Chemistry: Class 112 — Chapter 2: Organic Chemistry – Some Basic Principles and Techniques

Organic chemistry is the study of carbon-based compounds. Carbon has a unique property called catenation, which allows it to form covalent bonds with other carbon atoms to create long chains and rings. Historically, Berzelius proposed the "vital force theory," suggesting organic compounds could only be produced by living organisms. This was disproved in 1828 when F. Wöhler synthesized the organic compound urea from the inorganic compound ammonium cyanate:

The tetravalence of carbon and the shapes of its compounds are explained by orbital hybridisation:

• sp3 Hybridisation: Leads to a tetrahedral shape (e.g., methane, $\text{CH}_4$) with $109.5^\circ$ bond angles.
• sp2 Hybridisation: Leads to a trigonal planar shape (e.g., ethene, $\text{C}_2\text{H}_4$) with $120^\circ$ bond angles.
• sp Hybridisation: Leads to a linear shape (e.g., ethyne, $\text{C}_2\text{H}_2$) with $180^\circ$ bond angles.

The electronegativity and bond strength of carbon increase with the increase in s-character of the hybridised orbitals: $\text{sp} > \text{sp}^2 > \text{sp}^3$.

A pi (\pi) bond is formed by the sideways overlap of unhybridised p-orbitals on adjacent atoms. This overlap restricts rotation around the carbon-carbon double bond, providing reactive centers containing multiple bonds where electron-seeking reagents can attack.

Organic molecules can be represented in several ways:

• Complete Structural Formulas: Show every single covalent bond as a dash.
• Condensed Structural Formulas: Omit some or all covalent bonds, grouping identical units together (e.g., $\text{CH}_3(\text{CH}_2)_4\text{CH}_3$).
• Bond-line Formulas: Represent carbon-carbon bonds as zig-zag lines, where vertices and line ends represent carbon atoms saturated with hydrogen.
• Three-Dimensional Representation: Uses solid wedges to indicate bonds projecting out of the plane towards the observer, dashed wedges for bonds projecting away, and normal lines for bonds in the plane of the paper.

Organic compounds are broadly classified into:

• Acyclic or Open Chain Compounds: Also called aliphatic compounds, consisting of straight or branched chains.
• Cyclic or Closed Chain Compounds: Divided into:
  • Alicyclic: Ring compounds that behave like aliphatic compounds (e.g., cyclohexane).
  • Aromatic: Special cyclic compounds containing conjugated systems, classified into benzenoid (e.g., benzene, aniline) and non-benzenoid (e.g., tropone).
  • Heterocyclic: Ring systems containing heteroatoms like oxygen, nitrogen, or sulphur (e.g., furan, thiophene, pyridine).

A functional group is an atom or group of atoms joined in a specific manner that determines the chemical properties of an organic compound. A homologous series is a family of organic compounds containing the same functional group, where successive members differ by a $-\text{CH}_2-$ unit.

IUPAC rules for naming branched-chain alkanes include:

1. Identify the longest continuous carbon chain to determine the parent alkane.
2. Number the carbon atoms of the parent chain such that the branched carbon atoms get the lowest possible locant numbers.
3. Name the alkyl substituents and prefix them to the parent name in alphabetical order.
4. If identical substituents are present, use prefixes like di-, tri-, tetra-, separated by commas for locants.

When multiple functional groups are present, one is selected as the principal functional group based on the following priority order:

$-\text{COOH} > -\text{SO}_3\text{H} > -\text{COOR} > -\text{COCl} > -\text{CONH}_2 > -\text{CN} > -\text{CHO} > >\text{C}=\text{O} > -\text{OH} > -\text{NH}_2 > >\text{C}=\text{C}< > -\text{C}\equiv\text{C}-$

The remaining groups are treated as substituents and named using prefixes.

