Introduction to Organic Chemistry

• Organic chemistry focuses on compounds containing carbon, excluding certain carbon compounds like CO2, CO, metal carbonates, and bicarbonates.
• It is defined as the study of hydrocarbons and their derivatives.
• Hydrocarbons consist solely of carbon and hydrogen atoms.
• Hydrocarbon derivatives are formed when one or more hydrogen atoms in a hydrocarbon are replaced by other atoms or groups.

Historical Context and Synthesis

• Initially, a vital force was thought to be essential for producing organic compounds.
• In 1828, Friedrich Wöhler synthesized urea in the lab by heating ammonium cyanate, disproving this belief.
• Kolbe later synthesized acetic acid, and Berthelot synthesized methane in the laboratory.
• Nowadays, around 95% of organic compounds are synthesized in the lab.

Reasons for the Existence of a Large Number of Carbon Compounds:

• Tetravalency: Carbon always has a valency of four, forming 4 covalent bonds to achieve an octet configuration.
• Ability to form single and multiple bonds: Carbon can form single, double, and triple bonds with itself and other elements through sp³, sp², or sp hybridization.
• Catenation: Carbon exhibits catenation, the ability to self-link and form long chains and rings.
• Isomerism: Carbon compounds can exhibit isomerism, where compounds have the same molecular formula but different structural or spatial arrangements.

Structural Representation of Organic Compounds:

• Complete Structural Formula: Represents all bonds between atoms with dashes, with single, double, and triple bonds shown as single, double, and triple dashes, respectively.
• Condensed Structural Formula: Omits carbon-hydrogen bonds and some other bonds for simplification.
• Bond Line Representation: The simplest representation, where carbon and hydrogen atoms are not shown, and carbon-carbon bonds are depicted in a zig-zag fashion.
• Three-Dimensional Representation (Wedge Representation): Uses solid and dashed wedges to indicate bonds projecting out or away from the plane of the paper.

Classification of Organic Compounds:

• Organic compounds are broadly classified into acyclic (open chain) and cyclic (ring) compounds.
• Acyclic or Aliphatic Compounds: Feature carbon atoms joined in long chains, either straight or branched.
  • Saturated compounds contain only carbon-carbon single bonds.
  • Unsaturated compounds contain at least one carbon-carbon multiple bond.
    • Saturated hydrocarbons are called alkanes.
    • Unsaturated hydrocarbons are alkenes and alkynes.
• Cyclic or Ring Compounds: Carbon atoms are joined to form rings.
  • Homocyclic compounds contain rings made of only carbon atoms.
  • Heterocyclic compounds contain rings with atoms other than carbon (O, N, S).
• Homocyclic compounds are further divided into alicyclic and aromatic compounds.
  • Alicyclic compounds have at least one carbo-cyclic ring.
    • Alicyclic hydrocarbons include cycloalkanes, cycloalkenes, and cycloalkynes.
  • Aromatic compounds are specific types divided into benzenoid (containing a benzene ring) and non-benzenoid compounds.

Functional Groups

• Functional groups are atoms or groups of atoms (excluding hydrogen) bonded to carbon atoms, which give the organic compounds their chemical properties.
• Common functional groups include hydroxyl (-OH), amino (-NH2), halo (-X), aldehyde (-CHO), ketone (>CO), carboxyl (-COOH), ether (-O-), cyano (-CN), and nitro (-NO2).

Homologous Series

• A homologous series is a group of organic compounds where adjacent members differ by a -CH2 group.
• Members of a homologous series, called homologues, share similar chemical properties and show gradation in physical properties.

Nomenclature of Organic Compounds:

• Organic compounds have common names and IUPAC names.
• IUPAC names systematically identify the parent hydrocarbon and attached functional groups.
• IUPAC names consist of a word root (indicating the number of carbon atoms) and a suffix or prefix.

