Study Notes on Organic Chemistry

Organic Chemistry: Some Basic Principles and Techniques

Objectives

  • Understand the reasons for the tetravalence of carbon and shapes of organic molecules.

  • Write structures of organic molecules in various formats.

  • Classify organic compounds.

  • Name compounds according to IUPAC and derive their structures from provided names.

  • Understand the concept of organic reaction mechanisms.

  • Explain the influence of electronic displacements on structure and reactivity of organic compounds.

  • Recognize types of organic reactions.

  • Learn purification techniques for organic compounds.

  • Write chemical reactions for qualitative analysis of organic compounds.

  • Understand principles of quantitative analysis of organic compounds.

General Introduction

  • Importance of Organic Compounds: Vital for sustaining life, includes DNA, proteins, fuels, polymers, and medicines.

  • History: Organic chemistry is about 200 years old. The distinction between organic (plant/animal-derived) and inorganic compounds (mineral-derived) emerged in 1780.

  • Vital Force Theory: Proposed by Berzilius; rejected after F. Wohler synthesized urea (NH2CONH2) from ammonium cyanate (NH_4CNO).

  • Modern Synthesis: Demonstrated by synthesis of acetic acid by Kolbe and methane by Berthelot from inorganic materials.

8.1 Tetravalence of Carbon: Shapes of Organic Compounds

8.1.1 Shapes of Carbon Compounds

  • Molecular Structure Importance: Fundamental to understanding organic compounds.

  • Hybridization:

    • sp3 Hybridization: Forms single bonds; e.g. Methane (CH_4)

    • sp2 Hybridization: Forms double bonds; e.g. Ethene (C2H4)

    • sp Hybridization: Forms triple bonds; e.g. Ethyne (C2H2)

  • Bond Characteristics:

    • The more s character in a hybrid orbital, the greater the bond strength and shorter the bond length.

    • Electronegativity increases with greater s character.

8.1.2 Pi Bonds Characteristics

  • Formation: Requires parallel orientation of adjacent p orbitals.

  • Rotation Restriction: Rotation around C=C double bonds is restricted due to overlap of p orbitals.

  • Reactive Centers: Pi bonds often are sites for chemical reactions.

Problem 8.1: Bond Count

  • (a) HC≡CCH=CHCH_3

    • ext{σC-C: 4; σC–H: 6; πC=C: 1; πC≡C: 2}

  • (b) CH2=C=CHCH3

    • ext{σC–C: 3; σC–H: 6; πC=C: 2}

8.3 Structural Representations of Organic Compounds

8.3.1 Structural Formulas

  • Types:

    • Lewis Structures (dot structures) represent valence electrons.

    • Condensed Structure: e.g. CH3(CH2)6CH3 represents long-chain structures.

    • Bond-Line Structure: Carbon and hydrogen atoms are often not shown; zig-zag lines denote carbon chains.

Problem 8.4

  • Expanding Condensed Formulas into Complete Structural Formulas:

    • (a) CH3CH2COCH2CH3

    • (b) CH3CH=CH(CH2)3CH3

8.3.2 Three-Dimensional Representation

  • 3D Models: Using wedges and dashes to represent stereochemistry. Solid wedge (out of the paper), dashed wedge (into the paper).

  • Models:

    • Framework models: Bonds only.

    • Ball-and-stick models: Atoms and bond representations.

    • Space-filling models: Size of atoms depicted.

8.4 Classification of Organic Compounds

8.4.1 Types of Organic Compounds

  1. Acyclic (Open Chain) Compounds: Aliphatic compounds (e.g. straight or branched).

  2. Cyclic (Closed Chain) Compounds:

    • Alicyclic: Carbon atoms in rings (e.g. Cyclopropane).

    • Aromatic: Include compounds with benzenoid structures (e.g. Benzene).

8.4.2 Functional Group

  • Definition: A specific atom or group of atoms that confer chemical properties.

  • Examples: Hydroxyl (–OH), aldehyde (–CHO), carboxylic acid (–COOH).

8.5 Nomenclature of Organic Compounds

  • IUPAC System: Developed for systematic naming correlated with structure.

  • Alkanes: Hydrocarbons with only single bonds, ending in -ane.

  • Common names: Legacy names still widely used (e.g., citric acid).

8.5.1 Deriving Names

  • Select the parent chain, identify functional groups, and assign positions using the lowest locant rule.

  • Branched alkanes involve naming substituents and their positions using prefixes.

8.5.2 IUPAC Naming Examples

  1. Unbranched Alkanes:

    • C1 : CH4, ext{Methane}

  2. Branched Alkanes: Prefixes (e.g. methyl, ethyl) denote substitutions.

Problem 8.7

  • Structures and IUPAC names for selected hydrocarbons.

8.6 Isomerism

  • Isomerism: Existence of compounds with the same formula but different properties.

  • Types:

    • Structural Isomerism: Different bonding arrangements.

    • Stereoisomerism: Same bonds but different spatial orientation.

Problem 8.10: Structural Formula Solutions

  • Examples provided for different isomeric forms.

8.7 Organic Reaction Mechanisms

  • Reagents: Molecules that participate in reactions, categorized as substrates and attacking reagents.

  • Types of Cleavage:

    • Heterolytic: Yields charged species (carbocations/carbanions).

    • Homolytic: Yields free radicals.

Problem 8.11: Curved Arrow Notation

  • Represents the movement of electrons during reaction steps.

8.8 Methods of Purification

  • Common Techniques:

    1. Sublimation

    2. Crystallisation

    3. Distillation (fractional and reduced pressure)

    4. Chromatography

8.8.1 Sublimation

  • Separates sublimable compounds from non-sublimable impurities.

8.8.2 Crystallisation

  • Based on differing solubility of compounds.

8.8.3 Distillation

  • Separates volatile liquids based on boiling point differences.

8.8.4 Differential Extraction

  • Utilizes solubility differences in two immiscible liquids.

8.8.5 Chromatography

  • A separation technique for complex mixtures based on differential adsorption and partitioning.

8.9 Qualitative Analysis of Organic Compounds

Detection Techniques

  1. Carbon and Hydrogen: Oxidation to CO2 and H2O respectively.

  2. Nitrogen: Lassaigne's Test; conversion to sodium salts for testing.

  3. Sulphur, Halogens, Phosphorus: Reaction with specific reagents to yield identifiable precipitates.

8.10 Quantitative Analysis

  • Determines mass percentage of elements in organic molecule.

Estimation Techniques

  • Combustion analysis for carbon and hydrogen.

  • Kjeldahl and Dumas methods for nitrogen.

Summary

  • Understanding electronic effects, hybridization, and functional groups is key in organic chemistry. Employ techniques for analysis and naming to identify organic structures systematically.