Chemistry Practical for B. Sc. Part II) - Dr Deepak Pant (read e book txt) 📗

B. Sc. CBZ with Biotech 2nd Year
(Chemistry Practical)

1. Qualitative Analysis of Organic Compounds (2-33)
2. Inorganic Quantitative Analysis (33-45)
3. Inorganic Synthesis (46-54)


Qualitative Analysis of Organic Compounds.
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.
Practical Notes
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.
General Scheme of Analysis
A. Preliminary Tests
(a) Note physical characteristics - solid, liquid, colour and odour.
(b) 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 (Lassaigne's Test)
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.
Theory
It is a general test for the detection of halogens, nitrogen & sulphur in an organic compound. These elements are bonded covalently in the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution, and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne's extract.
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.**
Test for Nitrogen
The carbon and nitrogen present in the organic compound on fusion with sodium metal give sodium cyanide (NaCN) soluble in water. This is converted in to sodium ferrocyanide by the addition of sufficient quantity of ferrous sulphate .Ferric ions generated during the process reacts with ferrocyanide to form blue precipitate of ferric ferrocyanide.


Test for chlorine
Chlorine present in the organic compound forms sodium chloride on fusion with sodium metal. Sodium chloride, extracted with water, can be easily identified by adding silver nitrate solution after acidifying with dil. Nitric acid.


Test for sulphur
If sulphur is present in the organic compound, sodium fusion will convert it into sodium sulphide. Sulphide ions are readily identified by sodium nirtoprusside.

The 'fusion' filtrate which should be clear and colourless, is used for the SPECIFIC TESTS DESCRIBED BELOW:
1. To a portion (2 mL) of the 'fusion' filtrate add 0.2 g of powdered ferrous sulphate crystals. Boil the mixture for a half a minute, cool and acidify by adding dilute sulphuric acid dropwise. Formation of a bluish-green precipitate (Prussian blue) or a blue solution indicates that the original substance contains nitrogen. If no precipitate appears, allow to stand for 15 minutes, filter and inspect filter paper.
2. SULPHUR (SULPHIDE)
To the cold 'fusion' filtrate (1 mL) add a few drops of cold, freshly prepared, dilute solution of sodium nitroprusside. The latter may be prepared by adding a small crystal of the solid to 2 mL of water. Production of a rich purple colour indicates that the original substance contains sulphur. This test is very sensitive. Only strong positive results are significant.
3. HALOGENS (HALIDES)
Acidify a portion (1 mL) of the 'fusion' filtrate with 2N nitric acid, and if nitrogen and/or sulphur are present, boil for 1 - 2 minutes.* Cool and add aqueous silver nitrate (1 mL), compare with a blank. Formation of a heavy, white or yellow precipitate of silver halide indicates halogen. If a positive result is obtained: acidify the remaining portion of the 'fusion' filtrate with dilute sulphuric acid, boil and cool. Add carbon tetrachloride (1 mL) and a few drops of freshly prepared chlorine water. Shake the mixture.
(a) If the carbon tetrachloride layer remains colourless - indicates chlorine.
(b) If the carbon tetrachloride layer is brown - indicates bromine.
(c) If the carbon tetrachloride layer is violet - indicates iodine.
*If nitrogen and/or sulphur are also present, the addition of silver nitrate to the acidified 'fusion' solution will precipitate silver cyanide and/or silver sulphide in addition to the silver halides. The removal of hydrogen cyanide and/or hydrogen sulphide is effected by boiling the 'fusion' solution. GROUP CLASSIFICATION TESTS

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:


SOLUBILITY TABLE
REAGENT AND TEST CLASS GROUP OF COMPOUNDS
Soluble in cold or hot water. (If the unknown is soluble do NOT perform solubility tests below) Neutral, acidic or basic. (Test with litmus or universal indicator paper) Lower members of series. Neutral, e.g. alcohols; Acidic, e.g. acids, phenols; Basic, e.g. amines
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, acid and alkali Neutral Hydrocarbons, nitrohydro-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.
NOTE:
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.
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.
Tests for unsaturation
1. Cold dilute potassium permanganate solution.
2. Solution of bromine in carbon tetrachloride.
Tests for compounds containing nitrogen
1. Amines
(a) Nitrous acid.
(b) Confirmatory tests.
2. Compounds which give amines or ammonia on acid or alkaline hydrolysis:
Amides, substituted amides, anilides, nitriles.
3. Compounds which give amines on reduction:
Nitro, nitroso, azo, hydrazo, nitriles.
Tests for compounds containing C, H and possibly oxygen
1. Carboxylic acids
Na2CO3 or NaHCO3 solution liberate carbon dioxide.
2. Phenols
(a) Sodium hydroxide solution (soluble). Insoluble in and no CO2 from NaHCO3 (except when electron attracting groups present, e.g. 2,4-dinitrophenol).
(b) Ferric chloride solution.
(c) Bromine water.
3. Aldehydes and Ketones
(a) 2,4-dinitrophenylhydrazine (as Brady's reagent) for C=O.
(b) Iodoform test for CH3CO-.
4. Aldehydes only (reducing properties)
(a) Fehling's solution.
(b) Tollen's reagent (ammoniacal AgNO3 solution).
(c) Jones reagent.
5. Alcohols
(a) Lucas' reagent to distinguish I, II and III alcohols.
(b) Jones reagent.
(c) Metallic sodium (use dry liquid and dry tube).
6. Sugars
(a) Molisch's test.
7. Esters
(a) Hydroxamic acid test.
(b) Hydrolysis.


________________________________________
Write up of the identification of an unknown organic compound

Compound containing C, H ,N, Halogens, S

Physical characteristics ...................... (solid, liquid, gas, colour, odour, etc.)

Ignition test .............................. (aromatic or aliphatic)

Physical constant ........................ (boiling point or melting point)

Solubility tests (in tabular form)

Group classification tests (in tabular form)
Test Observation Inference

From the above tests and