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The Schiff test is an early
The further reaction of the Schiff reagent with aldehydes is complex with several research groups reporting multiple reaction products with model compounds. In the currently accepted mechanism, the pararosaniline and bisulfite combine to yield the "decolorized" adduct with sulfonation at the central carbon as described and shown. The free, uncharged aromatic amine groups then react with the aldehyde being tested to form two aldimine groups; these groups have also been named for their discoverer as
accessed 8 March 2013. n German/ref> Prior formation of classical bisulfite adducts of the tested aldehyde may, when the adducts are stable, give rise to false negative tests such as in the case of testing for the aldehydic terminus of glucose. Schiff's reagent on reaction with Acetaldehyde gives pink colour. Such an imine-mediated mechanism was first proposed by Paul Rumpf (1908–1999) in 1935, and experimental evidence was provided by Hardonk and van Duijn in 1964. In 1980, Robins, Abrams and Pincock provided substantial NMR evidence for the mechanism, leading to its general acceptance.Robins, J.H., Abrams, G.D. & Pincock, J.A. (1980) "The structure of Schiff reagent aldehyde adducts and the mechanism of the Schiff reaction as determined by nuclear magnetic resonance spectroscopy," ''Canadian Journal of Chemistry'', 58 (4) : 339–347. Stoward had examined the mechanism in 1966 and, on the whole, considered this mechanism to be correct. A second, earlier mechanism continues to appear in the literature. The mechanism was proposed in 1921 by the eminent German organic chemist Heinrich Wieland and his student Georg Scheuing (1895–1949).Puchtler, Holde; Meloan, Susan N.; Brewton, Barbara R. (1975) "On the history of basic fuchsin and aldehyde-Schiff reactions from 1862 to 1935," ''Histochemistry'', 41 (3) : 185–194. Bisulphite was believed to react with the available
What is Schiff's reagent?
{{Organic reactions Chemical tests
The Schiff test is an early organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
named reaction developed by Hugo Schiff
Hugo (Ugo) Schiff (26 April 1834 – 8 September 1915) was an Italian naturalized chemist. The son of a Jewish businessman and brother of the physiologist Moritz Schiff was German by nationality. He discovered Schiff bases and other imines, ...
, and is a relatively general chemical test
In chemistry, a chemical test is a qualitative or quantitative procedure designed to identify, quantify, or characterise a chemical compound or chemical group.
Purposes
Chemical testing might have a variety of purposes, such as to:
* Determine ...
for detection of many organic aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...
s that has also found use in the staining of biological tissues. The Schiff reagent is the reaction product of a dye formulation such as fuchsin and sodium bisulfite
Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite in fact is not a real compound, but a mixture of salts that dissolve in water to give solutions ...
; pararosaniline (which lacks an aromatic
In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...
methyl group) and new fuchsin (which is uniformly mono-methylated ''ortho'' to the dye's amine functionalities) are not dye alternatives with comparable detection chemistry.
In its use as a qualitative test for aldehydes, the unknown sample is added to the decolorized Schiff reagent; when aldehyde is present a characteristic magenta
Magenta () is a color that is variously defined as pinkish- purplish- red, reddish-purplish-pink or mauvish- crimson. On color wheels of the RGB (additive) and CMY (subtractive) color models, it is located exactly midway between red and bl ...
color develops. Schiff-type reagents are used for various biological tissue staining
Staining is a technique used to enhance contrast in samples, generally at the microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissues), in cytology (microscopic study of cells), and in ...
methods, e.g. Feulgen stain and periodic acid-Schiff stain. Human skin also contains aldehyde functional groups in the termini of saccharide
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...
s and so is stained as well.
Mechanism
Fuchsin solutions appear colored due to the visible wavelengthabsorbance
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
of its central quinoid structure—see also for example viologen —but are "decolorized" upon sulfonation Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution. Aryl sulfonic acids are used as detergents, dye, and drugs.
