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

and wash the test tube with dilute nitric acid. Any silver fulminate present, which is highly explosive when dry, will be destroyed.
(c) Jones Reagent (See section under alcohols).
5. Alcohols
The tests for the hydroxyl group not only detect the presence of the group, but may also indicate whether it is primary, secondary or tertiary.
(a) Jones Reagent (CrO3-H2SO4 in H2O)
This reagent distinguishes primary and secondary alcohols from tertiary alcohols; the test is based on the much greater resistance to oxidation of tertiary alcohols compared to the other two types. Aldehydes also give a positive test.
Place 1 mL of acetone in a test tube and dissolve one drop of a liquid or ca 10 mg of a solid alcohol or aldehyde in it. Add one drop of the reagent to the acetone solution and shake the tube to mix the contents. Primary and secondary alcohols react within two seconds as indicated by the disappearance of the orange colour of the reagent and the formation of a green or blue-green precipitate or emulsion.
Tertiary alcohols do not react even after 3 minutes.
(I) RCH2OH -> RCHO -> RCO2H

(II) R2CHOH -> R2C=O

(III) R3COH -> no visible reaction.
(b) Lucas' Reagent [ZnCl2 - conc. HCl]
This reagent converts alcohols into the corresponding alkyl chlorides. Zinc chloride (a Lewis acid) increases the reactivity of alcohols towards acid. The test depends on the rate of reaction of primary, secondary, and tertiary alcohols with the reagent at room temperature.
(I) RCH2OH -> no reaction at room temperature.

(II) R2CHOH -> R2CHCl + H2O (1 hour or maybe longer)

(III) R3COH -> R3CCl + H2O (immediately)
To 1 mL of the alcohol in a small test tube add 6 mL of Lucas' reagent at room temperature. Close the tube with a cork, shake and allow to stand.
(i) Primary alcohols - the aqueous phase remains clear (except allyl alcohol - droplets after 7 minutes).
(ii) Secondary alcohols - very slow reaction (~ 1 hour or maybe longer) when droplets of alkyl chloride may be seen.
(iii) Tertiary alcohols - very fast reaction and droplets of the alkyl chloride formed almost immediately.
6. Sugars, Carbohydrates
Molisch's Test
This is a general test for carbohydrates. Dissolve 20 - 30 mg of the compound in 2 mL water and add 0.5 mL of the reagent (a 20% solution of 2-naphthol in ethanol). Pour 2 mL of concentrated sulphuric acid from a dropper carefully down the side of the tube so that the acid forms a layer beneath the aqueous solution without mixing with it. A red colouration, changing to dark purple forms at the interface. Carry out a second test on a blank solution.
7. Esters
Hydroxamic acid test
R-CO-OR' + H2N-OH -> R-CO-NH-OH + R'-OH
Esters react with hydroxylamine in the presence of sodium hydroxide to form the sodium salt of the corresponding hydroxamic acid. On acidification and addition of ferric chloride the magenta-coloured iron (III) complex of the hydroxamic acid is formed.
It is always advisable to ensure that an unknown compound does not give a colour with iron (III) chloride before carrying out the hydroxamic acid test.
Procedure for hydroxamic acid test
(a) Ferric chloride test
Dissolve a drop or a few small crystals of the compound in 1 mL of 95% ethanol (rectified spirit) and add 1 mL of M hydrochloric acid. Note the colour produced when 1 drop of 5% iron (III) chloride is added to the solution. If a pronounced violet, blue, red or orange colour is produced, the hydroxamic acid test described below is NOT APPLICABLE.
(b) Hydroxamic acid test
Mix 1 drop or several small crystals (ca 0.05 g) of the compound with 1 mL of 0.5 M hydroxylamine hydrochloride in 95% ethanol and add 0.2 mL of 6 M aqueous sodium hydroxide. Heat the mixture to boiling and after the solution has cooled slightly add 2 mL of M hydrochloric acid. If the solution is cloudy, add 2 mL of 95% ethanol. Observe the colour produced when 1 drop of 5% iron (III) chloride solution is added. If the resulting colour does not persist, continue to add the reagent dropwise until the observed colour pervades the entire solution. Usually only 1 drop of the iron (III) chloride solution is necessary. Compare the colour with that produced in test (a). A positive test will be a distinct burgundy or magenta colour as compared with the yellow colour observed when the original compound is tested with iron (III) chloride solution in the presence of acid. It is often advisable to conduct in parallel the test with, say, ethyl acetate, to ensure that the conditions for this test are correct.
THE PREPARATION OF DERIVATIVES OF ORGANIC COMPOUNDS
The preliminary examination and group classification tests indicate the particular class (functional group) to which an unknown organic compound may belong. Further characterisation and identification depends on the selection and preparation of a suitable solid derivative and accurate determination of its melting point (best, between 90 - 150 ).
The following table lists some of the classes of organic compounds and a selection of derivatives that may be prepared to characterise them. Check with the tables of melting points in Vogel which derivatives are most suitable for the characterisation of your particular compound.


