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Inductive Effect and Qualitative Tests for Carboxylic Acids, Amide and Ester


Inductive effects

The electron-releasing or electron-withdrawing species will generate a similar negative or positive charge when they are attached to the atoms of a chain (a carbon chain). The inductive effect occurs when a molecule develops a permanent dipole.

Inductive effects can determine whether a compound is acidic or basic. Acidity is decreased by electron-donating groups, while acidity is increased by electron-withdrawing groups. It has an electron-drawing property because RCOO- is delocalized and has a negative charge. Electron-rich R would lead to instabilities in the conjugate base due to inter-electronic repulsions.

Types

Negative inductive effect (-I effect)

Their electronegative charge is due to halogen and carbon atoms having unequal electron distributions. Positive charge is therefore flowing through carbon molecules. When an atom with a negative charge acquires a permanent dipole, electron-withdrawing induction produces permanent dipoles in molecules. -I effects are related to this phenomenon.

Positive inductive effect (+I effect)

Positive Inductive Effects are a process in which an electron is donated to or released from a carbon chain by a species of chemical that incline to donate electrons.

Carboxylic acids, esters, and amides

Qualitative tests

Several tests are performed to identify carboxylic acid.

Action with blue litmus

Whenever carboxylic acids are present, blue litmus turns red.

Action with bicarbonates and carbonates

Carbonates and bicarbonates dissolve in carboxylic acids, undergoing decomposition. CO2 is then emitted rapidly.

Carboxylic acid mechanism

Sodium carbonate and carboxylic acids undergo effervescent reactions that produce carbon dioxide. An aqueous solution of sodium bicarbonate does not produce effervescence from most phenols. The reaction between sodium bicarbonate and carboxylic acids containing sodium carbonate releases carbon dioxide from Na2CO3 or NaHCO3.

Formation of ester

Alcohol and carboxylic acid are heated in order to create the anhydride. Carboxylic acids, such as HCl or sulfuric acid, create ester molecules. Esterification is an irreversible chemical reaction.

Fluorescein test

A dicarboxylic group is present if it is present. A dicarboxylic compound is heated to produce an acid anhydride. Combining resorcinol and anhydride forms a fluorescent dye.

Reaction with FeCl3

Iron trichloride precipitates some carboxylic acids when they react with carboxylic acids. The reaction produces puffy precipitates of acetic acid, for example.

For amides and ester

Alkalines Hydrolysis

The test for amides and esters is performed with this reaction. Using procedure A, you can identify amides. In case you have an ester, use procedure B. (Remember that esters that boil at higher temperatures may not react.)

Note - Skin, eyes, and mucous membranes are severely damaged by Sodium hydroxide, which is toxic and corrosive. Be careful if you come into contact with it.

Procedure A for Amides

  • In a test tube containing 4 mL of 6 M sodium hydroxide, place 0.1 g of the unknown liquid (3 drops).
  • Apply 2 drops of copper(II) sulfate to a small piece of filter paper and secure it over the top of the tube.
  • Watch the filter paper for any color change when you boil the mixture for a minute or two.
  • If a carboxylic acid precipitates, save the precipitate for later use as a derivative. Acidify the solution with 6 M HCI.
The filter paper will be blue if it contains an amide. A reaction occurs between the copper (II) sulfate and the free amine. After acidification of an amide hydrolysis mixture, it may precipitate or separate into a liquid phase (the carboxylic acid). Try heating the mixture for longer times or repeating the reaction at 200 degrees Celsius using 20% KOH in glycerine if the results are inconclusive.

Procedure B for esters

  • One gram of the unknown (1 mL of the solution) is dissolved in 10 mL of 6 M sodium hydroxide in a round-bottom flask with 30 minutes of reflux time (or until the solution is homogenous).
  • A separate organic layer or residue may remain in the mixture, which should be heated longer until it disappears or until no further reaction can be detected. Hydrolysis of most esters boiling at 110 °C takes 30 minutes; esters boiling at higher temperatures may require several hours.
  • Using 6 M sulfuric acid, acidify the aqueous solution after cooling the reaction mixture. Remove any organic layer if there is one. As a derivative, save the precipitated carboxylic acid.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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