Fehling's trial

Storage bottles for Fehling's solutions, around 1904 ( Zucker-Museum Berlin)

The Fehling sample is used to detect reducing agents , e.g. B. of aldehydes and reducing sugars .

history

The detection reaction published by Hermann Fehling in 1848 enabled the quantitative determination of sugar in urine by titration . This was for the diagnosis of diabetes (diabetes) is important. Previously, this was only possible qualitatively through simple taste testing or fermentation , later also quantitatively through polarimetry . Although the Fehling test has been an integral part of school chemistry for many years, its informative value has also been controversial for many years: The explanation of the positive Fehling test for reducing sugars - oxidation of the aldehyde group to the carboxyl group - which is common in chemistry lessons - contradicts the observation that fructose reacts faster than glucose and mannose. The primary oxidation product of the reaction of a copper (II) salt solution with glucose is not the corresponding gluconic acid or gluconate, but glucosone (2-ketoglucose). The latter is further oxidized by C − C bond cleavage under the reaction conditions. This fact has been known for almost 90 years, but has not caught on in textbooks and textbooks. In student experiments, the very similar Benedict reaction to the Fehling's test is preferable, as the risk assessment makes it clear that the same results are achieved when using less hazardous chemicals ( sodium carbonate instead of sodium hydroxide ).

Fehling's solution

Copper tartrate complex with square-coordinated copper

Fehling reaction, negative on the left, positive on the right (precipitation of copper (I) oxide)

To carry out the Fehling test, two solutions are used as detection reagents , which according to Hermann Fehling are called "Fehling I" and "Fehling II".

• The light blue Fehling's solution I is a dilute copper (II) sulfate solution .
• The colorless Fehling's solution II is an alkaline potassium-sodium-tartrate-tetrahydrate solution .

After combining equal volumes of both Fehling's solutions, the Fehling's reagent has a characteristic dark blue color due to the complex formation of the Cu (II) ions with the tartrate ions. The tartrate is a complexing agent: Due to the high complex stability, the solubility product of the copper (II) hydroxide is no longer reached. If the copper (II) ions were not bound in a complex, the OH ^- ions would react with the copper (II) ions to form the poorly soluble blue copper (II) hydroxide Cu (OH) ₂ , and the desired detection reaction would then not be possible take place more.

${\displaystyle \mathrm {2\ [C_{4}H_{4}O_{6}]^{2-}+Cu^{2+}+2\ OH^{-}\longrightarrow [Cu(C_{4}H_{3}O_{6})_{2}]^{4-}+2\ H_{2}O} }$

Salts of tartaric acid and copper (II) ions react to form the deep blue copper tartrate complex and water.

The addition of glycerine before filling up with water extends the shelf life of a self-made solution.

After adding the test substance, the solution is heated. This accelerates the detection reaction according to the RGT rule . The monosaccharides are detected in their open-chain form, as the oxidisability of the aldehyde group is used here, which is bound as a hemiacetal in the ring forms. The open-chain form is in chemical equilibrium with the various ring forms. For example, in the case of glucose in aqueous solution, less than 0.1% of the sugar molecules are in open-chain form.

The copper (II) ions are then first reduced to yellow copper (I) hydroxide (CuOH) and then dehydrated to copper (I) oxide (Cu ₂ O), which precipitates out as a red-brown precipitate. According to the old version, aldehydes are oxidized to carboxylic acids.

Not least because of the formation of a solid product, the equilibrium of this reaction is almost entirely on the part of the carboxylic acid. As a result, further sugar molecules are converted into the open-chain form until the reaction is practically complete: according to the old version

${\displaystyle \alpha {,}\beta {\text{-Glucose (Halbacetal)}}\ \rightleftharpoons \ {\text{offenkettige Form (Aldehyd)}}\ {\xrightarrow {\text{Oxidation}}}\ \mathrm {Glucons{\ddot {a}}ure} }$

As already shown in the history of the Fehling test, according to current knowledge, gluconic acid is not produced, but glucosone (2-ketoglucose). The latter is further oxidized by C − C bond cleavage under the reaction conditions.

With longer heating or with simpler aldehydes such as formaldehyde or acetaldehyde , elemental copper can also be formed.

