Grade 11
  1. Basic concepts of chemistry  1.1. Importance of Chemistry  1.2. Nature and scope of chemistry  1.3. laws of chemical combination  1.3.1. Law of conservation of mass  1.3.2. Law of definite proportions  1.3.3. Law of multiple proportions  1.3.4. Law of gaseous volume  1.3.5. Avogadro's Law  1.4. Dalton's atomic theory  1.5. Mole concept and molar mass  1.6. Percent composition  1.7. Empirical and molecular formula  1.8. Stoichiometry and Stoichiometric Calculations  1.9. Limiting reagent concept
  2. Structure of the atom  2.1. Discovery of the electron, proton, and neutron  2.2. Atomic Model  2.2.1. Thomson's model  2.2.2. Rutherford's model  2.2.3. Bohr's model  2.3. Quantum mechanical model of the atom  2.4. Dual nature of matter and radiation  2.5. Heisenberg's uncertainty principle  2.6. Quantum numbers  2.7. Atomic orbitals and their shapes  2.8. Electronic configuration of elements  2.9. Aufbau principle  2.10. Pauli's exclusion principle  2.11. Hund's maximum multiplicity rule  2.12. de Broglie's hypothesis  2.13. Photoelectric effect
  3. Classification of elements and periodicity in properties  3.1. Historical development of the periodic table  3.2. Modern Periodic Law and Periodic Table  3.3. Periodic trends in properties  3.3.1. Atomic and Ionic Radius  3.3.2. Ionization enthalpy  3.3.3. Electron gain enthalpy  3.3.4. Electronegativity  3.3.5. Valency  3.3.6. Metallic and Non-Metallic Properties  3.3.7. Oxidation states  3.4. Unusual Properties of the First Elements
  4. Chemical Bonding and Molecular Structure  4.1. Kossel–Lewis approach to chemical bonding  4.2. Ionic or electrovalent bond  4.3. Covalent bond and its features  4.4. Bond parameters  4.5. VSEPR Theory  4.6. Valence bond theory  4.7. Hybridization and its types  4.8. Molecular orbital theory  4.8.1. Bond order and stability  4.9. Hydrogen bonding
  5. States of matter  5.1. Intermolecular forces  5.2. Gas Laws  5.2.1. Boyle's law  5.2.2. Charles's Law  5.2.3. Avogadro's Law  5.2.4. Dalton's law of partial pressure  5.2.5. Graham's law of spread  5.3. Ideal Gas Equation and Applications  5.4. Real gases and deviations from ideal behaviour  5.5. Kinetic molecular theory of gases  5.6. Liquefaction of gases  5.7. Properties of Liquids  5.8. Surface tension and viscosity
  6. Thermodynamics  6.1. System and environment  6.2. Types of systems in thermodynamics  6.3. Types of procedures  6.4. First law of thermodynamics  6.5. Internal energy and enthalpy  6.6. Heat capacity and specific heat  6.7. Hess's law of constant heat summation  6.8. Bond dissociation enthalpy  6.9. Enthalpy of formation and combustion  6.10. Enthalpy of solution and neutralization  6.11. Spontaneity and the second law of thermodynamics  6.12. Gibbs free energy and its applications
  7. Balance  7.1. The concept of balance  7.2. Equilibrium constant and its applications  7.3. Le Chatelier's principle  7.4. Ionic equilibrium in solutions  7.5. Acid and Base Theory  7.5.1. Arrhenius concept  7.5.2. Bronsted-Lowry concept  7.5.3. Lewis concept  7.6. pH Scale and pOH  7.7. Common ion effect  7.8. Hydrolysis of salts  7.9. Buffer solutions and their mechanism of action  7.10. Solubility equilibrium of sparingly soluble salts
  8. Redox reactions  8.1. Oxidation and reduction concepts  8.2. Oxidation number and its applications  8.3. Redox reactions in terms of electron transfer  8.4. Balancing redox reactions (oxidation number and ion-electron methods)  8.5. Types of redox reactions  8.6. Electrochemical Cells and Redox Reactions
  9. Hydrogen  9.1. Position of Hydrogen in the Periodic Table  9.2. Isotopes of hydrogen  9.3. Preparation of Hydrogen  9.4. Properties of Hydrogen  9.5. Hydrides and their classification  9.6. Water and heavy water  9.7. Hydrogen peroxide and its properties  9.8. Uses of Hydrogen and Its Compounds
  10. S-block elements (alkali and alkaline earth metals)  10.1. Group 1 Elements - Properties and Trends  10.2. Group 2 Elements - Properties and Trends  10.3. Unusual properties of lithium and beryllium  10.4. Important compounds of sodium and their uses  10.5. Important Compounds of Magnesium and Calcium
  11. Some p-block elements  11.1. General Introduction to p-Block Elements  11.2. Group 13 Elements  11.2.1. Boron and its compounds  11.3. Group 14 Elements  11.3.1. Carbon and its allotropes  11.3.2. Important Compounds of Carbon and Silicon
  12. Organic Chemistry - Some Basic Principles and Techniques  12.1. Classification and Nomenclature of Organic Compounds  12.2. Structural representation of organic compounds  12.3. Classification of organic reactions  12.4. Electronic Effects in Organic Chemistry  12.4.1. Inductive effect  12.4.2. Resonance effect  12.4.3. Hyperconjugation  12.5. Reaction mechanism and intermediate species  12.6. Purification and qualitative analysis of organic compounds
  13. Hydrocarbons  13.1. Hydrocarbons  13.1.1. Preparation and Properties of Alkenes  13.2. alkene  13.2.1. Preparation and Properties of Alkenes  13.2.2. Electrophilic addition reactions  13.3. Alkynes  13.3.1. Preparation and Properties of Alkynes  13.4. Aromatic hydrocarbons  13.4.1. Structure and properties of benzene  13.4.2. Electrophilic substitution reactions of benzene
  14. Environmental Chemistry  14.1. Environmental Pollution  14.2. Atmospheric pollution and the greenhouse effect  14.3. Water pollution and treatment methods  14.4. Soil pollution and its effects  14.5. Industrial waste and its treatment  14.6. Strategies for pollution control

