Quantitative Tests For Identifying Organic Functional Groups.

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QUANTITATIVE TESTS FOR IDENTIFYING ORGANIC FUNCTIONAL GROUPS

Introduction:

Aldehydes and ketones can undergo a variety of reactions, but they share many chemical properties with other related compounds such as acids and esters. It is possible to detect the presence of an aldehyde or ketone by reaction with 2,4 dinitrophenylhydrazine this test only gives a positive result with aldehydes and no reaction with ketones, nevertheless carry out the tollen’s reagent to find out whether the ketone is present.

Alcohols are hydrocarbons with -OH replacing one or more hydrogen atoms. Alcohols have physical properties intermediate between those of hydrocarbons and water. Depending on where the - OH group appears, alcohols are classified as primary (1 o) [on a carbon attached to only one other carbon], secondary (2 o) [on a carbon attached to two other carbons], or tertiary (3o)[on a carbon attached to three other carbons].

It is important that test tube is dry in order to carry out a test of finding out the presence of OH group, since water contains an –OH group and can therefore produce fumes of HCl which would invalidate the test. As the addition of phosphorus pentachloride is not a specific test for alcohols only.  The usual oxidising agent is acid. K2Cr2O7 which turns from orange to green (Cr3+). The reactions involve the loss of the hydroxyl hydrogen together with a hydrogen atom from the adjacent alkyl group.

With primary alcohols, the initial aldehyde product may be oxidised further to a carboxylic acid, particularly if heat or excess oxidising agent is used, but ketones cannot be readily oxidised (involves breaking C-C bond).

carboxylic acids

Carboxylic acids are weak acids

CH3COOH  + H2O                    CH3COO–   + H3O+

Neutralized by a base

CH3COOH  + NaOH                    CH3COO–   Na+ + H2O

The reaction with sodium hydrogencarbonate is used as a test for carboxylic acids. All carboxylic acids tend to react with sodium hydrogencarbonate to release carbon dioxide, this then distinguishes them from weaker acids such phenols.

Esters are most often identified by their "fruity" odors but some are rather unpleasant smelling, so this is no sure guide. Esters react with hydroxylamine hydrochloride to yield a compound which can complex with Fe 3+ to give a colored species Esters react with water and acid catalyst Split into carboxylic acid and alcohol

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                  O

                      ||                                    H+

        H  C—O—CH2CH3  +  H2O               O

                                                                                                 ||                                                

                                                                    ...

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