In
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 ...
, Fehling's solution is a chemical
reagent used to differentiate between water-soluble
carbohydrate
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'' ...
and
ketone
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
()
functional group
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 ...
s, and as a test for
reducing sugars and non-reducing sugars, supplementary to the
Tollens' reagent test. The test was developed by German chemist
Hermann von Fehling 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, and Fehling's B, which is a colorless solution of aqueous
potassium sodium tartrate (also known as
Rochelle salt) made strongly alkaline with
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly corrosive base (chemistry), ...
. 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
reagent is a tartrate complex of Cu
2+, which serves as an
oxidizing agent. 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.'' 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.][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 Viollette solution (eponymous for (1823–1894)) and the Soxhlet solution (eponymous for
Franz Ritter von Soxhlet (1848–1926)), both containing tartrate, and Soldaïni's solution (eponymous for Arturo Soldaïni), which instead contains carbonate.
Barfoed's test is also related and similar to Fehling's test (eponymous for
Christen Thomsen Barfoed (1815–1889)).
Use of the reagent
Fehling's solution can be used to distinguish
aldehyde vs
ketone
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
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 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 or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
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 and other reducing sugars (e.g., maltose). It will give a positive result for
aldose monosaccharides (due to the oxidisable aldehyde group) but also for
ketose
In organic chemistry, 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 keto ...
monosaccharides, as they are converted to
aldoses by the base in the
reagent, and then give a positive result.
Fehling's can be used to screen for
glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
in
urine
Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...
, thus detecting
diabetes. Another use is in the breakdown of starch to convert it to glucose syrup and
maltodextrins in order to measure the amount of
reducing sugar, thus revealing the
dextrose equivalent (DE) of the
starch sugar.
Formic acid (HCO
2H) also gives a positive Fehling's test result, as it does with
Tollens' (eponymous for
Bernhard Christian Gottfried Tollens (1841 – 1918)) test and Benedict's solution also. The positive tests are consistent with it being readily oxidizable to
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
.
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 included:
:
RCHO + 2 Cu(C4H4O6)2^2- + 5 OH- -> RCOO- + Cu2O + 4 C4H4O6^2- + 3 H2O
See also
*
Barfoed's test
References
External links
*
{{DEFAULTSORT:Fehling's Solution
Biochemistry detection methods
Carbohydrate methods
Chemical tests
Coordination complexes
Copper(II) compounds
Oxidizing agents
Analytical reagents