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The formose reaction, discovered by
Aleksandr Butlerov Alexander Mikhaylovich Butlerov (Алекса́ндр Миха́йлович Бу́тлеров; 15 September 1828 – 17 August 1886) was a Russian chemist, one of the principal creators of the theory of chemical structure (1857–1861 ...
in 1861, and hence also known as the Butlerov reaction, involves the formation of
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double s ...
s from
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section Fo ...
. The term formose is a
portmanteau A portmanteau word, or portmanteau (, ) is a blend of wordscatalyzed Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
by a base and a divalent metal such as
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
. The intermediary steps taking place are
aldol reaction The aldol reaction is a means of forming carbon–carbon bonds in organic chemistry. Discovered independently by the Russian chemist Alexander Borodin in 1869 and by the French chemist Charles-Adolphe Wurtz in 1872, the reaction combines two car ...
s, reverse aldol reactions, and aldose-ketose isomerizations. Intermediates are glycolaldehyde,
glyceraldehyde Glyceraldehyde (glyceral) is a triose monosaccharide with chemical formula C3 H6 O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism. The word comes ...
,
dihydroxyacetone Dihydroxyacetone (; DHA), also known as glycerone, is a simple saccharide (a triose) with formula . DHA is primarily used as an ingredient in sunless tanning products. It is often derived from plant sources such as sugar beets and sugar cane, an ...
, and
tetrose A tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde functional group in position 1 (aldotetroses) or a ketone functional group in position 2 (ketotetroses). File:DErythrose Fischer.svg , D-Erythrose File:DThreose Fischer ...
sugars. In 1959, Breslow proposed a mechanism for the reaction, consisting of the following steps: The reaction exhibits an induction period, during which only the nonproductive Cannizzaro disproportionation of formaldehyde (to methanol and formate) occurs. The initial dimerization of formaldehyde to give glycolaldehyde (1) occurs via an unknown mechanism, possibly promoted by light or through a free radical process and is very slow. However, the reaction is autocatalytic: 1 catalyzes the condensation of two molecules of formaldehyde to produce an additional molecule of 1. Hence, even a trace (as low as 3 ppm) of glycolaldehyde is enough to initiate the reaction. The autocatalytic cycle begins with the aldol reaction of 1 with formaldehyde to make glyceraldehyde (2). An aldose-ketose isomerization of 2 forms
dihydroxyacetone Dihydroxyacetone (; DHA), also known as glycerone, is a simple saccharide (a triose) with formula . DHA is primarily used as an ingredient in sunless tanning products. It is often derived from plant sources such as sugar beets and sugar cane, an ...
(3). A further aldol reaction of 3 with formaldehyde produces tetrulose (6), which undergoes another ketose-aldose isomerization to form aldotetrose 7 (either threose or erythrose). The retro-aldol reaction of 7 generates two molecules of 1, resulting in the net production of a molecule of 1 from two molecules of formaldehyde, catalyzed by 1 itself (autocatalysis). During this process, 3 can also react with 1 to form
ribulose Ribulose is a ketopentose — a monosaccharide containing five carbon atoms, and including a ketone functional group. It has chemical formula . Two enantiomers are possible, -ribulose (-erythro-pentulose) and -ribulose (-erythro-pentulose). ...
(4), which can isomerize to give rise to
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this compou ...
(5), an important building block of
ribonucleic acid Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohyd ...
. The aldose-ketose isomerization steps are promoted by chelation to calcium. However, these steps have been shown to proceed through a
hydride shift A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular reaction. The name ''sigmatropic'' is the result of a compounding of the long ...
mechanism by isotope labeling studies, instead of via an intermediate enediolate, as previously proposed.


Significance

The formose reaction is of importance to the question of the
origin of life In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
, as it leads from simple
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section Fo ...
to complex sugars like
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, , is a component of the ribonucleotides from which RNA is built, and so this compou ...
, a building block of
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohyd ...
. In one experiment simulating early Earth conditions, pentoses formed from mixtures of formaldehyde, glyceraldehyde, and
borate A borate is any of several boron oxyanions, negative ions consisting of boron and oxygen, such as orthoborate , metaborate , or tetraborate ; or any salt with such anions, such as sodium metaborate, and disodium tetraborate . The name also ...
minerals such as
colemanite Colemanite (Ca2B6O11·5H2O) or (CaB3O4(OH)3·H2O) is a borate mineral found in evaporite deposits of alkaline lacustrine environments. Colemanite is a secondary mineral that forms by alteration of borax and ulexite. It was first described in ...
(Ca2B6O115H2O) or
kernite Kernite, also known as rasorite, is a hydrated sodium borate hydroxide mineral with formula . It is a colorless to white mineral crystallizing in the monoclinic crystal system typically occurring as prismatic to acicular crystals or granular mass ...
(Na2B4O7). However, issues remain with both the thermodynamic and kinetic feasibility of binding pre-made sugars to a pre-made nucleobase, as well as a method to selectively employ ribose from the mixture. Both
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section Fo ...
and glycolaldehyde have been observed spectroscopically in
outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
, making the formose reaction of particular interest to the field of
astrobiology Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investig ...
. It must be carefully controlled, otherwise the alkaline conditions will cause the aldoses to undergo the
Cannizzaro reaction The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid. ...
.


References

{{Reflist Carbohydrates Organic reactions Origin of life RNA Name reactions