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Practical Organic chemistry IIIdentification of Organic compounds

An Introduction

Prepared by :

Pshtiwan Ghareeb Ali

Bsc. In Pharmacy

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The identification and characterization of the structures of unknown substances are an important part of organic chemistry.

To establish and identify the structure of organic compounds there are two methods.

The first and older method is to check the purity of unknown, and then to run certain examinations of physical properties (Physical state, odor, color, M.P. , B.P.) ; ignition ; elemental analysis; solubility : PH test and classification tests to identify the functional groups of the compound following by the formation of one or more derivatives to confirm the exact nature of the desired compound .

The second method is to determine the spectral properties of the desired organic compound by using mainly IR; C13-NMR, H-NMR and MS spectroscopy.

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Systematic identification steps

1. Preliminary classification by physical state, color, and odor*.

2. Melting-point or boiling-point determination; other physical data*.

3. Purification, if necessary*.

4. Elemental analysis, if necessary*.

5. Determination of solubility behavior in water and in acids and bases*.

6. Simple preliminary tests: ignition (combustion)*.

7. Application of relevant chemical classification tests*.

8. Preparation of derivatives, if required

9. Inspection of tables for possible structure(s) of unknown; elimination of unlikely compounds

* For procedures and detail of those tests, Refer to the first course lecture slides

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Important notes

•The solubility tests usually suggest or eliminate severalpossible functional groups.

•Choose only those tests that the solubility tests suggestmight be meaningful.

•When you are performing a test that is new to you, it isalways good practice to run the test separately on botha known substance and the unknown at the same time.This practice lets you compare results directly.

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•Do not perform the chemical tests either haphazardly or in a methodical, comprehensive sequence. Instead, use the tests selectively.

•You should also examine the tables of unknowns in Appendix 1 carefully

•The boiling point or the melting point of the unknown may eliminate the need for many of the tests.

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•Once the melting or boiling point, the solubilities, and the main chemical classification tests have been made, you should be able to identify the class of compound (aldehyde, ketone, and so on).

•At this stage, with the melting point or boiling point as a guide, you can compile a list of possible compounds from one of the appropriate tables in Appendix 1. It is very important to draw out the structures of compounds that fit the solubility, classification tests, and melting point or boiling point that were determined.

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Classification Tests ( Functional group tests)

•The classic scheme involves performing a number of chemical tests on a substance, each of which is specific for a type of functional group.

•You may normally do these tests quickly, and they are designed so that the observation of a color change or the formation of a precipitate indicates the presence of a particular functional group.

•The results of these tests usually allow the assignment of the unknown to a structural class such as alkene, aldehyde, ketone, or ester, for example.

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The following factors should be considered when performing qualitative classification tests for functional groups:

• A compound may contain more than one functional group, so the complete series of tests must be performed unless you have been told that the compound is monofunctional.

• Careful attention is required when the functional group tests are performed. Record all observations, such as the formation and color of any solid produced as a result of a test.

• Some of the color-forming tests occur for several different functional groups. Although the expected color is given in the experimental procedures, the observed color may be affected by the presence of other functional groups.

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• It is of utmost importance to perform a qualitative test on both the unknown and a known compound that contains the group being tested.

Some functional groups may appear to give only a slightly positive test, and you will find it helpful to determine how a compound known to contain a given functional group behaves under the conditions of the test being performed.

It is most efficient and reliable to do the tests on standards at the same time as on the unknown. In this manner, inconclusive positive tests may be interpreted correctly. Because aliphatic compounds are sometimes more reactive than aromatic ones, it is wise to perform a test on both of these types of standards along with the unknown.

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• The results obtained from the elemental analysis and the solubility tests can be used in deciding which functional group tests should be performed initially and which should not be done at all. The following examples illustrate the use of the preliminary work in making these decisions:

a. A classification test for an amine should be applied first if a compound is found to be soluble in dilute hydrochloric acid and to contain nitrogen.

b. The test for a phenol should be performed on an unknown that is soluble in dilute sodium hydroxide but insoluble in dilute sodium bicarbonate.

c. The tests for alkyl or aryl halides should be omitted if the elemental analysis indicates the absence of halogen.

d. The tests for amines, amides, nitriles, and nitro compounds need not be performed if nitrogen is absent, as shown by the elemental analysis.

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• You should use a logical approach for deciding which tests are needed. The result obtained from one test, whether positive or negative, often has a bearing on which additional tests should be done.

A random “hit-or-miss” approach is wasteful of time and often leads to erroneous results. Another error commonly made by beginners in qualitative organic analysis is to omit the tests for functional groups and immediately attempt preparation of a derivative. This tactic has a very low probability for success.

For example, trying to make a derivative of a ketone is certain to fail if the unknown is actually an alcohol. You should continue performing the different classification tests until you have defined the nature of the functional group(s) present in the unknown as completely as possible! This will minimize unproductive efforts at derivatization of the compound.

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Preparation of derivatives

One of the principal tests for the correct identification of an unknown compound is to convert the compound by a chemical reaction to another known compound. This second compound is called a derivative.

The best derivatives are solid compounds, because the melting point of a solid provides an accurate and reliable identification of most compounds. Solids are also easily purified through crystallization.

Tables of unknowns and derivatives are listed in Appendix 1. Procedures for preparing derivatives are given in Appendix 2.

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Spectroscopy

Spectroscopy is probably the most powerful and modern tool available to the chemist for determining the structure of an unknown compound. It is often possible to determine the structure through spectroscopy alone.

On the other hand, there are also situations for which spectroscopy may not be of much help, and the traditional methods must be relied on. For this reason, you should not use spectroscopy to the exclusion of the more traditional tests but rather as a confirmation of those results. Nevertheless, the main functional groups and their immediate environ-mental features can be determined quickly and accurately with spectroscopy.

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The following example illustrates how classification tests and preparation of derivatives are used to identify an unknown compound.

Now suppose that two derivatives, the 2,4-dinitrophenylhydrazone and the semi-carbazone were prepared from the unknown and found to melt at 93–95 °C and 69–71 °C.

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From the experimental facts that were given, some of these compounds may be eliminated as possibilities for the following reasons: 4 because it contains a halogen, 5 because it contains a carbon-carbon double bond, and 6 because it is not a methyl ketone. Examining Table 25.1 reveals that the derivatives of only one of the ketones under consideration, namely 3-methyl-2-pentanone (3), melt at these temperatures. Hence, the identity of the unknown is deduced.

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Tests for Unsaturation and aromaticity

Tests for Aldehyde and ketones

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Derivatives for aldehyde and ketone

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Carboxylic acid

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Phenol

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Amines

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Derivatives of amines

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Alcohols

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Derivatives of Alcohol

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Esters

Preparation of derivatives

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