CLASS OF COMPOUND DERIVATIVES
1. ALCOHOLS 3,5-dinitrobenzoate
2. PHENOLS benzoate, acetate, bromo-derivative
3.ALDEHYDES AND KETONES semicarbazone, 2,4-dinitrophenyl-hydrazone, oxime
4. ACIDS anilide, amide, p-toluidide.
5. AMINES benzoyl, acetyl and sulphonamide derivatives

METHODS FOR THE PREPARATION OF DERIVATIVES
ALCOHOLS
(i) 3,5-Dinitrobenzoates
3,5-Dinitrobenzoyl chloride is usually partially hydrolysed and should be prepared in the pure state by heating gently a mixture of 3,5-dinitrobenzoic acid (1 g) and phosphorus pentachloride (1.5 g) in a dry test tube, until it liquifies (5 min).* The liquid is poured on a dry watch glass and allowed to solidify. The phosphoryl chlorides are removed by pressing the solid with a spatula on a wad of filter paper. The residual acid chloride is suitable for immediate use in the preparation of the derivatives.
*Work under fume hood. Fumes are irritating to the eyes and nose.
The 3,5-dinitrobenzoyl chloride is mixed with the alcohol (0.5 - 1 mL) in a loosely corked dry test tube and heated on a steam bath for about 10 min. Secondary and tertiary alcohols require up to 30 min. On cooling add 10 mL sodium hydrogen carbonate solution, stir until the ester crystallises out, and filter at the pump. Wash with a little carbonate solution, water and suck dry. Recrystallise from the minimum hot ethanol or light petroleum. Cool slowly to avoid the formation of oily droplets of your ester.
PHENOLS
(i) Benzoates (Schötten-Baumann method).
To the phenol (0.5 g) is added 5% sodium hydroxide (10 mL) in a well-corked boiling tube or a small conical flask. Benzoyl chloride (2 mL) is added in small quantities at a time, and the mixture shaken vigorously with occasional cooling under the tap or in ice-water. After 15 min the solid benzoate separates out: the solution should be alkaline at the end of the reaction; if not alkaline, or if the product is oily, add a solid pellet of sodium hydroxide and shake again. Collect the benzoate, wash thoroughly with cold water, and recrystallise from alcohol or light petroleum.
(ii) Acetates
Acetates of many simple phenols are liquids; however, this is a suitable derivative for polyhydric and substituted phenols. The phenol (0.5 g) is dissolved in 10% sodium hydroxide solution and an equal quantity of crushed ice is added, followed by acetic anhydride (2 mL). The mixture is vigorously shaken in a stoppered test tube until the acetate separates. The product is filtered and recrystallised from alcohol.
(iii) Bromo derivatives
The phenol (0.3 g) is suspended in dilute hydrochloric (10 mL) and bromine water added dropwise until no more decolourisation occurs. The bromo derivative which precipitates out is filtered off and recrystallised from alcohol.
ALDEHYDES AND KETONES
(i) Semicarbazones
Dissolve semicarbazide hydrochloride (1 g) and sodium acetate (1.5 g) in water (8 - 10 mL), add the aldehyde or ketone (0.3 mL) and shake. Shake the mixture for a few minutes and then cool in ice-water. Filter off the crystals, wash with a little cold water and recrystallise from methanol or ethanol.
(ii) 2,4-Dinitrophenylhydrazones
Suspend 0.25 g of 2,4-dinitrophenylhydrazine in 5 mL of methanol and add 0.5 mL of concentrated sulphuric acid cautiously. Filter the warm solution and add a solution of 0.2 g of the carbonyl compound in 1 mL of methanol. Recrystallise the derivative from methanol, ethanol or ethyl acetate.