Compounds with the same molecular formula but different structural arrangements are called structural isomers:

• Chain Isomerism: Difference in the carbon skeleton (e.g., pentane and isopentane).
• Position Isomerism: Difference in the position of a functional group or substituent (e.g., propan-1-ol and propan-2-ol).
• Functional Group Isomerism: Difference in the nature of the functional group (e.g., ethanol and dimethyl ether).
• Metamerism: Unequal distribution of alkyl chains on either side of a polyvalent functional group (e.g., methoxypropane and ethoxyethane).

Compounds with the same molecular formula and connectivity but different spatial orientations of atoms. It is classified into geometrical isomerism and optical isomerism.

• Heterolytic Cleavage: The shared pair of electrons remains with one of the fragments, producing a carbocation (positively charged carbon) or a carbanion (negatively charged carbon).
• Homolytic Cleavage: Each fragment takes one electron from the shared pair, producing highly reactive neutral species called free radicals.

• Nucleophiles: Electron-rich species that donate an electron pair to electron-deficient centers (e.g., $\text{HO}^-$, $\text{H}_2\text{O}$, $\text{NH}_3$).
• Electrophiles: Electron-deficient species that accept an electron pair from reactive sites (e.g., $\text{NO}_2^+$, $\text{Cl}^+$, $\text{BF}_3$).

• Inductive Effect: Permanent displacement of $\sigma$-electrons along a carbon chain due to electronegativity differences. It can be electron-withdrawing ($-I$) or electron-donating ($+I$).
• Resonance Effect: Polarity produced in a molecule by the interaction of two $\pi$-bonds or a $\pi$-bond and a lone pair of electrons. It includes $+R$ and $-R$ effects.
• Electromeric Effect: A temporary effect involving the complete transfer of a shared pair of $\pi$-electrons to one of the atoms in the presence of an attacking reagent.
• Hyperconjugation: A permanent stabilizing interaction involving the delocalisation of $\sigma$-electrons of a $\text{C}-\text{H}$ bond of an alkyl group directly attached to an atom of an unsaturated system or a carbocation.

• Sublimation: Used to separate sublimable compounds from non-sublimable impurities.
• Crystallisation: Based on differences in solubilities of the compound and impurities in a suitable solvent.
• Distillation: Used to separate volatile liquids from non-volatile impurities or liquids with sufficient differences in boiling points. Variations include:
  • Fractional Distillation: For liquids with close boiling points.
  • Distillation under reduced pressure: For liquids that decompose at or below their boiling points.
  • Steam Distillation: For steam-volatile, water-immiscible substances.
• Differential Extraction: Separates an organic compound from an aqueous solution using an organic solvent in which the compound is more soluble.
• Chromatography: Based on differential migration of components through a stationary phase under the influence of a mobile phase. Includes Adsorption Chromatography (Column and Thin Layer) and Partition Chromatography (Paper).

• Carbon and Hydrogen: Detected by heating the compound with copper(II) oxide, converting carbon to $\text{CO}_2$ (turns lime water milky) and hydrogen to $\text{H}_2\text{O}$ (turns anhydrous copper sulphate blue).
• Lassaigne's Test: Used to detect Nitrogen, Sulphur, and Halogens by fusing the organic compound with sodium metal to form soluble sodium salts ($\text{NaCN}$, $\text{Na}_2\text{S}$, $\text{NaX}$).

• Carbon and Hydrogen: Estimated by Liebig's combustion method, measuring the masses of $\text{CO}_2$ and $\text{H}_2\text{O}$ produced.
• Nitrogen: Estimated using either the Dumas Method (collecting nitrogen gas) or the Kjeldahl Method (converting nitrogen to ammonia, which is titrated against standard acid).
• Halogens: Estimated by the Carius Method, where the compound is heated with fuming nitric acid and silver nitrate to precipitate silver halides.
• Sulphur: Oxidized to sulphuric acid and precipitated as barium sulphate ($\text{BaSO}_4$).
• Phosphorus: Precipitated as ammonium phosphomolybdate or magnesium pyrophosphate.
• Oxygen: Usually calculated by subtracting the sum of percentages of all other elements from 100.