Redox reaction

Since the sample substance is oxidized by reducing the copper (II) ions, the overall reaction, as with all redox reactions , can be broken down into an oxidation and reduction reaction . For the sake of simplicity, the following example does not take into account that the copper ions are actually present in a complex with tartrate ions ( copper tartrate ):

Oxidation:

${\displaystyle \mathrm {R\!-\!CHO+2\,OH^{-}\longrightarrow R\!-\!COOH+H_{2}O+2\,e^{-}} }$

An aldehyde group is oxidized in the basic to a carboxylic acid.

${\displaystyle \mathrm {R{-}COOH+OH^{-}\longrightarrow R{-}COO^{-}+H_{2}O} }$

Since the reaction takes place in an alkaline environment, the resulting carboxy group is deprotonated by hydroxide ions to form the carboxylate group in the sense of an acid-base reaction .

Reduction:

${\displaystyle \mathrm {2\,Cu^{2+}+2\,OH^{-}+2\,e^{-}\longrightarrow 2\ CuOH\longrightarrow Cu_{2}O\downarrow +H_{2}O} }$

Copper (II) ions and hydroxide ions react to form copper (I) hydroxide, which further dehydrates to copper (I) oxide.

Redox reaction:

${\displaystyle \mathrm {2\,Cu^{2+}+R{-}CHO+5\,OH^{-}\longrightarrow Cu_{2}O\downarrow +R{-}COO^{-}+3\,H_{2}O} }$

Copper (II) ions and aldehyde groups react in a basic environment to form copper (I) oxide, carboxylates and water.

Limits

As a rule, Fehling's solution does not oxidize ketones , which allows a distinction to be made between an aldehyde and a ketone . This does not apply to α-hydroxy ketones , e.g. B. keto sugars such as fructose . These have one or more OH groups in the immediate vicinity of the carbonyl group of the ketone: Due to the enediolate ions formed in alkaline solution (cf. ketol-enediol tautomerism ), these have a reducing effect as well as "real" aldehydes, so they also carry them Fehling's solution to the copper (I) oxide deposition described above.

In addition, the Fehling reaction with reducing sugars does not generally follow the simple stoichiometry shown above , since oxidation products are formed which themselves have a further reducing effect ( ketoaldehydes , hydroxy diketones and products of retro aldol reactions ), so that in the end a mixture is more diverse Reaction products is present.

In the case of sucrose , the Fehling reaction is negative, as the aldehyde group is blocked due to the 1,2-glycosidic bond and thus cannot have a reducing effect.

Further detection reactions for aldehydes

• Angeli Rimini reaction
• Schiff's sample with the Schiff's reagent
• Tollens sample (silver mirror sample) with the Tollens reagent
• Benedict reagent
• Nylander's reagent

See also : List of organic detection reactions

Individual evidence

1. ↑ H. Fehling: Quantitative determination of sugar in urine. Archives of Physiological Medicine, 1848, 7: 64-73.
2. ↑ I. Munk: For the quantitative determination of the sugar and the so-called reducing substances in the urine by means of Fehling's solution , 1886, doi : 10.1007 / BF01925199
3. ↑ Prof. Blume's tip of the month March 2006 (tip no. 105): Diabetes - definitely a topic for chemistry classes
4. ↑ J. Büttner: Scientific methods in the clinical laboratory of the 19th century and their influence on clinical thinking. In: Reports on the history of science . doi : 10.1002 / 1522-2365 (200206) 25: 2 <93 :: AID-BEWI93> 3.0.CO; 2-G <93 :: AID-BEWI93> 3.0.CO; 2-G
5. ↑ Holger Fleischer: Misinterpretation of the Fehling's test for reducing sugars - From observation in chemistry class to evidence against the oxidation of the aldehyde group . In: CHEMKON . tape 24 , no. 1 , January 1, 2017, p. 27–30 , doi : 10.1002 / ckon.201610283 .
6. ^ ^{A b} A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1447.
7. ↑ Differentiation between aldehydes and ketones
8. ↑ Hans Beyer: Textbook of Organic Chemistry ; Leipzig 1968; Pp. 153, 316, 329.

Web links

Commons : Fehling-Probe  - collection of images, videos and audio files

Wikibooks: Practical course in organic chemistry / Fehling's test  - learning and teaching materials