The analysis and identification of unknown organic compounds constitutes a very important aspect of experimental organic chemistry.
There is no definite set procedure that can be generally applied to organic qualitative analysis. Various books have different approaches, but a systematic approach based on the scheme given below will give good results.
Students should, however, consult the laboratory manual and Textbook of Practical Organic Chemistry, A.I. Vogel (4th Edition).
Before outlining the general scheme, one or two points of practical importance should be noted.
(a) Quantities of substance for tests. For most tests about 0.1 g solid or 0.1 - 0.2 mL (2 - 3 drops) of liquid material (NOT MORE) should be used.
(b) Reagents likely to be met within organic analysis are on the reagent shelves. Students are advised to develop a general knowledge of the physical characteristics of common organic compounds. If in doubt about the expected result of a test between a certain compound and a reagent, carry out a trial test with a known compound and compare with the unknown.
(c) Quantities of substance derivatives. Students have wasted much time and material in the past by taking too large a quantity of substance for preparation of a derivative. In general, 0.5 - 1 g (or 0.5 - 1 mL) of substance gives the most satisfactory results.
If a practical book instructs one to use larger quantities (3 - 4 g or more), the quantities should be scaled down to 1 g or 1 mL of the unknown substance and corresponding quantities of reagents should be used.
A. Preliminary Tests
(a) Note physical characteristics - solid, liquid, colour and odour.
(b) Perform an ignition test (heat small amount on metal spatula) to determine whether the compound is aliphatic or aromatic (i.e. luminous flame - aliphatic; sooty flame - aromatic).
B. Physical Constants
Determine the boiling point or melting point. Distillation is recommended in the case of liquids (see Appendix 3). It serves the dual purpose of determining the b.p., as well as purification of the liquid for subsequent tests.
C. Analysis for elements present
At C10 level, the elements present will be told to you, but read up the method.
D. Solubility tests
The solubility of the unknown in the following reagents provides very useful information. In general, about 3 mL of the solvent is used with 0.1 g or 0.2 mL (2 - 3 drops) of the substance. The class of compound may be indicated from the following table:
REAGENT AND TEST CLASS GROUP OF COMPOUNDS Soluble in cold or Neutral, acidic or Lower members of series. hot water. (If the basic. (Test with Neutral, e.g. alcohols; unknown is litmus or universal Acidic, e.g. acids, phenols; soluble do NOT indicator paper). Basic, e.g. amines. perform solubility tests below) __________________________________________________________________ Soluble in dil. HCl Basic Most amines (except III amines with only aromatic groups). __________________________________________________________________ Soluble in dil. NaOH Acidic Most acids, most phenols. __________________________________________________________________ Soluble in NaHCO3 Strongly acidic Most carboxylic acids. __________________________________________________________________ Insoluble in water, Neutral Hydrocarbons, nitrohydro- acid and alkali carbons, alkyl or aryl halides, esters and ethers. Higher molecular weight alcohols, aldehydes and ketones. _________________________________________________________________
E. Group Classification Tests
From the previous tests it is often possible to deduce the functional groups present in the unknown compound. Consult i.r. spectra when available.
Individual tests are then performed to identify and confirm the functional groups present.
1. Students are strongly advised against carrying out unnecessary tests, since not only are they a waste of time but also increase the possibility of error. Thus it is pointless to first test for alcohol or ketone in a basic compound containing nitrogen! Instead tests for amines, etc. should be done on such a compound.
2. A systematic approach cannot be overemphasised in group classification tests to avoid confusion and error.
F. Consultation of Literature
Once the functional group has been identified, reference is made to tables in a book on organic analysis, for assessing possibilities and for the preparation of suitable solid derivatives.
It should be noted that whilst two substances with the same functional group may sometimes have very similar b.p. or m.p., solid derivatives canusually be chosen from the literature, with m.p. differences of about 10 (or more), which distinguish between the two possibilities.
Example:
COMPOUND B.P. DERIVATIVES (M.P.)
2,4-DNPH SEMICARBAZONE
Diethyl ketone 102 156 139
Methyl n-propyl ketone 102 144 112
G. Preparation of derivatives
The final characterisation of the unknown is made by the preparation of suitable solid derivatives. The derivative should be carefully selected and its m.p. should preferably be between 90 - 150 for ease of crystallisation and m.p. determination.
Preparation of one derivative should be attempted. The derivative should be purified by recrystallisation, dried and the m.p. determined. Derivatives should be submitted correctly labelled for assessment together with the record.
Recording of Results
The results should be recorded in a systematic manner. Results should be recorded in the practical book at the time (not written up afterwards).
A record should be made of every test carried out, no matter whether a NEGATIVE RESULT HAS BEEN OBTAINED.
Test, observation and inference should be given.
At the conclusion of the analysis a brief summary of results should be included, giving the name, b.p. or m.p., and formula of the analysed compound.
Qualitative Analysis for Elements (for reference only)
In organic compounds the elements commonly occurring along with carbon and hydrogen, are oxygen, nitrogen, sulphur, chlorine, bromine and iodine. The detection of these elements depends upon converting them to water-soluble ionic compounds and the application of specific tests.
Lassaigne's Sodium Fusion Test
C, H, O, N, S, X NaX NaCN -> Na2S NaCNS
PROCEDURE
Place a piece of clean sodium metal, about the size of a pea into a fusion tube. Add a little of the compound (50 mg or 2 - 3 drops).* Heat the tube gently at first, allowing any distillate formed to drop back onto the molten sodium. When charring begins, heat the bottom of the tube to dull redness for about three minutes and finally plunge the tube, while still hot, into a clean dish containing cold distilled water (6 mL) and cover immediately with a clean wire gauze.**
*For liquids it is better to first melt the sodium add the liquid drop by drop.
**CAUTION: The tube shatters, and any residual sodium metal reacts with water. Stir the mixture, boil for 1 - 2 minutes, on a tripod and filter hot through a fluted paper.
The 'fusion' filtrate which should be clear and colourless, is used for the SPECIFIC TESTS DESCRIBED BELOW: