What is Fehling’s Solution?

What is Fehling’s Solution? The simple answer to this question is—Fehling’s Solution is a mixture of two other chemicals, generally known as Fehling A and Fehling B solutions. Fehling A is copper sulphate solution, and Fehling B is Rochelle salt (potassium sodium tartrate solution). Both the solutions (Fehling A and Fehling B) are mixed in equal quantities to make Fehling’s solution. Fehling’s solution should be prepared just before its use; it should not be prepared beforehand. It is dark blue in colour and alkaline in nature. It is a mild oxidising agent.

Preparation of Fehling’s Solution

• Fehling A is an aqueous solution of copper sulphate. It is prepared by dissolving pentahydrated copper sulphate in distilled water and then adding some drops of dilute sulphuric acid. Fehling A is also blue in colour. The copper present in solution A imparts blue colour to Fehling A solution.

• Fehling B is Rochelle salt. It is prepared by dissolving potassium tartrate in an aqueous solution of sodium hydroxide. It is also known as potassium sodium tartrate solution. It is a colourless solution and acts as a chelating agent.

• In labs, these two solutions are prepared beforehand and stored. When needed, Fehling’s solution is prepared by mixing these two solutions in equal volumes. It is always freshly prepared.

Fehling A  (blue colour)       

CuSO4 +  2NaOH  →  Cu(OH)2  +  Na2SO4

Fehling B  (colourless)

Copper tartrate complex in Fehling’s solution (dark blue colour)

• Fehling’s solution in chemistry is used to find out the carbonyl group, which is present in an organic compound as a ketone functional group or as an aldehyde functional group. Ketones do not react with Fehling’s solution (a few exceptions are there). Aldehydes do react with Fehling’s solution and give a reddish-brown coloured precipitate

• Fehling’s solution is also used to test whether sugar is reducing or non-reducing in nature

Fehling’s Test 

The basic principle behind this test is that aldehydes are oxidised to carboxylic acids in the presence of copper ions. If a compound contains an aldehyde group, after adding Fehling’s solution to it, we get a red precipitate of copper oxide that indicates the presence of an aldehyde group. No reaction occurs in case the carbonyl moiety is present as a ketone functional group in the compound. It is a common test performed in laboratories to detect compounds having aldehyde and ketone functional groups. 

Procedure 

• In the first step, Fehling A and Fehling B solutions are mixed in equal volumes in a test tube. The dark blue solution, thus obtained, is known as Fehling’s solution

• In another dry test tube, the organic compound is added for the detection of the carbonyl group (aldehyde or ketone) present. Then, the freshly prepared Fehling’s solution is added to it and heated in a water bath for some time

• If reddish-brown precipitate appears, then the compound has an aldehyde functional group. If no reddish-brown precipitation occurs, then it shows the presence of ketone moiety in the compound

• In this redox reaction, Cu2+ of Fehling’s solution is converted to Cu(I) oxide, which is insoluble in water and gives reddish-brown precipitate

Preparation of Fehling’s Solution

Copper sulphate Pentahydrate  +   NaOH   +   Potassium Sodium Tartrate     →     Fehling’s Solution    

Detection Of Aldehyde

Cu2 + (from tartrate complex)   +  Aldehyde   →  Carboxylic acid    +   Cu2O (red ppt)

Reducing Sugars and Non-reducing Sugars  

Generally, carbohydrates are classified into reducing and non-reducing sugars. Fehling’s solution in chemistry is used to differentiate between reducing and non-reducing sugars. 

• Those sugar molecules that reduce Fehling’s solution {Cu2+ to Cu(I) oxide} are known as reducing sugars. These sugars have a free aldehyde group or the hemiacetal group that makes them reducible. They also reduce Tollen’s reagent. Example: glucose

• Non-reducing sugars are the sugars that do not reduce Fehling’s solution and Tollen’s reagent. The aldehyde group in these sugars is not free to react. Example: sucrose

You should also know,

• Formic acid is the only acid that gives a red precipitate with Fehling’s solution. This is because of the presence of aldehyde group type hydrogen in formic acid

• Aromatic aldehydes do not give positive results with Fehling’s solution.

• Glucose and fructose both give red precipitate on reacting with Fehling’s solution. This is because they transform into an enediol intermediate

• Ketones with a hydroxyl group (-OH) at their alpha carbon also respond to Fehling’s test

Conclusion

Fehling’s solution is a freshly prepared mixture of Fehling A and Fehling B solutions. Fehling’s test in chemistry is one of the most common tests to detect the presence of a carbonyl group as an aldehyde functional group. Carbohydrates that reduce Fehling’s solution are known as reducing sugars.

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