(iii) Oximes
Hydroxylamine hydrochloride (0.5 g) is dissolved in water (2 mL). 10% sodium hydroxide (2 mL) and the carbonyl compound (0.2 - 0.3 g) dissolved in alcohol (1 - 2 mL) are added, the mixture warmed on a steam bath for 10 min and then cooled in ice. Crystallisation is induced by scratching the sides of the test tube with a glass rod. The oximes may be crystallised from alcohol.
ACIDS
(i) Amides, anilides and p-toluidides
The acid (0.5 g) is refluxed with thionyl chloride (2 - 3 mL) in a fume cupboard for about 30 mins.* It is advisable to place a plug of cotton wool in the top of the reflux condenser to exclude moisture. The condenser is removed and the excess of thionyl chloride is distilled off (b.p. 78 ). The acid chloride thus produced is treated with concentrated ammonia solution (5 mL) or aniline (0.5 - 1 mL) or p-toluidine (0.5 - 1 g), when the solid derivative separates out. It is collected and recrystallised from alcohol adding decolourising charcoal if found necessary.
*Alternately use PCl5 to form the acid chloride.
AMINES
(i) Acetyl derivatives (acetamides)
Reflux gently in a small dry flask under a dry condenser the amine (1 g) with acetic anhydride (3 mL) for 15 min. Cool the reaction mixture and pour into 20 mL cold water. Boil to decompose the excess acetic anhydride. Cool and filter by suction the insoluble derivative. Recrystallise from ethanol.
(ii) Benzoyl derivatives (benzamides)
Suspend 1 g of the amine in 20 mL of 5% aqueous sodium hydroxide in a well-corked flask, and add 2 mL benzoyl chloride (fume hood!), about 0.5 mL at a time, with constant shaking. Shake vigorously for 5 - 10 min until the odour of the benzoyl chloride has disappeared. Ensure that the mixture remains alkaline. Filter off the solid derivative, wash with a little cold water and recrystallise from ethanol.
(iii) Benzenesulphonamides
To 1 g of the amine in 20 mL of 5% sodium hydroxide solution in a well-corked flask add 1 mL benzenesulphonyl chloride (fume hood!). Shake the mixture until the odour of the sulphonyl chloride disappears. Check that the solution is alkaline. Acidify if necessary to obtain the precipitated derivative. Concentrated hydrochloric acid added dropwise should be used. Filter the product, wash with a little cold water and suck dry. Recrystallise from ethanol.


Inorganic Quantitative Analysis
Gravimetric Estimation of Ba2+, Fe2+, Zn2+ and Cu2+
All Gravimetric analyses rely on some final determination of weight as a means of quantifying an analyte. Since weight can be measured with greater accuracy than almost any other fundamental property, gravimetric analysis is potentially one of the most accurate classes of analytical methods available. These methods are among the oldest of analytical techniques, and they may be lengthy and tedious. Samples may have to be extensively treated to remove interfering substances. As a result, only a very few gravimetric methods are currently used in environmental analysis.
There are four fundamental types of gravimetric analysis: physical gravimetry, thermogravimetry, precipitative gravimetric analysis, and electrodeposition. These differ in the preparation of the sample before weighing