 – 2008)
Editor: Anton Sirota
ISBN 978-80-8072-092-6
Copyright © 2009 by IUVENTA – IChO International Information Centre, Bratislava, Slovakia
You are free to copy, distribute, transmit or adapt this publication or its parts for unlimited teaching purposes,
however, you are obliged to attribute your copies, transmissions or adaptations with a reference to "The
Competition Problems from the International Chemistry Olympiads, Volume 2" as it is required in the
chemical literature. The above conditions can be waived if you get permission from the copyright holder.
Issued by IUVENTA in 2009
with the financial support of the Ministry of Education of the Slovak Republic
Number of copies: 250
Not for sale.
International Chemistry Olympiad
International Information Centre
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Búdková 2
811 04 Bratislava 1, Slovakia
Phone: +421-907-473367
Fax: +421-2-59296123
E-mail: anton.sirota@stuba.sk
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Native editorial preface
Preface
Preface
Preface
Preface
This publication contains the competition problems (Volume 2) from the 21st – 40th
International Chemistry Olympiads (IChO) organized in the years 1989 – 2008 and is a
continuation of the publication that appeared last year as Volume 1 and contained
competition problems from the first twenty IChOs. The whole review of the competition
tasks set in the IChO in its fourty-year history is a contribution of the IChO International
Information Centre in Bratislava (Slovakia) to the development of this world known
international competition. This Volume 2 contains 154 theoretical and 46 practical
competition problems from the mentioned years. The review as a whole presents
altogether 279 theoretical and 96 practical problems.
In the elaboration of this collection the editor had to face certain difficulties because
the aim was not only to make use of past recordings but also to give them such a form
that they may be used in practice and further chemical education. Consequently, it was
necessary to make some corrections in order to unify the form of the problems. However,
they did not concern the contents and language of the problems.
Unfortunately, the authors of the particular competition problems are not known and
due to the procedure of the creation of the IChO competition problems, it is impossible to
assign any author's name to a particular problem. As the editor I would appreciate many
times some discussion with the authors about any critical places that occurred in the text.
On the other hand, any additional amendments to the text would be not correct from the
historical point of view. Therefore, responsibility for the scientific content and language of
the problems lies exclusively with the organizers of the particular International Chemistry
Olympiads.
Some parts of texts, especially those gained as scanned materials, could not be used directly and thus, several texts, schemes and pictures had to be re-written or created again. Some solutions were often available in a brief form and necessary extent only, just for the needs of members of the International Jury.
Recalculations of the solutions were made in some special cases only when the numeric results in the original solutions showed to be obviously not correct. Although the numbers of significant figures in the results of several solutions do not obey the criteria generally accepted, they were left without change.
In this publication SI quantities and units are used and a more modern method of
chemical calculations is introduced. Only some exceptions have been made when, in an
effort to preserve the original text, the quantities and units have been used that are not SI.
There were some problems with the presentation of the solutions of practical tasks,
because most of the relatively simple calculations were based on the experimental results
of contestants. Moreover, the practical problems are accompanied with answer sheets in
the last years and several additional questions and tasks have appeared in them that were
not a part of the text of the original experimental problems. Naturally, answer sheets could
not be included in this publication and can only be preserved as archive materials.
When reading the texts of the IChO problems one must admire and appreciate the work of those many known and unknown people – teachers, authors, pupils, and organizers – who contributed so much to development and success of this important international competition.
I am sure about the usefulness of the this review of the IChO problems. It may serve not only as archive material but, in particular, this review should serve to both competitors and their teachers as a source of further inspiration in their preparation for this challenging competition.
Bratislava, July 2009
Anton Sirota, editor
Условие
The dehydrogenation of succinate to fumarate in the Krebs cycle is catalysed by succinic dehydrogenase. Flavine-adenine-dinucleotide, FAD, acts as a coenzyme.
The stereochemistry of this reaction can be studied with 2,3-dideuteriosuccinates. These compounds may be prepared from related acids or by catalytic deuteration, using palladium on coal, of maleic or fumaric acid.