IUPAC Nomenclature - Word Roots for Carbon Chains:

• 1 carbon: Meth-
• 2 carbons: Eth-
• 3 carbons: Prop-
• 4 carbons: But-
• 5 carbons: Pent-
• 6 carbons: Hex-
• 7 carbons: Hept-
• 8 carbons: Oct-
• 9 carbons: Non-
• 10 carbons: Dec-
• 11 carbons: Undec-
• 12 carbons: Dodec-

IUPAC Nomenclature - Suffixes and Prefixes:

• Primary suffixes indicate saturation or unsaturation.
  • Alkanes use -ane
  • Alkenes use -ene
  • Alkynes use -yne
• Secondary suffixes indicate the type of functional group and can also be indicated as prefixes.

Nomenclature of Branched Chain Alkanes:

• An alkyl group is a branch formed by removing a hydrogen atom from an alkane.
• IUPAC rules for naming branched chain alkanes include:
  • Selecting the longest continuous carbon chain as the parent chain.
  • Numbering the parent chain to give branched carbon atoms the lowest possible numbers.
  • Prefixing alkyl group names with the appropriate position number to the parent alkane name.
  • Listing different alkyl groups alphabetically.
  • Using prefixes like di-, tri-, tetra- for multiple identical substituent groups, separated by commas.
  • If two substituents are in equivalent positions, the lower number is given to the one that comes first alphabetically.

IUPAC Nomenclature - Functional Groups

• Select the longest continuous chain containing the functional group.
• Number the carbon atoms so the functional group gets the lowest possible number.
• For functional groups containing a carbon atom (-CHO, -CN, -COOH, etc.), numbering should start from that carbon atom.
• Use the appropriate suffix or prefix to indicate the functional group

IUPAC Nomenclature - Multiple Functional Groups

• Identify the principal functional group and name the compound based on it.
• Name remaining functional groups as substituents using appropriate prefixes.
• The order of preference for principal functional groups is: -COOH, -SO3H, -COOR, -COCl, -CONH2, -CN, -CHO, >CO, -OH, -NH2, >C=C<, -C≡C-.
• Alkyl, phenyl, halogens, nitro, and alkoxy groups are always prefix substituents.

Isomerism

• Isomerism is the phenomenon where compounds have the same molecular formula but different structural or spatial arrangements.
• Isomers have different physical and chemical properties.
• Types of isomerism include structural and stereo isomerism.
  • Structural isomers have the same molecular formula but different structural formulas.
    • Chain isomers differ in the carbon chain.
    • Position isomers differ in the position of the substituent or side chain.
    • Functional group isomers differ in the functional group.
    • Metamers differ in the carbon chain around the functional group.
  • Stereoisomers have the same molecular formula but different spatial arrangements of atoms.
    • Geometrical isomerism
    • Optical isomerism

Organic Reaction Mechanism

• In an organic reaction, a substrate reacts with a reagent to form intermediates and products.
• The sequential account of steps is the reaction mechanism.
• A covalent bond can undergo fission by homolysis or heterolysis.

Fission of a Covalent Bond

• Homolysis: Each bonded atom receives one electron from the shared pair, resulting in free radicals.
  • Free radicals are species with an odd or unpaired electron.
  • Tertiary free radicals are more stable than secondary, which are more stable than primary.
• Heterolysis: One atom retains the shared pair of electrons.
  • This forms a carbocation (carbonium ion) and a carbanion.
  • Carbocations are electron-deficient and positively charged.
  • Carbanions have a shared pair of electrons and a negative charge.
• Organic reactions via homolysis are free radical or homopolar reactions.
• Reactions via heterolysis are ionic, heteropolar, or polar reactions.

Substrate and Reagent

• The substrate is the molecule with carbon involved in new bond formation.
• The reagent is the other molecule or species involved in the reaction.

Nucleophiles and Electrophiles

• A nucleophile (:Nu-) is electron-rich and seeks a nucleus or electron-deficient center.
• An electrophile (E+) is electron-deficient and seeks an electron-rich center.

