Reactions and characterisation of aliphatic aldehydes

The following reactions are characteristic of aliphatic aldehydes those which are shared by ketones, due to the presence of the carbonyl group, are given under Aliphatic Ketones (Section 111,74). 

Do not heat the silver solution or allow it to stand even for a few hours, since explosive silver fulminate may be formed. The ammoniacal solution of silver nitrate is prepared by treating 3 ml. of 0-lN silver nitrate solution with very dilute ammonia solution dropwise until the precipitate which is first formed just redissolves. 

It is preferable to use Tollen s ammoniacal silver nitrate reagent, which is prepared as follows Dissolve 3 g. of silver nitrate in 30 ml. of water (solution A) and 3 g. of sodium hydroxide in 30 ml. of water (solution B). When the reagent is requir, mix equal volumes (say, 1 ml.) of solutions A and JB in a clean test-tube, and add dilute ammonia solution drop by drop until the silver oxide is just dissolved. Great care must be taken in the preparation and use of this reagent, which must not be heated. Only a small volume should be prepared just before use, any residue washed down the sink with a large quantity of water, and the test-tubes rinsed with dilute nitric acid. 

Preparation of Fehling s solution. Solution No. 1. Dissolve 34-64 g. of A.R. copper sulphate crystals in water containing a few drops of dilute sulphuric acid, and dilute the solution to 500 ml. 

Solution No. 2. Dissolve 60 g. of pure sodium hydroxide and 173 g. of pure Rochelle salt (sodium potassium tartrate) in water, filter if necessary through a sintered glass funnel, and make up the filtrate and washings to 500 ml. 

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