1 ORGANIC QUALITATIVE ANALYSIS INTRODUCTION The aim of the qualitative analysis is to identify a given organic substance and to confirm its nature by performing a set of known standard reactions with it The entire process of this analysis is based on the two important theoretical concepts namely Homologous series and functional group Millions of known organic compounds are easily and effectively classed into limited number of 25 to 30 groups depending on their nature of functional group present A compound belonging to a particular class will exhibit characteristic reactions of the atoms are groups present in it When compounds of a class are arranged in the order of their molecular weight a homologous series result Compounds of a particular homologous series show a similarity in chemical reactions and gradation in physical properties Thus a compound s position in a homologous series reveals its physical characteristics and chemical nature In general organic compounds are considered to be derivatives of hydrocarbons aliphatic or aromatic The substituent s or linking atoms are groups in an organic molecules are invariably inorganic atoms are groups such as halogens oxygen and hydrogen nitrogen and hydrogen sulphur etc which leads to the formation of halogen compounds alcohols aldehydes ketones esters amines amides thiols etc A good understanding of the reactions of these simple class of organic compounds is an essential requirement before attempting to take up qualitative analysis of substance Hence the analysis includes a number of stages For accurate analysis of a substance one must follow a systematic procedure Besides this over all knowledge of reactions and careful observations are also a must for an analyst In any qualitative Organic Analysis the systematic procedure includes the following steps 1 Identification of a compound and its purity Generally a sharp melting point indicates that the substance is pure In case of a liquid its boiling point is determined Comparison of these values with the standard literature value would confirm its nature and purity At the undergraduate level this test is not usually recommended in practical classes 2 Qualitative Elements Test or Lassaigne s Test Presence or absence of elements such as nitrogen halogens and sulphur as confirmed by Lassaigne s Test will shed more light on the nature of the class of compounds to which it belongs For instance if nitrogen is present one may test for amines amides nitro compounds etc In case of absence of nitrogen one need not attempt to perform the 1 2 experiments belonging to these classes one can readily test the compound for acids phenols esters etc 3 Solubility test Solubility of an organic substance illustrates the exact nature of the substance more easily For this purpose the given organic substance is dissolved in simple solutions and solvents H2O NaOH dilute HCl sodium bi carbonate NaHCO3 etc The presence or absence of certain class of compounds can be ascertained by means of this test 4 Nature of class of compounds or functional group test In addition to the test described in the sections 1 2 and 3 the performance of specific reactions will be helpful to assign the nature of an organic molecule on a firm basis The class to which an organic compound belongs is determine by this method For example performance of a few general reactions will give a clue whether the compound is a carboxlic acid or not Moreover by carrying out a specific reaction one can distinguish whether the given compound is monocarboxylic or dicarboxylic acid 5 Preparation of derivatives Conformation of the presence of a functional group in an organic compound is conclusively established only after preparing a solid derivative for the compound and determining its melting point and comparing it with the literature value In all organic qualitative analysis the preparation of a suitable solid derivative is required as a last proof of the functional group In the following pages a systematic procedure for organic qualitative analysis is described It comprises three sections of reactions discussed earlier When an unknown organic compound is subjected to the following tests in the order its identification will be much easier I Preliminary Tests a Physical characteristics b Solubility test c Heating on a nickel spatula d Identification of the elements present e Heating with soda lime f Treatment with 10 NaOH aqueous solution g Treatment with dil HCl h Treatment with conc H2SO4 i Treatment with saturated NaHCO3 solution j Test with Fehling s solution k Test with Tollen s reagent l Reactions and coloration with Ferric chloride m Treatment with 2 4 dinitrophenyl Hydrazine 2 II Test for Functional Groups present Confirmatory tests III Derivative for the functional group present ORGANIC QUALITATIVE ANALYSIS SCHEME OF PROCEDURE FOR IDENTIFYING CHARACTERISTIC GROUPS 3 The colour and appearance of the substance are noted The odour of the substance is noted A small amount of the substance is shaken up with about 1ml of water and the solution is tested with litmus I Preliminary Tests Yellow solid or liquid Brown or dark coloured solid or liquid Colourless solid or liquid Pleasant smell Phenolic smell Fishy smell Kerosene like smell Pungent or irritating smell Soluble in water and the solution is i Acid to litmus ii Neutral Sparingly soluble or practically insoluble in water and the suspension is i Acidic to litmus ii iii Neutral to Basic to litmus 3 Presence of aromatic nitro compounds phenols aromatic amines etc Presence of acids esters aldehydes ketones hydrocarbons anilides etc Presence of esters ketones nitro compounds Phenols present Aromatic amines present Hydrocarbons aromatic compounds with halogen in the nucleus present Presence of aromatic compounds with halogen in the side chain etc Presence of aliphatic acids polyhydric phenols nitro phenols amine salts etc Presence of simple alcohols sugars urea etc Aromatic acids simple phenols nitro phenols etc Present Presence of Aromatic amines Presence of esters 1 2 3 4 Litmus Test A little of the substance is brought in contact with a moist ened litmus paper 4 litmus Blue litmus turned red Red litmus turned blue i ii iii No changes of colour takes place aldehyde ketones hydrocarbons amines anilides Presence of acids and phenols Presence of amines Presence of carbohydrates ketones esters hydrocarbons etc 5 Test for the Elements A dry small pellet of sodium is melted in an ignition tube by heating