Electron Displacement Effects

• Electron displacement can occur due to the influence of an atom or an attacking reagent.
• Important effects include the inductive effect, electromeric effect, resonance effect, and hyperconjugation.
  • Inductive Effect (I effect): Shifting of sigma electrons through a carbon chain due to differences in electronegativity.
    • It decreases with increasing distance and is permanent.
    • Substituents can be electron-withdrawing (-I effect) or electron-donating (+I effect).
  • Electromeric Effect (E effect): Complete transfer of π-electrons in a multiple bond in the presence of an attacking reagent.
    • It's a temporary effect, possible only in compounds with multiple bonds.
    • Can be positive (+E) or negative (-E), depending on where the electrons are transferred.
  • Resonance Effect (R effect) / Mesomeric Effect (M effect): Polarity produced by the interaction of two π-bonds or a π-bond and lone pair on an adjacent atom.
    • It's transmitted through the chain.
    • Can be positive (+R) or negative (-R), depending on the direction of electron transfer.
  • Hyperconjugation: Partial conjugation of sigma electrons of a C-H bond with an unsaturated system.
    • It's a permanent effect that stabilizes carbocations.

Types of Organic Reactions

• Substitution reactions
• Addition reactions
• Elimination reactions
• Rearrangement reactions

Purification of Organic Compounds

• Purification methods depend on the compound's nature and the impurities present.
  • Sublimation: Separates sublimable compounds from non-sublimable impurities by heating the solid directly to vapor.
  • Crystallization: Based on differences in solubility in a solvent. The impure compound is dissolved in a suitable solvent at high temperature, and then cooled to crystallize the pure compound.
  • Distillation: Separates liquids based on differences in boiling points.
  • Fractional distillation: Separates miscible liquids with different boiling points using a fractionating column for repeated vaporization and condensation.
  • Distillation under reduced pressure: Purifies liquids with high boiling points that decompose at their boiling point, by lowering the boiling point.
  • Steam Distillation: Separates steam volatile substances immiscible with water.
  • Differential Extraction: Separates a compound from an aqueous medium by shaking with an organic solvent, followed by separating the layers.
  • Chromatography: Separates mixtures by passing them through a stationary phase and allowing a mobile phase to move.
    • Adsorption Chromatography separates compounds based on their different degrees of adsorption on an adsorbent.
      • Column Chromatography involves packing a column with adsorbent and passing the mixture through.
      • Thin Layer Chromatography (TLC) involves separating substances over a thin layer of adsorbent on a glass plate. TLC uses a retardation factor (Rf) to measure each components seperation.
    • Partition Chromatography: Separates based on the continuous differential partitioning of components between stationary and mobile phases.
      • Paper Chromatography is a type of Partition chromatography.

Qualitative Analysis of Organic Compounds

• Involves detecting the presence of elements such as carbon, hydrogen, oxygen, nitrogen, sulphur, halogens, and phosphorus.

Detection of Carbon and Hydrogen

• Organic compound being heated with copper (II) oxide leads to carbon oxidizing to carbon dioxide and water, which can be tested to confirm their existence.

Detection of Nitrogen, Sulphur and Halogens (Lassaigne's Test)

• Involves fusing the organic compound with metallic sodium, boiling, and filtering, and testing the elements from the sodium fusion extract.

Quantitative Analysis of Organic Compounds

• Determining the percentage composition of elements in the compound.
  • Estimation of Carbon and Hydrogen: Liebig's combustion method involves burning a known mass of the compound in excess oxygen and measuring the amounts of water and carbon dioxide produced.
  • Estimation of Nitrogen:
    • Dumas method involves heating the compound with copper oxide in the presence of carbon dioxide and collecting the nitrogen gas produced.
    • Kjeldahl's method involves heating the compound with concentrated sulphuric acid, followed by distillation and titration to determine the amount of ammonia released
  • Estimation of Halogens (Carius method): Heating the compound with fuming nitric acid and silver nitrate in the Carius tube to precipitate the halogen as the corresponding silver halide.
  • Estimation of Sulphur (Carius method): Heating the compound with sodium peroxide or fuming nitric acid, and precipitating sulphur as barium sulphate for calculating its percentage.
  • Estimation of Phosphorus: Heating the compound with fuming nitric acid and precipitating phosphorus and determining the presence of phosphorus through molybdate
  • Estimation of Oxygen determined by the percentage of other element(s) and subtracting from 100.