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Yellow colour precipitate Essay

This is used in our experiment. 2,4-DNP reacts with the carbonyl group for a condensation reaction with the elimination of a water molecule. Take propanone as an example, The product formed is a yellow colour precipitate, so we can easily distinguish the presence of C=O group. This can also help us to identify the carbonyl compound as the precipitate collected has a sharp melting point. By using the melting point test, we can find out the melting point of the crystals formed and compare the result with a data book to find out the carbonyl compound. 2. Tollens’ reagent (Aldehyde only) The formula of this reagent is Ag(NH3)2+.

As this reagent is not very stable, it must be prepared freshly in laboratory. To prepare the reagent, aqueous ammonia can be added in a continuous fashion directly to silver nitrate solution. At first, silver oxide will be formed and precipitate out, but as more ammonia solution is added the precipitate dissolves and the solution becomes clear as diamminesilver(I) is formed. At this point the addition of the ammonia should be stopped. This reagent is used in the silver mirror test. In this test, when there is the presence of aldehyde group, there would be formation of silver mirror.

The equation of this reaction is as below [Ag(NH3)2]+ (aq) + e- i?? Ag (s) + 2 NH3 (aq) RCHO (aq) + 3 OH- i?? RCOO- + 2 H2O + 2 e- The aldehyde acts as an reducing agent where [Ag(NH3)2]+ was reduced to Ag(S) , the formation of silver mirror. This reaction is very useful to extinguish aldehyde from ketone as ketone does not show this reaction. Silver mirror formed in a flask 3. Fehling’ reagent Aldehydes are also oxidized by the Fehling’s solution. This reagent is also prepared freshly in the laboratory. It is made initially as two separate solutions, known as Fehling’s A and Fehling’s B.

Fehling’s A is a blue aqueous solution of copper(II) sulfate, while Fehling’s B is a clear solution of aqueous potassium sodium tartrate and a strong alkali (commonly sodium hydroxide). Equal volumes of the two mixtures are mixed together to get the final Fehling’s solution, which is a deep blue colour. The Copper (II) ion is reduced to copper (I) oxide which is a red ppt, and in some cases, to copper metal (copper mirror). This is also useful to distinguish aldehyde from ketone and aromatic aldehyde as both ketone and aromatic aldehyde does not show any reaction. Using IR spectrum

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Using the IR spectrum, if the compound contains a carbonyl (C=O) group, there will be a stretch in the wave number 1670-1820 with high intensity. Below is the IR spectrum of propanone Other preparation methods of ketones and aldehydes Kornblum-DeLaMare rearrangement The Kornblum-DeLaMare rearrangement is a rearrangement reaction in organic chemistry in which a primary or secondary organic peroxide is converted to the corresponding ketone and alcohol under base catalysis. Geminal halide hydrolysis The reactants are a geminal dihalide and water or a hydroxide. The reaction product is a ketone or an aldehyde.

The first part of the reaction mechanism consists of an ordinary nucleophilic aliphatic substitution to produce a gem-halohydrin. Ruzicka Large Ring Synthesis Formation of large ring alicyclic ketones from dicarboxylic acids by thermal decomposition of salts with metals of the second and fourth groups of the periodic table (Ca, Th, Ce) Nef reaction Carbonyl compounds can also be formed using the Nef reaction. The Nef reaction is an organic reaction describing the acid hydrolysis of a salt of a primary or secondary nitroalkane (1) to an aldehyde or a ketone (3) and nitrous oxide (4).

Precaution of this experiment 1. Concentrated sulphuric acid is highly corrosive and oxidizing, so it must be handled with care. 2. Acidified potassium dichromate is also very oxidizing, so it should also be treated carefully. 3. If any concentrated sulphuric acid or acidified potassium dichromate are in contact with our skin, we should wash it with running tap water immediately. 4. During the oxidation of propan-2-ol, safety goggles should be worn as this reaction is highly exothermic.

Reference: http://en.wikipedia.org/wiki/Chromic_acid.

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