organic chemistry

Organic Chemistry – Some Basic Principles and Techniques

After Studying This Unit, You Will Be Able To:

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

  • Write structures of organic molecules in various ways.

  • Classify organic compounds.

  • Name compounds according to IUPAC system of nomenclature and derive their structures from given names.

  • Understand the concept of organic reaction mechanisms.

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

  • Recognize the types of organic reactions.

  • Learn techniques for purification of organic compounds.

  • Write chemical reactions involved in qualitative analysis of organic compounds.

  • Understand principles involved in quantitative analysis of organic compounds.

General Introduction

  • Organic compounds are vital for sustaining life on Earth, including molecules like DNA and proteins.

  • They are found in materials such as clothing, fuels, polymers, dyes, and medicines.

  • The science of organic chemistry is approximately 200 years old.

Historical Context

  • Around 1780, chemists differentiated between organic compounds (from plants and animals) and inorganic compounds (from minerals).

  • Berzilius proposed the existence of a ‘vital force’ for organic compounds, a notion rejected in 1828 after F. Wohler synthesized urea (NH2CONH2) from ammonium cyanate (NH4CNO).

  • The synthesis of other organic compounds from inorganic sources, like acetic acid and methane, established organic compounds could be synthesized in labs.

  • The development of electronic theory of covalent bonding solidified organic chemistry’s foundation.

Tetravalence of Carbon: Shapes of Organic Compounds

8.2.1 The Shapes of Carbon Compounds

  • Carbon forms four covalent bonds (tetravalence).

  • The electronic configuration and hybridization of carbon’s s and p orbitals explain its bonding and molecular shapes.

  • Shapes are explained using hybridization:

    • Methane (CH4) - sp3 hybridization (tetrahedral structure).

    • Ethene (C2H4) - sp2 hybridization (trigonal planar structure).

    • Ethyne (C2H2) - sp hybridization (linear structure).

  • Hybridization impacts bond length and strength:

    • sp hybrid orbitals (50% s character) form shorter, stronger bonds than sp3 (25% s character).

    • Relative electronegativity increases with greater s character.

8.2.2 Characteristic Features of π Bonds

  • π bonds require parallel orientation of two p orbitals for proper overlap.

  • They restrict rotation around the double bond due to their planar structure.

  • The electron charge cloud of a π bond is located above and below the bonding atoms, making π bonds more reactive.

Problems

  • How many σ and π bonds are in:

    • (a) HC≡CCH=CHCH3: σC – C: 4; σC–H: 6; πC=C: 1; πC≡C: 2

    • (b) CH2=C=CHCH3: σC – C: 3; σC–H: 6; πC=C: 2

  • Determine hybridization in:

    • (a) CH3Cl: sp3

    • (b) (CH3)2CO: sp3, sp2

    • (c) CH3CN: sp3, sp

    • (d) HCONH2: sp2

    • (e) CH3CH=CHCN: sp3, sp2, sp2, sp

  • For the compounds:

    • (a) H2C=O: sp2 (trigonal planar)

    • (b) CH3F: sp3 (tetrahedral)

    • (c) HC≡N: sp (linear)

Structural Representations of Organic Compounds

8.3.1 Structural Formulas

  • Organic compounds can be represented by Lewis structures or dot structures.

  • Dash structural formulas use dashes to denote bonds:

    • Single bond: –

    • Double bond: =

    • Triple bond: ≡

  • Ethane (C2H6), ethene (C2H4), ethyne (C2H2), and methanol (CH3OH) can be represented in various structural forms:

    • Complete: CH3CH3, H2C=CH2, HC≡CH, CH3OH

    • Condensed: CH3(CH2)6CH3 (for a long chain)

8.3.2 Three-Dimensional Representation

  • 3D structures can be represented using wedges:

    • Solid wedge indicates a bond projecting out of the plane of paper (towards the observer).

    • Dashed wedge indicates a bond projecting away from the observer.

    • Normal lines denote bonds in the plane of the paper (—).

Molecular Models

  • Various types of models:

    • Framework model: Bonds shown, not atoms.

    • Ball-and-stick model: Atoms shown as balls, bonds as sticks.

