Part 2: Testing the presence of carbonyl group in the compound
- 2cm3 of 2,4-dinitrophenylhydrazine was added into a test tube.
- 3 drops of the product were added into the test tube.
- The crystals were collected by using a suction flask.
- Propan-2-ol had a smell of disinfectants.
- Propan-2-ol, sulphuric acid, the organic products were all colourless.
- 2,4-dinitrophenylhydrazine solution was yellow in colour.
- The sulphuric acid had an irritating smell.
- The anhydrous calcium chloride was in white granule form.
- Potassium dichromate (VI) was orange powder.
- After adding 4g potassium dichromate to 10 cm3 of deionized water, the solution was orange in colour and there was undissolved potassium dichromate solid remained.
- The addition of concentrated sulphuric acid to potassium dichromate solution was exothermic.
- The potassium dichromate solution turned from orange to green after adding it to propan-2-ol.
- After the addition of the oxidizing agent, the mixture in the pear-shaped flask existed in 2 layers. The top layer was green and the lower layer was dark green in colour.
- During the distillation, the reading of the thermometer rose to 65oC when the first drop of product was collected and then to 69oC after which the distillation was stopped.
- The crystal formed by reaction the product with 2,4-DNP was yellow in colour.
Since this experiment was only qualitative but not quantitative, so no calculations can be done.
The percentage yield must not be 100%. Why?
There are quite a few reasons for this.
- There was a loss of product during the extractions.
First, when we distilled out the propanone from the reaction mixture, some propanone might be vapoured and condense and stuck to the wall of the pear-shaped flask, the thermometer and other apparatus it passed through. So, the product obtained was slightly reduced.
Second, the boiling point of cyclohexene is only 56oC, so it is volatile. During the oxidation of propan-2-ol which is exothermic, some of the propanone might be vapourized and lost.
Third, we added anhydrous calcium chloride to the product to dehydrate the product. There might be some product that stayed with the calcium chloride granules after pouring the product out which led to a loss of product.
- There were side reactions and incomplete reactions.
First, the reaction might also be incomplete as in this reaction, it was quite hard to distinguish whether the oxidation has completed or not, so there might be propan-2-ol unreacted in the mixture in the pear-shaped flask.
Third, as concentrated sulphuric acid is a dehydrating agent, so it might dehydrate the propan-2-ol to form propene
With acidified potassium dichromate as an oxidizing agent, propene could be oxidized to form propan-1,2-diol.
This could cause the propan-2-ol to form propan-1,2-diol which wasted the raw material (propan-2-ol) which led to less amount of propanone formed and obtained.
With sulphuric acid as a catalyst, 2 propan-2-ol might react with each other to form ether with elimination of a water molecule. The equation is as below:
Why do we need to use ice bath?
This is because the boiling point of propanone is only 56oC, during the oxidation, the boiling point of propanone would be reached as the oxidation was very exothermic. If the iced water bath was not used, most of the propanone would be vaporized and escape from the reaction mixture. This causes loss of the product. Therefore, we need to use an ice bath during the oxidation to minimize the amount of propanone vaporized.
Why do we need to use anhydrous sodium chloride?
This is because even after distillation, the product might contain some water. We need to use anhydrous sodium chloride as a drying agent to extract the water molecule mixed in the organic product.
Why does the ketone further oxidize to form carboxylic acid?
Ketone may undergo further oxidation to form carboxylic acid with acidified potassium permanganate and under reflux. This reaction has very high activation energy because this requires breaking the strong C-C bond
Why do we use the suction flask instead of purely using the gravitational force to filter the crystals?
This is because if we do not use the suction flask or any negative pressure, the filtration process will be too slow. The suction flask provides a negative pressure and together with the gravitational force, the filtration will be much faster. The crystal and the filter paper can also be air dried quickly.
A picture of a suction flask
The filter paper is placed on the plate, and the filter paper is moistened with a solvent to prevent initial leakage. The liquid to be filtered is poured into the cylinder and drawn through the perforated plate by vacuum suction. In our experiment, we use running tap water to produce a suction force. Actually, there are many more methods to produce the negative pressure, such as lowering the pressure using a vacuum pump.
The main advantage in using this type of filtration is that it proceeds much more quickly (several orders of magnitude) than simply allowing the solvent to drain through the filter medium via the force of gravity.
Ketones and aldehydes
Ketones are versatile compounds which can be converted to a number of useful functional groups through reduction, nucleophilic addition or condensation reactions. Ketones and aldehydes are important series in preparation of other compounds and they are commonly prepared by oxidizing alcohol which is done in this experiment. Ketone also plays a very important part in organic synthesis. Ketones and aldehydes can be synthesised into many other chemicals. Reactions involving ketones include nucleophilic addition reactions to the carbon-oxygen double bond to form an –OH group in the compound with the addition of a nucleophilic group.
Testing carbonyl compound
There are lots of ways to test the existence of C=O in an organic compound.
- Using 2,4-dinitrophenylhydrazine
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.
- 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- → Ag (s) + 2 NH3 (aq)
RCHO (aq) + 3 OH- → 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
- 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 , while Fehling's B is a clear solution of aqueous and a strong alkali (commonly ).
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
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
The Kornblum–DeLaMare rearrangement is a in in which a primary or secondary is converted to the corresponding and under .
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)
Carbonyl compounds can also be formed using the Nef reaction.
The Nef reaction is an describing the of a salt of a primary or secondary (1) to an or a (3) and (4).
Precaution of this experiment
- Concentrated sulphuric acid is highly corrosive and oxidizing, so it must be handled with care.
- Acidified potassium dichromate is also very oxidizing, so it should also be treated carefully.
- If any concentrated sulphuric acid or acidified potassium dichromate are in contact with our skin, we should wash it with running tap water immediately.
- During the oxidation of propan-2-ol, safety goggles should be worn as this reaction is highly exothermic.
Here's what a teacher thought of this essay
This is a highly detailed, well laid out experiment with excellent alternative methods. This piece of work is 5*