organic chemistry Organic chemistry is a subdiscipline within chemistry involving the 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.Clayden, J ...

, Fehling's solution is a chemical

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

used to differentiate between water-soluble

carbohydrate 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 o ...

and

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...

()

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

s, and as a test for

reducing sugars A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reacti ...

and non-reducing sugars, supplementary to the

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 nit ...

test. The test was developed by German chemist

Hermann von Fehling Hermann von Fehling (9 June 1812 – 1 July 1885) was a German chemist, famous as the developer of Fehling's solution used for estimation of sugar. Biography Hermann von Fehling was born in Lübeck. With the intention of taking up pharmacy he ...

in 1849.

Laboratory preparation

Fehling's solution is prepared by combining two separate solutions: Fehling's A, which is a deep blue aqueous solution of

copper(II) sulfate Copper(II) sulfate, also known as copper sulphate, is an inorganic compound with the chemical formula . It forms hydrates , where ''n'' can range from 1 to 7. The pentahydrate (''n'' = 5), a bright blue crystal, is the most commonly encountered h ...

, and Fehling's B, which is a colorless solution of aqueous potassium sodium tartrate (also known as

Rochelle salt Potassium sodium tartrate tetrahydrate, also known as Rochelle salt, is a double salt of tartaric acid first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, France. Potassium sodium tartrate and monopotassium phospha ...

) made strongly alkali with

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and al ...

. These two solutions, stable separately, are combined when needed for the test because the copper(II) complex formed by their combination is not stable: it slowly decomposes into copper hydroxide in the alkaline conditions. The active

is a tartrate complex of Cu²⁺, which serves as an

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...

. The tartrate serves as a ligand. However, the coordination chemistry is complex and various species with different metal to ligand ratio have been determined.T. G. Hörner, P. Klüfers: ''The Species of Fehling's Solution.'' In: ''

Eur. J. Inorg. Chem. The ''European Journal of Inorganic Chemistry'' is a weekly peer-reviewed scientific journal covering inorganic, organometallic, bioinorganic, and solid-state chemistry. It is published by Wiley-VCH on behalf of Chemistry Europe. The journal, alon ...

'' 2016, S. 1798–1807, doi:10.1002/ejic.201600168.Fangfang Jian, Pusu Zhao, Qingxiang Wang: ''Synthesis and crystal structure of a novel tartrate copper(II) two-dimensional coordination polymer: _∞.'' In: ''J. Coord. Chem.'' 58, 2005, S. 1133–1138, doi:10.1080/00958970500148446.C. K. Prout, J. R. Carruthers, F. J. C. Rossotti: ''Structure and stability of carboxylate complexes. Part VII. Crystal and molecular structures of copper(II)meso-tartrate trihydrate and copper(II)d-tartrate trihydrate.'' In: '' J. Chem. Soc.'' A, Inorg. Phys. Theo., 1971, S. 3336–3342, doi:10.1039/J19710003336.R. Weiss, S. Vukojevi, C. Baltes, R. Naumann d'Alnoncourt, M. Muhler, M. Epple: ''Copper/Zinc L-Tartrates: Mixed Crystals and Thermolysis to a Mixture of Copper Oxide and Zinc Oxide That Is Catalytically Active in Methanol Synthesis.'' In: ''

'' 2006, S. 4782–4786, doi:10.1002/ejic.200600561.I. Quasim, A. Firdous, B. Want, S. K. Khosa, P. . Kotru: ''Single crystal growth and characterization of pure and sodium-modified copper tartrate.'' In: ''J. Cryst. Growth.'' 310, 2008, S. 5357–5363, doi:10.1016/j.jcrysgro.2008.09.021.N. D. Jespersen: ''Novel Copper-Tartrate Coordination Compounds.'' In: ''Anal. Let.'' 5, 1972, S. 497–508. Other methods of preparing comparable cupric-ion test-reagent solutions were developed at about the same time as Fehling's. These include the Violette solution (eponymous for Charles Violette) and the Soxhlet solution (eponymous for Franz von Soxhlet), both containing tartrate, and Soldaïni's solution (eponymous for Arturo Soldaïni), which instead contains carbonate.

Use of the reagent

Fehling's solution can be used to distinguish

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 gro ...

functional groups. The compound to be tested is added to the Fehling's solution and the mixture is heated. Aldehydes are oxidized, giving a positive result, but ketones do not react, unless they are α-hydroxy ketones. The bistartratocuprate(II) complex oxidizes the aldehyde to a

carboxylate In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge. Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...

anion, and in the process the copper(II) ions of the complex are reduced to copper(I) ions. Red copper(I) oxide then precipitates out of the reaction mixture, which indicates a positive result i.e. that

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

has taken place (this is the same positive result as with Benedict's solution). Fehling's test can be used as a generic test for

monosaccharides Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units ( monomers) from which all carbohydrates are built. They are usually colorless, water ...

and other reducing sugars (e.g., maltose). It will give a positive result for

aldose An aldose is a monosaccharide (a simple sugar) with a carbon backbone chain with a carbonyl group on the endmost carbon atom, making it an aldehyde, and hydroxyl groups connected to all the other carbon atoms. Aldoses can be distinguished from ...

monosaccharides (due to the oxidisable aldehyde group) but also for

ketose A ketose is a monosaccharide containing one ketone group per molecule. The simplest ketose is dihydroxyacetone, which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide ketoses are reducing sugars, be ...

monosaccharides, as they are converted to

aldoses An aldose is a monosaccharide (a simple sugar) with a carbon backbone chain with a carbonyl group on the endmost carbon atom, making it an aldehyde, and hydroxyl groups connected to all the other carbon atoms. Aldoses can be distinguished from keto ...

by the base in the

, and then give a positive result. Fehling's can be used to screen for

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellul ...

, thus detecting

diabetes Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ...

. Another use is in the breakdown of starch to convert it to glucose syrup and

maltodextrin Maltodextrin is a polysaccharide that is used as a food ingredient. It is produced from vegetable starch by partial hydrolysis and is usually found as a white hygroscopic spray-dried powder. Maltodextrin is easily digestible, being absorbed as ...

s in order to measure the amount of

reducing sugar A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reacti ...

, thus revealing the dextrose equivalent (DE) of the starch sugar.

Formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Est ...

(HCO₂H) also gives a positive Fehling's test result, as it does with Tollens' test and Benedict's test also. The positive tests are consistent with it being readily oxidizable to

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

. The solution cannot differentiate between benzaldehyde and acetone.

Net reaction

The net reaction between an aldehyde and the copper(II) ions in Fehling's solution may be written as: : RCHO + 2 Cu^2+ + 5 OH- -> RCOO- + Cu2O + 3 H2O or with the

tartrate A tartrate is a salt or ester of the organic compound tartaric acid, a dicarboxylic acid. The formula of the tartrate dianion is O−OC-CH(OH)-CH(OH)-COO− or C4H4O62−. The main forms of tartrates used commercially are pure crystalline ta ...

included: : RCHO + 2 Cu(C4H4O6)2^2- + 5 OH- -> RCOO- + Cu2O + 4 C4H4O6^2- + 3 H2O

References

External links

* {{DEFAULTSORT:Fehling's Solution Biochemistry detection methods Chemical tests Coordination complexes Copper(II) compounds Oxidizing agents Analytical reagents

Laboratory preparation

Use of the reagent

Net reaction

See also

References

External links