QUESTION NO. 11

White short notes on the following:
(a) Mesomeric effect
(b) Thin layer chromatography
(c) Purification of volatile organic compounds

(a) Mesomeric Effect

Mesomeric Effect (also known as Resonance Effect) describes the influence of delocalized electrons in a molecule on its electronic distribution and reactivity. It occurs due to the resonance structures that contribute to the overall electronic structure of a molecule.

Key Points:

• Definition: The mesomeric effect is the stabilization of a molecule due to the delocalization of π-electrons across multiple atoms.
• Types:
• +M Effect (Electron Donating): Groups that donate electron density through π-bonding, such as -OH, -NH₂, -OCH₃.
• -M Effect (Electron Withdrawing): Groups that withdraw electron density through π-bonding, such as -NO₂, -CN, -COOH.
• Examples:
• In benzene, the delocalized π-electrons stabilize the molecule.
• In carboxylate ions, resonance between the two oxygen atoms helps stabilize the negative charge.

Applications:

• Chemical Reactivity: The mesomeric effect influences the reactivity of compounds, affecting their stability and how they react with other substances.
• Spectroscopy: It affects the UV-Vis and NMR spectra of compounds due to changes in electron density.

(b) Thin Layer Chromatography (TLC)

Thin Layer Chromatography (TLC) is a simple and effective chromatographic technique used for separating and analyzing mixtures of compounds.

Key Points:

• Principle: TLC is based on the differential affinity of compounds for a stationary phase (usually a silica gel or alumina coated on a glass plate) and a mobile phase (a solvent or solvent mixture).
• Procedure:
• Preparation: Apply small spots of the sample mixture onto a TLC plate.
• Development: Place the plate in a developing chamber containing the mobile phase. The solvent moves up the plate by capillary action, carrying the components of the mixture with it.
• Visualization: After development, the separated spots are visualized using UV light or staining reagents.
• Applications:
• Qualitative Analysis: Identify the presence and purity of compounds.
• Monitoring Reactions: Track the progress of chemical reactions.
• Purity Testing: Determine the purity of substances.

Advantages:

• Simplicity: Requires minimal equipment and is easy to perform.
• Speed: Provides rapid separation and analysis.

(c) Purification of Volatile Organic Compounds

Purification of Volatile Organic Compounds (VOCs) involves techniques to isolate and purify compounds that readily evaporate at room temperature.

Key Methods:

1. Distillation:

• Principle: Utilizes differences in boiling points to separate compounds. The mixture is heated, and the vapor is condensed to separate the components.
• Types:
  • Simple Distillation: For compounds with significantly different boiling points.
  • Fractional Distillation: For compounds with closer boiling points, using a fractionating column to improve separation.

1. Steam Distillation:

• Principle: Used for heat-sensitive compounds. Steam is passed through the mixture, and the vapors are condensed and collected.
• Applications: Extraction of essential oils and natural products.

1. Vacuum Distillation:

• Principle: Reduces the pressure to lower the boiling points of compounds, allowing the distillation of high-boiling substances at lower temperatures.
• Applications: Purification of compounds that decompose at high temperatures.

Considerations:

• Boiling Points: Select the appropriate method based on the boiling points of the compounds.
• Decomposition: Avoid methods that may decompose the volatile compound.

Applications:

• Chemical Industry: Purification of chemicals used in pharmaceuticals, agriculture, and manufacturing.
• Environmental Analysis: Measurement and purification of VOCs in environmental samples.