To identify an organic unknown using basic cohesive chemical tests to discover the functional groups present in the unidentified chemical.There are six unknowns A-F including one of any of the following: Alcohol, Aldehyde, Ketone, carboxylic acid, ester or phenol. Each test used to identify the functional group must be dependant on the previous result. Therefore it is useful to use a flow chart to demonstrate the path of the test.
Positive NegativeAdd a solution of 2,4 dinitropheylhydrazineSilver Mirror Test Add Sodium Hydrogen +veCarbonate+ve -ve-ve+ve Add Sodium -veAdd Concentrated+ve HCl -veBut what makes these tests so conclusive. The first test is the reaction of 2,4 dinitrophenylhydrazine with the specific C=0 bond present in either an aldehyde or a ketone. The reaction creates a yellow precipitateand shows either a Aldehyde or a Ketone is present. A silver mirror test can then be carried out on the substance to find whether it is either a Aldehyde or a Ketone. Tollens reagent is added to the chemical in a test tube.
The tube is placed in a beaker of boiling water and once heated if a silver coating forms on the outside of the test tube the compound is an Aldehyde if no coating is made the substance is a Ketone. This silver coating is formed by the oxidising abilities of the Aldehyde which a Ketone doesn’t have.When the first test fails you must be testing one of the other four compounds. Carboxylic acid is one of these compounds and also happens to be the only compound in this collection that liberates Carbon dioxide from substances such as sodium hydrogen carbonate for example. When the white solid is added to the acid it begins to effervesce.
Limewater can then be added if the bubbles turn the limewater cloudy then CO2 is present which proves you have a Carboxylic Acid.However if you have one of the other three chemicals you can then begin distinguishing between the OH bond (present in both the Phenol and the Alcohol) and the Ester. Adding sodium in the presence of and OH bond hydrogen should be given off which can be tested with a lighted splint. If no hydrogen is given off you have an Ester but if it is Testing between a Alcohol or a Phenol is slightly more difficult as both have exactly the same functional group. Yet there is one minor difference between them that means you can tell a Phenol from an Alcohol.
Phenol acts as a very weak acid which will neutralise a strong base. By adding sodium hydroxide to the Phenol Sodium Phenoxide is made along with water. Then if concentrated HCl is added a milky emulsion is formed indicating the presence of Phenol. The test would prove inconclusive with an alcohol.
Equipment:* Several test tubes and a test tube holder* Tongs* Beaker* Bunsen burner, tripod, gauze and a heat mat* Pipettes* Spatula* Splints* 2,4 dinitropheylhydrazine (about 1ml)* Tollens reagent* Sodium Hydrogen Carbonate ( 1 spatula full)* Limewater (1ml)* Small piece of sodium* Concentrated NaOH and HCL (1ml each)* Safety Glasses* GlovesResults: Unknown compound E1. Negative with first test.2. CO2 released with sodium hydrogen carbonate.This shows that the compound is a carboxylic acid.
Accuracy, Reliability and Risk Factors:Accuracy and reliability relies on the correct selection of reagents and correct interpretation of the observations. This is achieved by research into the reagents to find which work best and which prove to be the most conclusive and what indications they give. Then when testing the compound it is important to notice precisely what the observations mean as the reliability of the whole experiment depends on good judgement. This judgement can be added by secondary test for example if you presume hydrogen is being released you can use a lighted splint to test to see if it is the case as if hydrogen is being liberated the splint should cause a small explosion or a pop. The risk factors involved are that many of the experiments use chemicals which can be classed as dangerous as they can be toxic or corrosive. Such chemicals require special care. For example when using a concentrated solution of HCl which is highly corrosive it is advisable to wear gloves and goggles to protect you from the acid. Other factors to take into account would be the use of equipment safely and accurately not only for the benefit of reliable results but also for your own protection.
Conclusion:The outcome of my investigation proved that compound E was a carboxylic acid. This was discovered as E didn’t react with 2,4 dinitrophenylhydrazine this meant that a ketone or aldehyde could be ruled out. However once sodium hydrogen carbonate was added CO2 was liberated thus testing positive to be a Carboxylic acid. After analysing the compounds spectra further identification could be made. First the mass spectrum result gave it a molecular mass of 74 units. The I.R shows as expected there is a Hydroxide group present and a C=O present which makes up the Carboxyl group. The nmr tells us that the compound has in total 6 hydrogen atoms present with a triplet between 0.
7-1.6 a R-CH3. A quartet between 2.
0-2.9 for and a singlet at 11.7 forThis suggests the following structure which also has a Mr of 74.This confirms the identity of the Carboxylic Acid as propanoic acid.
This is the third member of the homologous series of carboxylic acids. Propanoic acid like the other carboxylic acids comes from petroleum fractions and natural oils. It is a weak acid and so forms salts when reacted with metals, alkalis and carbonates. Metals also give off hydrogen, Alkalis produce water and Carbonates make carbon dioxide and water which is significant as it can be used to test for a carboxylic acid.The tests are possible due to the different ways in which the functional groups react. This means to discover what compound you have you need to know simply how each functional reacts and what reagents can take advantage of this. Due to the range of functional groups involving different mechanisms certain reagents will react in one way with one functional group and in a totally different way if at all with another.
For example 2,4 dinitropheylhydrazine reacts only with the C=O of a Aldehyde or Ketone forming a yellow precipitate in warm conditions and is unreactive with other functional groups. Tests like this can therefore give us conclusive evidence of approximately what a compound is or at least what functional group it has.Evaluation:The experiment succeeded in that it the aim of the experiment was successfully completed. However the chemical test on their own only lead to the correct identification of the functional group not of the specific compound.
In this case E was identified to be a carboxylic acid. But to further the identification and come up with a precise compound the spectra are required. Mass spectrum provides evidence of the compounds Relative Atomic Mass which in this case was 74. This on its own doesn’t give much extra evidence to what the compound exactly was but combined with the other evidence it provides a check to prove the identification wrong or right.
The I.R is slightly more useful at this stage. It shows us peaks at wave numbers of different functional groups identifying their presence. Although this should of all ready been deduced from the primary tests it acts as confirmation for the original conclusion.
In compound Es case there is a peak for an OH group and C=O group which confirms that it is indeed a carboxylic acid. The most useful spectra as far as I am concerned is the nmr. This shows peaks for different protons telling us exactly what atoms make up the compound.The nmr tells us that the compound has in total 6 hydrogen atoms present with a triplet between 0.
7-1.6 a R-CH3. A quartet between 2.0-2.9 for and a singlet at 11.7 forThis suggests the following structure which also has a Mr of 74.The Mr of 74 confirms this structure so together with the three spectra and the primary chemical tests there has been a positive identification of E showing it to be propanic acid.