Practical Organic Chemistry

Introduction to Practical Organic Chemistry involves the qualitative analysis of organic compounds is essential for identifying unknown substances. This process involves various tests and techniques to determine the functional groups, elements, and molecular structures present in a sample. 

Is practical organic chemistry important for JEE Advanced?

This chapter is important as it provides a clear view of the applications of properties of different organic compounds. These properties can be used to recognize the compounds by determining the results of such tests.

1.0Preliminary Tests:

Preliminary tests provide basic information about an organic compound's physical properties and chemical nature, guiding further analysis.

1. Physical Examination: 

2. Solubility Tests:

   Water Solubility: Dissolve the compound in water to determine if it's polar (e.g., alcohols, acids, amines) or non-polar (e.g., hydrocarbons).Organic Solvents: Test solubility in solvents like ethanol or ether to gauge polarity.

3. Flame Test:

   Burn the compound and observe the flame color (e.g., yellow for sodium, green or blue for copper) to indicate the presence of certain elements.

4. Melting Point (for solids) and Boiling Point (for liquids):

   Determine and compare the melting or boiling point to known values for potential identification.

5. Ignition Test:

   Heat the compound and observe if it chars, melts, or burns, noting any specific flame color or odor (e.g., sooty flame for aromatic compounds).

6. pH Test:

   Dissolve the compound in water and test the pH to determine if it is acidic, basic, or neutral, indicating possible functional groups.

2.0Element Detection:

1. Carbon and Hydrogen Detection: 

Combustion analysis produces CO₂ and H₂O, detected by lime water and anhydrous copper sulfate.

• $\mathrm{C}+2\mathrm{CuO}\stackrel{\Delta }{\to }2\mathrm{Cu}+{\mathrm{CO}}_2$

•   $2\mathrm{H}+\mathrm{CuO}\stackrel{\Delta }{\to }\mathrm{Cu}+{\mathrm{H}}_2\mathrm{O}$

• ${\mathrm{CO}}_2+\mathrm{Ca}(\mathrm{OH}{)}_2\stackrel{\Delta }{\to }{\mathrm{CaCO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}$

•   $5{\mathrm{H}}_2\mathrm{O}+\underset{\text{ White }}{\mathrm{CuSOl}}\stackrel{\Delta }{\to }\underset{\text{ Blue }}{{\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}}$

2. Nitrogen, Halogens  and Sulfur  Detection:

Nitrogen, sulfur, halogens, and phosphorus present in an organic compound are detected by "Lassaigne’s test." In this test, the elements in the compound are converted from their covalent form to their ionic form by fusing the organic compound with sodium metal. During this fusion process, the following reactions take place, where C, N, S, and X represent elements from the organic compound:

• $\mathrm{Na}+\mathrm{C}+\mathrm{N}\stackrel{\Delta }{\to }\mathrm{NaCN}$
• $2\mathrm{Na}+\mathrm{S}\stackrel{\Delta }{\to }{\mathrm{Na}}_2\mathrm{S}$
• $\mathrm{Na}+\mathrm{X}\text{ (halogens) }\stackrel{\Delta }{\to }\mathrm{NaX}$
• $\mathrm{Na}+\mathrm{S}+\mathrm{C}+\mathrm{N}\stackrel{\Delta }{\to }\mathrm{NaSCN}$

The cyanide, sulfide, and halide of sodium formed during sodium fusion are extracted from the fused mass by boiling it with distilled water. This extract is known as the sodium fusion extract or Lassaigne’s solution. This extract is then used for further qualitative analysis to detect the presence of these elements.

1. Test for Nitrogen-

Upon boiling the sodium fusion extract with freshly prepared ferrous sulfate (FeSO₄) solution and subsequent acidification with concentrated sulfuric acid, the appearance of a Prussian blue color confirms the presence of nitrogen.

6NaCN   +  FeSO₄          →          Na₄ [Fe(CN)₆]₃     +    Na₂SO₄ 

 FeSO₄   +  H₂SO₄            →            2Fe₂(SO₄)₃ 

 3Na₄ [Fe(CN)₆]  +  2Fe₂(SO₄)₃   →   Fe₄[Fe(CN)₆]₃      +   6Na₂SO₄

                                           Ferric Ferrocyanide (Prussian blue)

2. Test for Sulphur                                                                 

The sodium fusion extract is first acidified using acetic acid, then treated with lead acetate. The emergence of a black precipitate of lead sulfide confirms the presence of sulfur.

               Na₂S   +   (CH₃COO)₂Pb   →    PbS

                                                      Lead Sulphide  (Black)

When sodium fusion extract is treated with sodium nitroprusside, the appearance of a violet color indicates the presence of sulfur.

   Na₂S + Na₂[Fe(CN)₅ NO]   → Na₄[Fe(CN)₅NOS] 

                                                             (Violet/Purple)

3. When both Nitrogen and Sulfur are present in an organic compound

When both nitrogen and sulfur are present in an organic compound, sodium thiocyanate (NaSCN) is formed. This compound reacts with a neutral ferric chloride (FeCl₃) solution. The interaction between sodium thiocyanate and ferric chloride results in the formation of ferric thiocyanate, which exhibits a distinctive blood red color. This color change serves as a confirmatory test, indicating the simultaneous presence of nitrogen and sulfur in the organic compound.

