Schiff's Reagent:

   Preparation of Schiff Reagent:

1. 5g of basic fuchsin or rosaniline hydrochloride dissolve in 900ml of hot deionised water. 
2.  Cool to 50°C and slowly add 100ml of 1N HCl.
3. Cool to 25°C and add 10g of K2S2O5 (potassium metabisulfite or potassium pyrosulfite).
4. Shake for 3 minutes and keep in the dark place at RT(room temperature) for 24 hours. 
5. Add 5 grams of fine activated charcoal and shake for 3 minutes and filter. 
6. Till Clear solution is obtained.
7.  Store at 4°C in a bottle covered with foil. 

Q) An organic  liquid added to Schiff's reagent. The Pink colour is obtained the organic compound is

Ans) Schiff's reagent is decolourised by sulphurous acid.

• The schiff's reagent is then treated with the organic compound to give bright pink colour.

• Immediate pink/red colour indicates aliphatic aldehyde.

• This reagent is used to detect aldehyde or ketonic group.

• aliphatic ketone give reaction very slowly which may be neglected. Explained with e.g. below. 

• Aromatic aldehyde gives this test slowly, it takes time to give pink/ red colour. 

• But aromatic ketone give this test negative (no reaction).

• In books, we have seen that ketones gives negative test for schiff's reagent.  

• But Aliphatic Ketones gives very slow reaction or no reaction with schiff's reagent. 
• In comparison with aldehyde group in answers, ketones are considered as not giving pink/red colour. 
• For example, if there are four option like   1)formaldehyde                    2)acetaldehyde                              3)Acetone                                                         4) Benzaldehyde  and from this you have select organic compound not giving pink colour with schiff's reagent. 

   The answer is Acetone because An aliphatic Ketones reacts slowly with schiff's reagent and it is neglected and said as no pink colour obtained.

•  This reagent was named after the German chemist Hugo Schiff (1834–1915).

Aliphatic aldehyde:(fast reaction pink colour retains) 

This aldehydes do not have aromatic ring.

General formula for aldehyde:R-CH=O
Where, R is an open chain hydrocarbon.

Aromatic aldehyde:(slow reaction) 

This aldehydes have one ring with carbonyl group and one hydrogen.

General formula for aldehyde:R-CH=O
Where, R is an close chain hydrocarbon.

Some Other reagents:

Tollen's reagent:

• Both aliphatic and aromatic aldehyde reduces to gives silver mirror but only aromatic aldehyde reduces schiff's reagent. 

• Ketones gives no reaction with tollen's reagent. 

Fehling's reagent:

• Aliphatic aldehydes gives positive test, indicating cuprous oxide formed with red ppt. 

• Aromatic aldehydes does not give red ppt because it does contain free alpha hydrogen. 

• Example benzaldehyde(Ar-CHO) does not give red ppt because the carbonyl group in aromatic aldehyde is electron withdrawing group which pull electron from the electron rich benzene and the bond between the C-H becomes strong due to less polarity of C-H. So, free alpha hydrogen means no reaction for aromatic aldehyde. 

• Fehling's agent is comparatively weaker oxidizing agent than tollen's reagent. 

• ketones do not gives this test, unless it is an alpha hydroxyl ketone.