General Organic Chemistry & Techniques

1. Tetravalence of Carbon & 3D Molecular Representations

• Hybridisation & Geometry:
  • $sp^3$ (Methane, $\text{CH}_4$): Tetrahedral, $109.5^\circ$ bond angle, 25% $s$-character.
  • $sp^2$ (Ethene, $\text{C}_2\text{H}_4$): Trigonal planar, $120^\circ$ bond angle, 33.3% $s$-character.
  • $sp$ (Ethyne, $\text{C}_2\text{H}_2$): Linear, $180^\circ$ bond angle, 50% $s$-character.
• Electronegativity Influence: Greater $s$-character holds electrons closer to the nucleus:

  $$\text{Electronegativity: } \text{C}(sp) > \text{C}(sp^2) > \text{C}(sp^3)$$

• Bond-Line Notation Conventions: Carbon and hydrogen atoms attached to carbon are omitted. Vertices and line terminals represent carbon atoms filled with the required stoichiometric hydrogens.

2. IUPAC Nomenclature & Functional Group Priorities

• IUPAC Priority Hierarchy for Principal Functional Groups:

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

• Substituents Treated Exclusively as Prefixes: Halogens ($-\text{F}, -\text{Cl}, -\text{Br}, -\text{I}$), Nitro ($-\text{NO}_2$), Alkoxy ($-\text{OR}$), Alkyl ($-\text{R}$), and Phenyl ($-\text{C}_6\text{H}_5$).
• Lowest Locant Rule: Number from the chain end that yields lower locants at the first point of difference. If locant sets are identical from both ends, assign the lower locant based on alphabetical precedence of the substituents.

3. Structural & Stereoisomerism Classifications

• Chain Isomerism: Identical molecular formula with different carbon chain connectivity (e.g., pentane, isopentane, and neopentane for $\text{C}_5\text{H}_{12}$).
• Position Isomerism: Differences in the position of a functional group, multiple bond, or substituent on the same carbon frame (e.g., propan-1-ol and propan-2-ol).
• Functional Group Isomerism: Identical molecular formula with different functional classes:
  • Alcohols $\longleftrightarrow$ Ethers ($\text{C}_2\text{H}_6\text{O}$: ethanol vs methoxymethane)
  • Aldehydes $\longleftrightarrow$ Ketones ($\text{C}_3\text{H}_6\text{O}$: propanal vs propanone)
  • Carboxylic acids $\longleftrightarrow$ Esters ($\text{C}_2\text{H}_4\text{O}_2$: ethanoic acid vs methyl methanoate)
• Metamerism: Unequal distribution of alkyl groups on either side of the same polyvalent functional group (e.g., diethyl ether $\text{C}_2\text{H}_5\text{OC}_2\text{H}_5$ vs methyl propyl ether $\text{CH}_3\text{OC}_3\text{H}_7$).

4. Reactive Intermediates: Geometry & Stability

• Carbocation Stability Trend:

  $$\text{Triphenylmethyl} > \text{Tropylium} > \text{Allyl} \approx \text{Benzyl}$$

  $$3^\circ > 2^\circ > 1^\circ > \text{CH}_3^+$$

• Carbanion Stability Trend (Inverted):

  $$\text{CH}_3^- > 1^\circ > 2^\circ > 3^\circ$$

  Stabilised strongly by $-I$ and $-M$ electron-withdrawing groups (e.g., $\text{CCl}_3^- > \text{CF}_3^-$).

5. Electronic Displacements: Inductive, Resonance & Hyperconjugation

• Inductive Effect ($\pm I$): Permanent displacement of $\sigma$-electrons along a carbon chain due to electronegativity differences:
  • $-I$ Series: $-\text{NO}_2 > -\text{CN} > -\text{COOH} > -\text{F} > -\text{Cl} > -\text{Br} > -\text{I} > -\text{OH} > -\text{C}_6\text{H}_5$
  • $+I$ Series: $-(\text{CH}_3)_3\text{C} > -(\text{CH}_3)_2\text{CH} > -\text{CH}_2\text{CH}_3 > -\text{CH}_3$
• Resonance & Mesomeric Effect ($\pm R$ / $\pm M$): Interaction of two $\pi$-bonds or a $\pi$-bond with an adjacent lone pair:
  • $+R$ groups: $-\text{OH}, -\text{OR}, -\text{NH}_2, -\text{NHR}, -\text{Cl}, -\text{Br}$ (increase electron density at ortho/para sites).
  • $-R$ groups: $-\text{NO}_2, -\text{CHO}, \text{C=O}, -\text{CN}, -\text{COOH}$ (deactivate ring; meta-directing).
• Hyperconjugation ($\sigma-\pi$ conjugation): Delocalisation of $\sigma_{\text{C-H}}$ electrons into an adjacent vacant $p$-orbital or $\pi$-bond.

6. Modern Purification Techniques

7. Qualitative & Quantitative Elemental Analysis

• Lassaigne's Test Reactions:
  • Nitrogen:

    $$\text{Na} + \text{C} + \text{N} \xrightarrow{\Delta} \text{NaCN}$$

    $$6\text{CN}^- + \text{Fe}^{2+} \longrightarrow [\text{Fe}(\text{CN})_6]^{4-}$$

    $$3[\text{Fe}(\text{CN})_6]^{4-} + 4\text{Fe}^{3+} \xrightarrow{x\text{H}_2\text{O}} \text{Fe}_4[\text{Fe}(\text{CN})_6]_3 \cdot x\text{H}_2\text{O}\ (\text{Prussian blue})$$

  • Sulphur:

    $$\text{S}^{2-} + \text{Pb}^{2+} \longrightarrow \text{PbS}\ (\text{black ppt})$$

    $$\text{S}^{2-} + [\text{Fe}(\text{CN})_5\text{NO}]^{2-} \longrightarrow [\text{Fe}(\text{CN})_5\text{NOS}]^{4-}\ (\text{purple / violet})$$

  • Halogens:

    $$\text{X}^- + \text{Ag}^+ \longrightarrow \text{AgX} \quad (\text{White, yellow-white, or yellow ppt})$$

• Quantitative Analysis Formulations:
  • Carbon & Hydrogen (Liebig Method):

    $$\% \text{C} = \frac{12 \times m_{\text{CO}_2} \times 100}{44 \times m} \quad \text{and} \quad \% \text{H} = \frac{2 \times m_{\text{H}_2\text{O}} \times 100}{18 \times m}$$

  • Dumas Method (Nitrogen):

    $$\% \text{N} = \frac{28 \times V_{\text{STP}} \times 100}{22400 \times m}$$

  • Kjeldahl's Method (Nitrogen):

    $$\% \text{N} = \frac{1.4 \times M \times 2(V - V_1/2)}{m}$$

  • Carius Method (Halogens & Sulphur):

    $$\% \text{Halogen} = \frac{\text{Atomic mass of } X \times m_{\text{AgX}} \times 100}{\text{Molar mass of } \text{AgX} \times m} \quad \text{and} \quad \% \text{S} = \frac{32 \times m_{\text{BaSO}_4} \times 100}{233 \times m}$$