Questions
6.1
Represent in Fischer projections all stereoisomers obtained by catalytic deuteration of maleic acid and fumaric acid. How many stereoisomers are formed? Which are optically active? What are the relationships between the different stereoisomers? Represent their most stable conformations in Newman projections.
The proportion of dideuterated fumarate formed by the enzymatic dehydrogenation is when the starting dideuteriosuccinate is prepared from fumaric acid and when it is prepared from maleic acid. Assume that the molecules adopt their most stable conformation at the enzyme site.
6.2
What percentages of dideuterated fumarate would be obtained by a syn or an anti dehydrogenation? Assume that the theoretical percentages are or .
6.3
Deduce the stereochemistry of the enzymatic dehydrogenation.
Fumarase catalyses the addition of water to fumarate, giving -malate, or L-malate.
6.4
When dideuterofumarate reacts, the product is -dideuteromalate. Show schematically the addition of water, using the following representation of the plane containing the fumarate molecule.
Acetylcoenzyme A, , and glyoxylate react in the presence of malate synthetase to form -malate.
A similar reaction occurs between , an analogue of the thioester, and , an analogue of glyoxylate, in the presence of . It yields the -hydroxyester . The base produces an intermediate carbanion.
6.5
Write the different steps of the mechanism of this reaction.
Acetylcoenzyme A is prepared from acetic acid labelled with deuterium and tritium, , with the configuration shown below. and .
The -malate formed is dehydrated by an enzyme, in the reverse direction of the reaction in 6.4.
6.6
What is this enzyme?
The major product is monotritiated fumarate; is untritiated fumarate.
6.7
Represent, in Fischer projections, the two possible structures of the major -malate formed from the -labelled acetylcoenzyme A.
6.8
What products would be obtained starting from -labelled acetylcoenzyme A?
Source notes
The solution's contradictory I/II/III meso-isomer sentence and its abbreviated R,/S,/thr/eryt graphical labels are retained. The labelled acetylcoenzyme A scheme is retained under its printed 6.3 heading. The top hydration arrow in key 6.4 is labelled “fumarate” in the source. The chemical mechanism in key 6.5 is reproduced as bounded artwork because its native primes, ester groups and charges are internally inconsistent; it has not been redrawn as a corrected mechanism. The text under key 6.6 does not directly name the enzyme requested by question 6.6; no replacement answer is invented.
Решение
6.1
There are three isomers. Catalytic deuteration is syn.
The most stable conformation is the anti conformation, in which the groups are at the greatest distance from each other.
The printed text states: “I and II are enantiomeres, II and III are a meso isomer, thus there are in total three isomers (two enantiomers and one meso form).” Each threo isomer and the meso isomer are diastereomers.
6.2
Syn dehydrogenation of I or II gives fumarate without deuterium or with two deuterium atoms, in equal amounts (). Anti dehydrogenation gives monodeuterated fumarate. The value indicates anti dehydrogenation.
Syn dehydrogenation of the meso isomer gives only monodeuterated fumarate. Anti dehydrogenation gives normal fumarate and dideuterated fumarate. The experimental value from maleic acid is consistent with the meso isomer and anti dehydrogenation.
6.3
The enzymatic dehydrogenation is anti, as shown above. The following labelled acetylcoenzyme A reaction is also printed under this solution label:
6.4
“Stereospecific addition, syn or anti.” Syn addition gives or .
Anti addition gives -dideuteromalate.
6.5
6.6
“We would have (2R) (3R) if the enzyme did not produce enantiospecific addition. Thus: enantiospecific addition anti.”
6.7
Thus and or ; two projections are printed.
6.8
Starting from the configuration gives inversion at carbon 3 of L-malate. The native diagrams show two major-product projections and one minor-product projection, with nontritiated fumarate and monotritiated fumarate.
Оригинал в Архива: IChO 21st-40th.pdf · решение от архива: IChO 21st-40th.pdf · допълнителни данни: IChO 21st-40th.pdf · допълнителни данни: IChO 21st-40th.pdf