Stoichiometry ...
of the dye at its central carbon atom by sulfurous acid or its conjugate base, bisulfite. This reaction disrupts the otherwise favored delocalized extended pi-electron
In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbital ...
system and resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillat ...
in the parent molecule.
The further reaction of the Schiff reagent with aldehydes is complex with several research groups reporting multiple reaction products with model compounds. In the currently accepted mechanism, the pararosaniline and bisulfite combine to yield the "decolorized" adduct with sulfonation at the central carbon as described and shown. The free, uncharged aromatic amine groups then react with the aldehyde being tested to form two aldimine groups; these groups have also been named for their discoverer as Schiff base
In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( = alkyl or aryl, but not hydrogen). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimine ...
s ( azomethines), with the usual carbinolamine (hemiaminal
In organic chemistry, a hemiaminal (also carbinolamine) is a functional group or type of chemical compound that has a hydroxyl group and an amine attached to the same carbon atom: . R can be hydrogen or an alkyl group. Hemiaminals are intermediat ...
) intermediate being formed and dehydrated en route to the Schiff base. These electrophilic aldimine groups then react with further bisulfite, and the Ar-NH-CH(R)-SO3− product (and other resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillat ...
-stabilized species in equilibrium with the product) give rise to the magenta color of a positive test."Schiffsche Probe" (Schiff test), University of Bayreuth, Bavaria, Germanyaccessed 8 March 2013. n German/ref> Prior formation of classical bisulfite adducts of the tested aldehyde may, when the adducts are stable, give rise to false negative tests such as in the case of testing for the aldehydic terminus of glucose. Schiff's reagent on reaction with Acetaldehyde gives pink colour. Such an imine-mediated mechanism was first proposed by Paul Rumpf (1908–1999) in 1935, and experimental evidence was provided by Hardonk and van Duijn in 1964. In 1980, Robins, Abrams and Pincock provided substantial NMR evidence for the mechanism, leading to its general acceptance.Robins, J.H., Abrams, G.D. & Pincock, J.A. (1980) "The structure of Schiff reagent aldehyde adducts and the mechanism of the Schiff reaction as determined by nuclear magnetic resonance spectroscopy," ''Canadian Journal of Chemistry'', 58 (4) : 339–347. Stoward had examined the mechanism in 1966 and, on the whole, considered this mechanism to be correct. A second, earlier mechanism continues to appear in the literature. The mechanism was proposed in 1921 by the eminent German organic chemist Heinrich Wieland and his student Georg Scheuing (1895–1949).Puchtler, Holde; Meloan, Susan N.; Brewton, Barbara R. (1975) "On the history of basic fuchsin and aldehyde-Schiff reactions from 1862 to 1935," ''Histochemistry'', 41 (3) : 185–194. Bisulphite was believed to react with the available
aromatic amine
In organic chemistry, an aromatic amine is an organic compound consisting of an aromatic ring attached to an amine. It is a broad class of compounds that encompasses aniline
Aniline is an organic compound with the formula C6 H5 NH2. Consis ...
functional groups to form N-sulfinic acid groups, Ar-NH-SO2H, followed by reaction with aldehyde to form sulfonamide
In organic chemistry, the sulfonamide functional group (also spelled sulphonamide) is an organosulfur group with the structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this group is unreactiv ...
s, Ar-NH-SO2CH(OH)-R. The 1980 NMR data that allowed visualization of intermediates does not support this mechanism or the sulfonamides as the chromogenic product.
See also
*Tollens' reagent
Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitr ...
* Fehling's reagent
* 2,4-Dinitrophenylhydrazine
2,4-Dinitrophenylhydrazine (2,4-DNPH or DNPH) is the organic compound C6H3(NO2)2NHNH2. DNPH is a red to orange solid. It is a substituted hydrazine. The solid is relatively sensitive to shock and friction. For this reason DNPH is usually handle ...
* Fehling's solution
* Barfoed's test
* Benedict's reagent
References
External links
What is Schiff's reagent?
{{Organic reactions Chemical tests