Question

5. Write in your own words, the step-wise experimental procedure for the synthesis of the unknown organic compound.

6. How will the structure of your organic product be determined using the ¹HNMR data given/acquired? That is, what is the process that you will follow to identify the compound?

Answer 1

5. Characterizing and identifying unknown organic compounds is done by solubility tests and functional group tests. Based on their solubility behaviour and other visible effects like during color changes and precipitate formations during chemical reactions, we come to appropriate conclusions.

A) SOLUBILITY TESTS :

Step 1: Take your unknown organic compound and add water to it. Only low molecular weight compounds( fewer than five carbon atoms ) and oxygen containing compounds are soluble in water. Observe if the compound is soluble or not.

Step 2: If compound is soluble in water, then test the solution with pH paper. If it comes out to be an acidic, then it means the unknown compound is a low molecular weight carboxylic acid. If it comes out to be basic, then it means the unknown compound is low molecular weight amines. If the solution is observed to be neutral then it may be either low molecular weight aldehydes/ ketones/ alcohols.

Step 3: If compound is insoluble in water, then verify the unknown compound's solubility in 6M NaOH. Positive identification indicates the presence of high molecular weight carboxylic acids.

Step 4: If compound is insoluble in 6M NaOH then verify the solubility in 6M HCl. Now, positive identification means that the unknown compound is a high molecular amine. Negative identification indicates the compound is a high molecular aldehyde/ ketone/ alcohol or a high molecular alkane/ alkene/ alkyl halide.

Now, let's test this insoluble compound (Let's call this "X" ) to determine it's functional group.

B) FUNCTIONAL GROUP TESTS:

Step 1: Silver Nitrate Test for alkyl halides and carboxylic acids:

Test the coumpound with silver nitrate in ethanol. If it results in the formation of a yellow or white presipitate. This reaction is slow and occasional warming might be required.

NOTE: Now check if the above precipitate is soluble in dilute nitric acid. If soluble, then the unknown coumpound is a carboxylic acid. If insoluble, then the unknown compound is an alkyl halide.

Step 2: Beilstein Test for halides:

Heat "X" with copper oxide. If a volatile copper halide( imparts blue-green color to flame ) is formed, it means "X"  is an alkyl halide. Beilstein test cannot differentiate among chlorine, bromine and iodine.

Step 3: Lucas test for alcohols:

Mixture of hydrochloric acid and zinc chloride is called Lucas Reagent. Treat "X" with Lucas Reagent. Formation of alkyl chloride as an insoluble second layer indicates that "X" is an alcohol.

NOTE: Primary alcohols don't react at room temperature so it appears to be dissolving. Whereas secondary alcohols react slowly. When it comes to tertiary/ benzylic/ allylic alcohols, the reaction is spontaneous.

We can also use Chromic Acid Test for alcohols, even for low molecular weight and soluble/insoluble alcohols.

Step 4: Bromination test for alkenes:

Treat "X" with Bromine ( red in color). If a colorless product is formed, then it means that our "X" is an alkene and an addition reaction has taken place.

Step 5: Potassium PermanganateTest for aldehydes/ phenols/ aromatic amines/ some alcohols/ alkenes:

Treat "X" with Potassium Permanganate. If the purple color turns into a brown precipitate( MnO2), it means that our "X" is an unsaturated compound compound and oxidation has taken place.

Thus an unknown organic compound's functional group is found.

Furthur we can continue other tests to find the exact molecular formula of the compound based on the wights of precipitates or using NMR spectra etc.

6. Deducing the structure of a compound using the 1HNMR data is done by analysis of it's spectrum:

The peaks in the Nuclear Magnetic Resonance spectrum are important in determining the structure of compound. This is done by:

a) identifying the number of different 1H environments in the compound and

b) the ratio of 1H in each of these environments.

Let's observe some NMR spectra based on the data values and relate the peaks in the spectra to the possible organic compound.

Based on these 1HNMR peaks, we can also find the exact isomer of a compound if the formula is given.