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

observations the given compound is probably a
.........................(acid, phenol, aldehyde, etc.)

Consultation of literature (Possibilities) M.P. of derivative
(a)

(b)

(c)

Preparation of derivative (method of preparation)

Observed m.p. of derivative
Lit. m.p. of derivative

Result
Compound No. ........................ is ............................
(give formula)


TESTS FOR FUNCTIONAL GROUPS
I. UNSATURATED COMPOUNDS
Two common types of unsaturated compounds are alkenes and alkynes characterised by the carbon-carbon double and triple bond, respectively, as the functional group. The two common qualitative tests for unsaturation are the reactions of the compounds with (a) bromine in carbon tetrachloride and (b) potassium permanganate.
(a) 2% Bromine in carbon tetrachloride
Dissolve 0.2 g (or 0.2 mL) of the compound in 2 mL of carbon tetrachloride or another suitable solvent and add the solution dropwise to 2 ml of 2% bromine solution in carbon tetrachloride and shake. e. g.

Rapid disappearance of the bromine colour to give a colourless solution is a positive test for unsaturation.
NOTE: The reagent is potentially dangerous. Keep it off your skin and clothes; protect your eyes and nose.
(b) 2% Aqueous potassium permanganate
Dissolve 0.2 g (or 0.2 mL) of the substance in 2 mL of water (acetone may also be used as solvent). Add the potassium permanganate solution dropwise and observe the result.
e.g.

For a blank determination, count the number of drops added to 2 mL of acetone before the colour persists. A significant difference in the number of drops required in the two cases is a positive test for unsaturation.
II. COMPOUNDS CONTAINING NITROGEN
1. Amines
(a) Reaction with nitrous acid Dissolve the amine (0.5 mL) in concentrated acid (2.0 mL) and water (3 mL) and cool the solution to 0 - 5 in an ice-bath for 5 minutes. Add a cold solution (ice-bath) of sodium nitrite (0.5 g) in water (2.0 mL) from a dropper, with swirling of the test tube, still keeping the mixture in the ice-bath.

AMINE REACTION

Aliphatic N2 evolved.
RNH2 + HNO2 -> ROH + N2 + H2O
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Aromatic Diazonium salt is formed.
ArNH2 + HNO2 -> ArN=N+
Add the cold diazonium solution and with swirling
to a cold solution of 2-naphthol (0.2 g) in 5% NaOH
solution (2 mL). An orange-red azo dye is formed.
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aliphatic and Yellow oily nitrosamines are generally formed.
aromatic R2NH + HNO2 -> R2N-NO
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(b) Reaction with benzenesulphonyl chloride
Benzenesulphonyl chloride reacts with primary and secondary but not with tertiary amines to yield substituted sulphonamides.
e.g. (a) C6H5SO2Cl + H-NHR + NaOH -> C6H5SO2NHR + NaCl + H2O
(b) C6H5SO2Cl + H-NR2 + NaOH -> C6H5SO2NR2 + NaCl + H2O
The substituted sulphonamide formed from a primary amine dissolves in the alkali medium whilst that produced from a secondary amine is insoluble in alkali.
Place 0.5 mL (or 0.5 g) of the compound, 15 - 10 mL of 5% NaOH and 1 mL of benzenesulphonyl chloride in a test tube, stopper the tube and shake until the odour of the sulphonyl chloride has disappeared. The solution must be kept alkaline (if no reaction has occurred, the substance is probably a tertiary amine).
If a precipitate appears in the alkaline solution, dilute with about 10 mL of water and shake; if the precipitate does not dissolve, a secondary amine is indicated.
If there is no precipitate, acidify it cautiously to congo red with concentrated hydrochloric acid (added dropwise): a precipitate is indicative of a primary amine.
2. Amides R-CO-NH2
Simple primary amides can be decomposed by boiling with alkali and thereby evolving ammonia.
e.g. CH3-CO-NH2 + NaOH -> CH3-CO2- Na+ + NH3 ¬
Boil 0.5 g of the compound with 5 mL of 10% sodium hydroxide solution and observe whether ammonia is evolved.
III. COMPOUNDS CONTAINING C, H AND POSSIBLY OXYGEN
1. Carboxylic acids - test with 5% aq. NaHCO3
R-CO2H + NaHCO3 -> R-CO2- Na+ + CO2 ¬ + H2O
Sodium hydrogen carbonate reacts with carboxylic acids to give the sodium salt of the acid and liberates carbon dioxide. If the acid is insoluble in water and the reaction is sluggish dissolve the acid in methanol and add carefully to a saturated sodium hydrogen carbonate solution, when a vigorous effervescence will be observed.
2. Phenols [Soluble in NaOH and produce no CO2 from NaHCO3]
(a) Bromine water
Phenols are generally highly reactive towards electrophilic reagents and are readily brominated by bromine water. e.g.

