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The analysis and identification of unknown organic compounds constitutes a very important aspect of experimental organic chemistry. It is very important to identify organic compounds correctly for during research or even analysis of compounds in real world (Ex – Blood tests) there is absolutely no scope for mistake.

There is no definite set procedure that can be generally applied to organic qualitative analysis. Basically, you have separate tests available to identify and distinguish the different organic compounds. Various books have different approaches, but a systematic approach based on the scheme given below will give good results.

Tests to differentiate important organic compounds

Chemistry has a vast expanse!

Solubility Table

Firstly, we have a look at Solubility tests. These simple tests could help you in elementary classification of compounds under observation & differentiate important organic compounds from one another.

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

Test for alkenes

Due to the fact that they undergo electrophilic addition with bromine, alkenes will decolourise bromine water.

To test:

  • Add few drops on bromine water to your sample.
  • Shake the mixture.
  • If:
    • The bromine decolourises then an alkene is present.
    • The bromine does not decolourise no alkene is present.

Test for haloalkanes

If you heat haloalkanes with dilute sodium hydroxide, halide ions are produced due to a nucleophilic substitution reaction.

To test:

  • Add aqueous sodium hydroxide to your sample.
  • Heat the mixture and then allow it to cool.
  • Add dilute nitric acid followed by aqueous silver nitrate.
  • If:
    • A white precipitate that is soluble in dilute ammonia forms then a chloroalkane was present.
    • A cream precipitate that is soluble in concentrated ammonia forms then a bromoalkane was present.
    • A yellow precipitate that is insoluble in ammonia forms then an iodoalkane was present.

Test for aldehydes

It is possible to oxidised aldehydes using a mild oxidising agent like Tollen’s reagent or Fehling’s solution.

Tollen’s reagent:

  • Add Tollen’s reagent to your sample.
  • Heat the mixture.
  • If a silver mirror forms then an aldehyde is present.

Fehling’s solution:

  • Add Fehling’s solution to your sample.
  • Heat the mixture.
  • If a brick-red precipitate forms an aldehyde is present.

Test for carboxylic acids

Carboxylic acids are able to free carbon dioxide from carbonates.

To test:

  • Add sodium carbonate solution to your sample.
  • If effervescence is produced and the gas causes the lime water to become milky then a carboxylic acid is present.

Test for amines

Amines are alkaline.

To test:

  • Add universal indicator to your sample.
  • If it turns blue/purple and has a fishy smell then an amine is present.

Test for alcohols

When sulphuric acid is present, alcohols will react with carboxylic acids to produce esters.

To test:

  • Add ethanoic acid followed by sulphuric acid to your sample.
  • Heat the mixture.
  • If the mixtures begins to smell sweet and fruity then an alcohol was present.

Distinguishing tests for primary, secondary and tertiary alcohols

All react differently when oxidised:

  • primary alcohols form aldehydes
  • secondary alcohols form ketones
  • tertiary alcohols cannot be oxidised

To test:

  • Add dilute sulphuric acid and potassium dichromate to your mixture.
  • Warm the mixture gently.
  • If:
    • the mixture turns green than a primary or secondary alcohol is present.
    • the mixture does not turn green then a tertiary alcohol is present.
  • If the mixture turns green then add Fehling’s mixture and heat again.
  • If:
    • a brick-red precipitate forms then a primary alcohol was present.
    • no brick-red precipitate forms then a secondary alcohol was present.

Practical Organic Chemistry is a vast subject and the reactions are limitless. Hoping this article will help you differentiate important organic compounds from each other, here are some tips on inorganic qualitative analysis. After all, the more you explore, the more you learn! All the best!

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