Grade 11 → Organic Chemistry - Some Basic Principles and Techniques ↓


Reaction mechanism and intermediate species


Introduction to reaction mechanisms

In organic chemistry, it is important to understand how reactions occur. This involves studying reaction mechanisms. The reaction mechanism is a step-by-step description of the process by which reactants are converted into products, providing details about the breaking and formation of bonds and changes in atomic valency.

Example: Consider the reaction between ethene and bromine to form 1,2-dibromoethane:

C2H4 + Br2 → C2H4Br2

This mechanism involves several steps, such as the formation of a bromonium ion intermediate and its subsequent reaction with the bromide ion.

Role of intermediate species

Intermediate species are transient species within a reaction mechanism that are not present at the beginning or end of the reaction but are important in the transformation process. They often participate in the initial stages as reactive entities that quickly convert into more stable species.

Example: In the same example as ethene and bromine:

An intermediate, the bromonium ion, is formed:

C2H4 + Br2 → C2H4Br+

This intermediate reacts with the bromide ion (Br-) to give the final product:

C2H4Br+ + Br- → C2H4Br2

Steps in the reaction mechanism

Understanding a mechanism involves identifying the individual steps a reaction goes through. Each step within the mechanism is typically marked by a distinct series of bond-breaking and bond-forming events.

Early stages

An elementary step is a simple reaction that describes one of these steps. It involves molecular processes that cannot be broken down further. Elementary steps are simple and usually involve only a few molecules.

Each step in a mechanism can be classified as monomolecular, bimolecular, or intermolecular based on the molecularity, which refers to the number of reactant species involved in the reaction step.

Types of preparatory stages

  • Unimolecular: It involves a single molecule that undergoes transformation.
  • Bimolecular: In this there is collision and reaction between two molecules.
  • Thermomolecular: Rare because of the low probability that three molecules collide and react.

Determination of the reaction mechanism

Determining the mechanism of a chemical reaction involves a combination of experimental evidence and theoretical insight. Clues from a variety of sources can help chemists propose plausible mechanisms.

Rate laws and mechanisms

The rate law for a reaction, which relates the rate of the reaction to the concentrations of the reactants, can provide insight into the mechanism. The rate law is derived based on the slowest step in the mechanism, often called the rate-determining step or rate-limiting step.

Example: For a reaction where the velocity law is:

Rate = k[A][B]

This reveals a bimolecular interaction between species A and B in the rate determining step.

Evidence from intermediate investigation

The detection of intermediate elements could strongly support the proposed mechanism. Techniques such as spectrometry, magnetic resonance or even trapping experiments could provide evidence of these transient species.

Illustrative system example: SN1 reaction

The SN1 (unimolecular nucleophilic substitution) mechanism is a classic example in organic chemistry that illustrates the concepts of reaction mechanisms and intermediates.

SN1 mechanism phase

  1. Formation of carbocation: The reaction begins with the dissociation of the leaving group, forming a carbocation intermediate.
  2. Nucleophile attack: The nucleophile attacks the carbocation, forming the final substitution product.

The SN1 mechanism is characterized by the formation of a carbocation intermediate, which plays an important role in determining the rate and stereochemistry of the reaction.

Visualization example:

R3C—X → R3C+ + X- R3C+ + Nucleophile → R3C—Nucleophile

Here, X is the leaving group, and the nucleophile can be any molecule that is capable of donating an electron pair.

Illustrative mechanism example: E2 reaction

E2 (binomolecular elimination) mechanism is another example, in which a hydrogen and a replacing group are removed from adjacent carbon atoms to form a double bond.

E2 mechanism stage

  1. Proton abstraction: The base removes a proton adjacent to the carbon with the leaving group.
  2. Leaving group departure: The leaving group departs simultaneously as the double bond is formed.

Visualization example:

Base + RCH2—CH2X → RCH=CH2 + BaseH + X-

This coordinated mechanism results in the formation of alkanes.

Types of reaction intermediates

Understanding intermediates is important in organic chemistry, as they often determine the pathway a reaction will take to completion.

Carbocations

Carbocations are positively charged carbon species that are often sp2 hybridized and planar. They are common intermediates in mechanisms such as SN1 and E1 reactions.

Example of carbocation:

CH3—CH+—CH3

It is a secondary carbocation, known for its relative stability and ability to rearrange.

Radicals

Radicals are species that have an unpaired electron, which makes them highly reactive. Radicals are key intermediates in many halogenation reactions.

Example of radical:

•CH3

The methyl radical is often involved in radical chain reactions.

Carbanions

Carbanions are negatively charged species on the carbon atom. They act as strong nucleophiles and bases in many organic reactions.

Example of carbanion:

CH3C-H2

Carbanions can undergo a variety of nucleophilic substitution reactions.

Visualization of the system

When studying mechanisms, visual tools and formulas help to gain a deeper understanding of the transformation process. Representing the motion of the electron using curved arrows helps to understand the mechanism at a glance.

Curved arrow explanation:

In reaction mechanisms, curved arrows show the movement of electron pairs. They start at the source of the electrons, such as a lone pair or a bond, and point toward the atom or bond where the electrons are moving.


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