Dissolve or suspend about 0.05 g of the compound in 2 mL of dilute hydrochloric acid and add bromine water dropwise until the bromine colour remains. A white precipitate of the bromophenol may form. Solid bromophenol derivatives can be used for the confirmation of the structure of a phenol (cf the preparation of derivatives).
(b) Ferric chloride test
Most phenols react with iron (III) chloride to form coloured complexes. The colours vary - red, purple, blue or green - depending on various factors, e.g. the phenolic compound used, the solvent, concentration. Since some phenols do not give colours, a negative test must not be taken as significant without supporting information.
Dissolve 0.05 g of the compound in 2 mL water (or a mixture of water and ethanol if the compound is not water-soluble) and add an aqueous solution of ferric chloride dropwise. Observe any colour changes which may occur.
3. Aldehydes and ketones
(a) 2,4-Dinitrophenylhydrazine (as Brady's reagent)
A test for the carbonyl group (C=O) in aldehydes and ketones. 2,4- Dinitrophenylhydrazine gives sparingly soluble yellow or red 2,4-dinitrophenylhydrazones with aldehydes and ketones.

Add 3 mL of the reagent to 2 drops of the compound in a test tube and shake. If no precipitate forms immediately, warm and allow to stand for 5 - 10 minutes. A crystalline precipitate indicates the presence of a carbonyl compound.
The bench reagent is very dilute and is intended for qualitative tests only and should not be used in the preparation of a derivative for identification purposes.
(b) Iodoform test for CH3CO-
Dissolve 0.1 g (or 5 drops) of the compound in 2 mL of water; if it is insoluble in water add sufficient dioxan to produce a homogeneous solution. Add 2 mL of 5% NaOH solution and then introduce the potassium iodide - iodine reagent dropwise with shaking until a definite dark colour of iodine persists. Allow to stand for 2 - 3 minutes; if no iodoform separates at room temperature, warm the test tube in a beaker of water at 60 . Add a few more drops of the iodine reagent if the faint iodine colour disappears. Continue the addition of the reagent until a dark colour is not discharged after 2 minutes heating at 60 . Remove the excess of iodine by the addition of a few drops of dilute sodium hydroxide solution with shaking, dilute with an equal volume of water, and allow to stand for 10 minutes. The test is positive if a yellow precipitate of iodoform is deposited. Filter off the yellow precipitate, dry upon pads of filter paper and determine the m.p. Iodoform melts at 120 (it can be recrystallised from methanol- water).
The reaction is given by acetaldehyde and simple methyl ketones. Alcohols containing the CH3CHROH group will be oxidised under the reaction conditions and also give a positive test.
4. Aldehydes only (reducing properties).
(a) Fehling's solution
Aldehydes reduce Fehling's solution to yellow or red copper (I) oxide.
Preparation of the reagent: Mix equal volumes of Fehling's solution solution I (aqueous alkaline potassium tartrate) and Fehling's solution II (copper sulphate solution).
Add 2 drops (or 0.05 g) of the compound and 2 - 3 drops of the reagent and heat on a boiling water bath for 3 - 4 minutes.
The test is positive for aliphatic aldehydes, but is often indecisive for aromatic aldehydes where Jones' Reagent is often useful (see 5).
(b) Tollen's reagent (Ammonical silver nitrate solution)
Aldehydes are readily oxidised to carboxylic acids and will reduce Tollen's reagent to produce a silver mirror on the inside of a clean test tube.
FIRST clean up a test tube with a little hot nitric acid (fume cupboard) and rinse with distilled water.
Preparation of the reagent: To 1 mL of silver nitrate solution add a few drops of sodium hydroxide. Then add dilute ammonium hydroxide dropwise until the precipitate just dissolves.
Add 2 - 3 drops of the compound in methanol to 2 - 3 mL of Tollen's solution contained in a very clean test tube. If no reaction takes place in the cold, warm gently in a water bath.
CAUTION: After the test, pour the contents of the test tube into the sink