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 matter, organic materials, i.e., matter in its various forms that contain ...

named reaction A name reaction (or named reaction) is a chemical reaction named after its discoverer(s) or developer(s). Among the tens of thousands of organic reactions that are known, hundreds of such reactions are typically identified by the eponym. Well-known ...

developed by Hugo Schiff, and is a relatively general

chemical test In chemistry, a chemical test is a qualitative property, qualitative or Quantitative property, quantitative procedure designed to identify, quantify, or characterise a chemical compound or substituent, chemical group. Purposes Chemical testing m ...

for detection of many organic

aldehyde In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...

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 Fuchsine (sometimes spelled fuchsin) or rosaniline hydrochloride is a magenta dye with chemical formula C20H19N3·HCl.

and

sodium bisulfite Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite is not a real compound, but a mixture of salts that dissolve in water to give solutions compose ...

;

pararosaniline Pararosaniline, pararosaniline free base, Basic Red 9, or C.I. 42500 is an organic compound with the formula . It is the free base form of pararosaniline hydrochloride, , a magenta solid with a variety of uses as a dye. It is one of the four comp ...

(which lacks an

aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated as ...

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 purple-red color. On color wheels of the RGB color model, RGB (additive) and subtractive color, CMY (subtractive) color models, it is located precisely midway between blue and red. It is one of the four colors of ink used in colo ...

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 Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...

methods, e.g.

Feulgen stain Feulgen stain is a staining technique discovered by Robert Feulgen and used in histology to identify chromosomal material or DNA in cell specimens. It is darkly stained. It depends on acid hydrolysis of DNA, therefore fixating agents using st ...

and

periodic acid-Schiff stain Periodicity or periodic may refer to: Mathematics * Bott periodicity theorem, addresses Bott periodicity: a modulo-8 recurrence relation in the homotopy groups of classical groups * Periodic function, a function whose output contains values tha ...

. Human skin also contains aldehyde

functional groups In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...

in the termini of

saccharide A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' m ...

s and so is stained as well.

Mechanism

Fuchsin solutions appear colored due to the visible wavelength

absorbance 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 log ...

of its central quinoid structure—see also for example

viologen Viologens are organic compounds with the formula (C5H4NR)2n+. In some viologens, the pyridyl groups are further modified. Viologens are called so, because these compounds produce violet color on reduction iolet + Latin ''gen'', generator of ...

—but are "decolorized" upon

sulfonation In organic chemistry, aromatic sulfonation is a reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid () group. Together with nitration and chlorination, aromatic sulfonation is a widely used electrophilic aromatic substi ...

of the dye at its central carbon atom by sulfurous acid or its conjugate base, bisulfite. This reaction disrupts the otherwise favored

delocalized In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...

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 orbitals ...

system and

resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

in the parent molecule. Schiffs Reagenz

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 In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bo ...

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 aldim ...

s (

azomethine 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 aldimin ...

s), 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 In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carr ...

aldimine groups then react with further bisulfite, and the Ar-NH-CH(R)-SO₃⁻ product (and other

-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, Germany
accessed 8 March 2013. n German/ref> Prior formation of classical

bisulfite adduct The bisulfite ion (IUPAC-recommended nomenclature: hydrogensulfite) is the ion . Salts containing the ion are also known as "sulfite lyes". Sodium bisulfite is used interchangeably with sodium metabisulfite (Na2S2O5). Sodium metabisulfite diss ...

s 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 Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

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 Heinrich Otto Wieland (; 4 June 1877 – 5 August 1957) was a German chemist. He won the 1927 Nobel Prize in Chemistry for his research into the bile acids. Career In 1901 Wieland received his doctorate at the University of Munich while studying ...

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 Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be a ...

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 Chemical structure, structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this gro ...

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 In chemistry, the term chromogen refers to a colourless (or faintly coloured) chemical compound that can be converted by chemical reaction into a compound which can be described as "coloured" (a chromophore). There is no universally agreed definiti ...

product.

References

External links

What is Schiff's reagent?
{{Organic reactions Carbohydrate methods Chemical tests

Mechanism

See also

References

External links