Structure Identification and POC Revision Notes - IIT JEE/NEET Preparation | Nucleon

Structure Identification and POC

  • Degree of Unsaturation
  • The presence of double bonds or rings within a molecule is indicated by a quantity called degree of
    unsaturation.

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  • Monohalogenation
  • When an alkane or a cycloalkane is treated with halogen (Cl2, Br2, F2, I2), a photochemical reaction
    takes place, in which a C–H bond cleaves and a C–X bond is formed. In such reactions if one H-atom
    is substituted by one halogen atom, then this is known as monohalogenation reaction.

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  • Catalytic Hydrogenation
  • Alkenes, Alkynes, polyenes or polyynes can be hydrogenated by using catalysts Ni/Pt/Pd at room temperature.
    All Carbon–Carbon  bonds(C=C, CC) get hydrogenate. The reaction can’t be stopped at any
    intermediate stage

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  • Ozonolysis
  • Ozonolysis reaction is used to determine the position of C=C, CC in a molecule. In this reaction Alkene,
    Alkyne and polyalkene on ozonolysis undergo oxidative cleavage. It is of two types

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  • Test for acidic/active hydrogen
  • When any compound release H2 gas after reaction with sodium or potassium or alkali metals or sodamide
    then this reflect the presence of acidic hydrogen.

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  • Test for unsaturation
  • (i) Bromine water test ( Br2 + H2O, Red-brown solution) :
    This is used to distinguish between saturated (alkane) and unsaturated (alkene/alkyne) hydrocabon

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  • Baeyer Reagent
  • Cold dil. alkaline KMnO4 Pink/purple solution) :
    This is also used to distinguish between saturated (alkane) and unsaturated (alkene/alkyne) hydrocabon

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  • Test for terminal Alkyne
  • (i) Tollen’s Reagent [AgNO3 + NH4OH or {Ag(NH3)2}+ OH] :
    Tollen’s Reagent gives white precipitate (Silver alkynide) with terminal alkyne.

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  • Test for alcohols
  • (i) Lucas Reagent test (Conc. HCl + anhydrous ZnCl2)
    * It gives white turbidity or cloudiness with alcohols (OH groups attached with sp3 carbon).
    * Lucas Reagent used to distinguish between 1°, 2°, 3° alcohol because 1°, 2°, 3° alcohols react with different
    rate.

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  • Test for phenol or enol
  • Neutral FeCl3 test :
    It form coloured complex with phenol or enol (OH groups attached with sp2 carbon

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  • Test for carbonyl compounds
  • Carbonyl compouds (all aldehydes and ketones) give yellow–orange precipitate with 2,4–DNP. It is also
    knwon as Brady’s reagent

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  • Test for Aldehydes
  • (i) Tollen’s Reagent [AgNO3 + NH4OH or {Ag(NH3)2}+ OH] :
    Tollen’s Reagent gives silver mirror or Black precipitate with aldehydes

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  • Iodoform Test
  • Reagents : I2 + NaOH or NaOI Where R = H, alkyl, aryl group
    Acetaldehyde, all methyl ketones, ethyl alcohol and all methyl alcohols give Iodoform test

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  • Test for acids and esters and amides
  • (i) Sodium bicarbonate test (NaHCO3)
    All the acids which are stronger than H2CO3 give CO2 gas with NaHCO3.

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  • Test for Amines
  • (i) NaNO2 + aqueous HCl test it is used to distinguish between 1º, 2º and 3º amines and also
    distinguish between aliphatic and aromatic primary amines

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  • Carbyl amine test
  • 1° Amine R–CH2–NH2 CHCl3 KOH R–CH2–N  C + 3KCl + 3H2O
    (unpleasent smell of isocyanide)

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  • Hofmann mustard oil test
  • It is a test for 1º amine and aniline. Primary amine reacts with carbon disulphide to form dithioalkyl carbamic
    acids which decompose on heating with mercuric chloride (HgCl2) to give alkyl isothiocyanates having
    smell like mustard oil .

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  • Hinsberg reagent
  • 1° Amine R–NH2 + PhSO2Cl
    Pyridine
    R–NH–SO2–Ph
    Base
    Compund is soluble in base

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  • Test of Nitro group
  • Nitroalkane & nitrobenzene gives black ppt on reduction with Zn and ammonium chloride followed by
    treating with Tollen’s reagent. This is also said to be Mulliken’s test.

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  • Detection of Carbon and Hydrogen
  • Carbon and hydrogen are detected by heating the compound with copper(II) oxide. Carbon present in
    the compound is oxidised to carbon dioxide (tested with lime-water, which develops turbidity) and
    hydrogen to water (tested with anhydrous copper sulphate, which turns blue

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  • Detection of Other Elements
  • Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by “Lassaigne’s
    test”. The elements present in the compound are converted from their covalent form to their ionic form
    by fusing the organic compound with sodium metal

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  • Test for Nitrogen
  • The sodium fusion extract is boiled with iron(II) sulphate and then acidified with dilute sulphuric acid.
    The formation of Prussian blue or green colour confirms the presence of nitrogen. Alternatively FeCl3
    and dil. HCl may be added

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  • Test for Sulphur
  • (a) The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black
    precipitate of lead sulphide indicates the presence of sulphur.
    Na2S + (CH3COO)2Pb  PbS
    Black

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  • Test for Halogens
  • The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white
    precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate,
    sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate,
    insoluble in ammonium hydroxide shows the presence of iodine

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  • Test for Phosphorus
  • The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the
    compound is oxidised to phosphate

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