Organic Chemistry – Some Basic Principles and Techniques: Comprehensive NEET Chemistry Formulae

1. Tetravalence of Carbon and Hybridization

1.1 Hybridization in Organic Molecules

• sp³ Hybridization: Found in molecules like methane ($C{H}_4$). Carbon forms four sigma bonds, leading to a tetrahedral geometry.
• • Formula: $\text{Hybridization state (n)}=\frac{1}{2}[V+X-C+A]$ Where:
  • • $V$ = Number of valence electrons of the central atom
    • $X$ = Number of monovalent atoms bonded to the central atom
    • $C$ = Charge on the cation
    • $A$ = Charge on the anion
• sp² Hybridization: Found in ethene ($C_2{H}_4$), with a planar triangular geometry.
• sp Hybridization: Found in acetylene ($C_2{H}_2$), leading to a linear geometry.

NEET Tip:

Always identify the hybridization state to determine the molecular shape and bond angles, which are commonly tested in NEET.

2. Chemical Bonding and Molecular Structure

2.1 Sigma and Pi Bonds

• Sigma ($\sigma$) Bond: Single covalent bond formed by the head-on overlap of atomic orbitals.
• Pi ($\pi$) Bond: Covalent bond formed by the side-to-side overlap of atomic orbitals.
• • Example: In ethene ($C_2{H}_4$), there is one $\sigma$ bond and one $\pi$ bond between the carbon atoms.

Common Misconception:

Students often confuse the sigma bond as being weaker than the pi bond. In reality, the sigma bond is stronger due to the direct overlap of orbitals.

3. Resonance and Its Effects

3.1 Resonance Structures

• Resonance Hybrid: The actual structure of a molecule, which is a combination of multiple resonance structures.
• • Example: Benzene ($C_6{H}_6$) has a resonance hybrid with delocalized $\pi$ electrons.

Formula Explanation:

Resonance provides stability to molecules by allowing delocalization of electrons, reducing the energy of the molecule.

Common Mistake:

Avoid drawing all resonance structures with equal energy. Not all contribute equally to the resonance hybrid.

4. Thermodynamics and Chemical Reactions

4.1 Gibbs Free Energy ($\Delta G$)

• Formula: $\Delta G=\Delta H-T\Delta S$ Where:
• • $\Delta G$ = Gibbs free energy change
  • $\Delta H$ = Enthalpy change
  • $\Delta S$ = Entropy change
  • $T$ = Temperature (in Kelvin)
• Application: Determines the spontaneity of a reaction. If $\Delta G$ is negative, the reaction is spontaneous.

NEET Problem-Solving Strategy:

Use Gibbs Free Energy to predict reaction spontaneity, especially when asked to determine if a reaction will proceed under given conditions.

4.2 Enthalpy of Formation ($\Delta H_f$)

• Formula: $\Delta H_f=\sum \Delta H_{\text{products}}-\sum \Delta H_{\text{reactants}}$ Where:
• • $\Delta H_f$ = Standard enthalpy of formation for a compound.

Real-life Application:

Understanding enthalpy helps in fields like material science where energy efficiency is crucial.

5. Reaction Mechanisms and Kinetics

5.1 Reaction Rate Equation

• Rate Law: $\text{Rate}=k[A{]}^m[B{]}^n$ Where:
• • $k$ = Rate constant
  • $[A],[B]$ = Concentrations of reactants
  • $m,n$ = Orders of reaction with respect to each reactant.

Common Mistake:

Students often forget that the rate law is determined experimentally and not from the stoichiometric equation.

Practice Questions

1. Question: Calculate the Gibbs free energy change at 298 K for a reaction where $\Delta H=-40,\text{kJ/mol}$ and $\Delta S=-0.1,\text{kJ/mol}\cdot \text{K}$.
2. • Solution: $\Delta G=-40-(298\times -0.1)=-40+29.8=-10.2,\text{kJ/mol}$ Since $\Delta G$ is negative, the reaction is spontaneous.
2. Question: Determine the hybridization of carbon in $C{H}_{3}CN$.
3. • Solution: In $C{H}_{3}CN$, the first carbon (in $C{H}_3$) is $s{p}^3$ hybridized, and the second carbon (in $CN$) is $sp$ hybridized.
3. Question: Identify the major product in the nitration of benzene.
4. • Solution: The major product is nitrobenzene ($C_6{H}_{5}N{O}_2$), as the nitro group substitutes at the ortho or para position.

This summary includes essential formulae and concepts related to the provided chapter. It focuses on key principles and their applications in a NEET context, ensuring students have a thorough understanding of the critical content.