                   Na + C + N + S    →    NaSCN  

     NaSCN + Neutral FeCl₃     →    Fe(SCN)₃   

                                                       (Blood red)

4. Test for Halogen

To test for halogens, the sodium fusion extract is first acidified with nitric acid and then treated with silver nitrate. 

• The formation of a white precipitate that dissolves in ammonium hydroxide indicates the presence of chlorine. 

                               AgNO₃ + NaCl   →   NaNO₃  +  AgCl (White precipitate)

• A yellowish precipitate that is sparingly soluble in ammonium hydroxide suggests the presence of bromine.

        AgNO₃ + NaBr     →  NaNO₃  +  AgBr (Light Yellow precipitate)

• Lastly, a yellow precipitate that is insoluble in ammonium hydroxide confirms the presence of iodine.

                                AgNO₃ + NaI     →  NaNO₃  +  AgI (Yellow precipitate)

Here is a summary of Element Detection for Nitrogen, Sulphur and Halogens

3.0Functional Group Analysis:

A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. The presence of a functional group gives the organic compound its particular chemical properties and reactivity.

1. Alcohols and Phenols: 

• Ferric chloride test

The reaction involves the coordination of the ferric ion (Fe³⁺) with the lone pair of electrons on the oxygen of the phenol's hydroxyl group. This interaction forms a colored complex, the nature of which can vary depending on the structure of the phenol, hence the different possible colors.

                   $$

$6\mathrm{PhOH}+{\mathrm{FeCl}}_3\to {\left[(\mathrm{PhO}{)}_6\mathrm{Fe}\right]}^3\theta +3\mathrm{HCl}$

$$

Violet / Purplecomplex

• Lucas test (cloudiness)

Procedure

Add 1-2 mL of Lucas reagent (Conc. HCl with ZnCl₂) to a clean test tube. Then, add a few drops of the suspected alcohol into the same test tube and gently shake to mix the contents. Observe the mixture at room temperature, noting any changes such as cloudiness or the formation of a separate layer, which indicates the presence and type of alcohol based on the reaction speed.

Here are chemical reactions involve in Lucas test

2. Aldehydes and Ketones:

• Tollen's test 
  

Tollen's test is used to identify aldehydes by using a solution known as Tollen's reagent, which consists of silver nitrate (AgNO₃) and ammonia (NH₄OH). The reagent reacts with aldehydes to form a silver mirror or a black precipitate of silver, indicating the presence of an aldehyde. 

Ketones do not react in this test, making it specific for detecting aldehydes. This test is commonly used in organic chemistry to differentiate aldehydes from other functional groups.

• Fehling's test

Fehling's test is used to identify reducing aldehydes, including reducing sugars. It involves two solutions: Fehling's A (copper(II) sulfate) and Fehling's B (potassium sodium tartrate in an alkaline solution). When mixed and heated with an aldehyde, the solution changes from blue to a brick-red precipitate of copper(I) oxide, indicating a positive result. 

Note- Ketones do not cause this color change (apart from alpha hydroxy ketone), making this test specific for aldehydes.

3. Carboxylic Acids: 

• Litmus test 

Litmus is a water-soluble dye extracted from certain lichens, and it is available in paper form as blue or red litmus paper. The color change in litmus paper occurs due to a change in the pH of the substance being tested.

The carboxylic acid group (−COOH) donates a hydrogen ion (H⁺) to the environment, making the solution acidic. The blue litmus paper has a dye that changes color in response to this increase in hydrogen ion concentration, shifting from blue to red under acidic conditions.

• Sodium Bicarbonate test (effervescence).

The sodium bicarbonate test is another simple yet effective chemical test used to identify carboxylic acids. This test takes advantage of the reaction between carboxylic acids and sodium bicarbonate (baking soda,NaHCO₃), which results in the production of carbon dioxide gas (CO₂), water, and a sodium salt of the acid.

HCl   +   NaHCO₃        →      NaCl + H₂CO₃   →  H₂O + CO₂ ­↑

RCOOH   +   NaHCO₃   →   RCO₂Na + H₂CO₃    →   H₂O + CO₂ ­­↑

RSO₃H   +   NaHCO₃     →     RSO₃Na + H₂CO₃    →  H₂O + CO₂ ­­↑

4. Amines: 

• NaNO₂ + aqueous HCl test

The test involving sodium nitrite (NaNO₂) and hydrochloric acid (HCl) is commonly referred to as the "diazo test" and is primarily used to distinguish primary aromatic amines from other types of amines. This test results in the formation of a diazonium salt when performed with primary aromatic amines, under cold conditions.

• Carbyl amine test (CHCl₃ + KOH)

The Carbylamine test, also known as the Hoffmann's Isocyanide Test, is a chemical reaction used to detect the presence of primary amines, particularly aliphatic and aromatic primary amines. This test is specific to primary amines and does not give a positive result with secondary or tertiary amines.

5. 4.0Test of Nitro Group

The Mulliken-Barker Test, often referred to simply as Mulliken’s Test, is a specific chemical test used to detect the presence of nitro groups in organic compounds, such as nitroalkanes and nitrobenzenes. 

This test involves a reduction step followed by a reaction with Tollen's reagent to confirm the presence of the nitro group.