    • Space-filling model: Atoms shown in proportion to their volume with no representation of bonds.

  • Computer graphics can also illustrate molecular structures.

Classification of Organic Compounds

8.4 Organic Compounds Classification

  • Organic compounds can be ranked as:

  1. Acyclic (Open-chain) or Alicyclic:

    • Straight or branched chain compounds (e.g., butane, cyclopropane, cyclohexane).

  2. Aromatic Compounds:

    • Special types (e.g., benzene, aniline) containing cyclic structures.

    • Can be heterocyclic (containing atoms other than carbon).

8.4.1 Functional Groups

  • Functional Groups are atoms/groups responsible for characteristic properties (e.g., hydroxyl -OH, aldehyde -CHO, carboxylic acid -COOH).

8.4.2 Homologous Series

  • Series of compounds with the same functional group differing by -CH2.

  • Examples include alkanes, alkenes, alkynes, etc.

Nomenclature of Organic Compounds

8.5 IUPAC Nomenclature

  • A systematic naming method to clearly identify organic compounds.

  • Names correlate structure with properties to deduce structure from the name.

  • Common names often refer to origin or properties, e.g., citric acid from citrus fruits.

8.5.1 Naming Alkanes

  • Names derived from the chain structure with the suffix ‘-ane’.

  • The prefix indicates the number of carbon atoms.

8.5.2 Naming Branched Chain Hydrocarbons

  • Branches are identified using alkyl group names.

  • Rules confirm systematic naming concerning position and identity of substituents.

Reactions and Mechanisms

8.7 Organic Reaction Mechanisms

  • Organic reactions involve substrates and reagents, cleaving covalent bonds to form products.

8.7.1 Fission of Bonds

  • Heterolytic Cleavage: Shared electrons stay with one atom, leading to carbocations or carbanions.

  • Homolytic Cleavage: Electrons split, creating radicals.

8.7.2 Substrate and Reagent

  • Substrate: Molecule involved in new bond formation.

  • Reagent: Assists in forming similar bonds.

  • Examples:

    • Nucleophiles: Electron-rich species seeking positive sites.

    • Electrophiles: Electron-poor species receiving electron pairs.

8.7.3 Electron Movements

  • Movements indicated by curved-arrow notation.

  • 8Single electron movement indicated by fishhook arrows.

8.7.4 Electron Displacement Effects

  • Inductive, Resonance, Electromeric, and Hyperconjugation Effects:

    • Impact of electron displacements influencing polarity and reactivity of organic molecules.

  • The inductive effect arises when bonds polarize over distance.

Purification of Organic Compounds

8.8 Techniques Used

  • Distillation, Crystallization, Differential Extraction, and Chromatography are primary methods.

    • Purity is checked via melting or boiling points.

8.8.1 Sublimation

  • Used to separate sublimable compounds from non-sublimable impurities.

8.8.2 Crystallization and 8.8.3 Distillation

  • Based on solubility and boiling point difference for purification.

8.8.4 Differential Extraction

  • Separation based on solubility in an organic solvent.

8.8.5 Chromatography

  • Used for separation and purity testing: Adsorption & Partition Chromatography.

Qualitative and Quantitative Analysis

8.9 Analysis of Organic Compounds

  • Detection methods include:

  1. Detection of Carbon and Hydrogen

    • Copper(II) oxide combustion producing water and CO2.

  2. Lassaigne's Test for detecting nitrogen, sulfur, and halogens.

  3. Quantitative Analysis methods for carbon, hydrogen, nitrogen, and halogens detailing systematic approaches.

8.10 Quantitative Analysis

  • Estimation methods for the percentage of elements, particularly nitrogen, sulfur, and phosphorus.

    • Dumas Method: Collecting nitrogen from oxidation process.

    • Kjeldahl's Method: NH3 absorption.

Summary

  • Fundamental concepts for molecules' structure and reactivity, organic reaction classification, and their mechanisms.

  • Techniques for purification, qualitative and quantitative analysis of compounds highlighted.

  • Focus on systematic naming in correlation with structure for clear communication in organic chemistry.