Notes-Isomerism & Different types of reactions

Isomerism Overview

• Definition: Isomerism is the phenomenon where two or more compounds have the same chemical formula but different chemical structures.

• Isomers: These compounds have identical chemical formulas but differ in properties and the arrangement of atoms in their molecules.

Types of Isomerism

• Primary Types: Two main categories of isomerism are Structural Isomerism and Stereoisomerism.

  • Structural Isomerism: Variations in how the atoms are connected within the molecule.

  • Stereoisomerism: Variations in the spatial arrangement of atoms in molecules.

Structural Isomerism

• Also known as constitutional isomerism.

• Characteristics:

  • Atoms and functional groups are linked in different ways.

  • Different structural isomers may have distinct IUPAC names.

Subtypes of Structural Isomerism

1. Chain Isomerism: Variations in the carbon skeleton (branching).

   • Example: C5H12 can have different branched structures.

2. Position Isomerism: Functional groups or substituents occupy different positions on the carbon chain.

   • Example: C3H7Cl.

3. Functional Isomerism: Compounds with the same formula but different functional groups.

   • Example: C3H6O.

4. Metamerism: Different alkyl chains around a functional group, often involving divalent atoms.

   • Example: C4H10O can exist as ethoxyethane and methoxy-propane.

5. Tautomerism: Dynamic equilibrium forms with proton and electron shifts.

   • Common form: keto-enol tautomerism.

6. Ring-Chain Isomerism: Compounds possess both open chain and cyclic structures.

   • Example: Propene and cyclopropane.

Stereoisomerism

• Definition: Caused by different arrangements of atoms or groups in space, maintaining the same structural formulas.

Types of Stereoisomerism

1. Geometrical Isomerism: Occurs due to restricted rotation around double bonds or cyclic structures.

2. Optical Isomerism: Involves compounds that can rotate the plane of polarized light.

Geometrical Isomerism in Alkenes

• Carbon atoms involved in a double bond (C=C) are sp2 hybridized, leading to fixed planar arrangements.

• No rotation occurs around the C=C bond due to the presence of p bonds.

• Examples: 2-Butene exists as:

  • Cis isomer: Similar groups on the same side.

  • Trans isomer: Similar groups on opposite sides.

Geometrical Isomerism in Cyclic Compounds

• Restriction of rotation in cyclic compounds also leads to geometrical isomerism.

  • Example: 1,2-dimethyl cyclopropane has two distinct isomers.

E/Z Naming System

• Priority Assignment: Groups attached to sp2 double bond carbons are assigned priority based on atomic number.

• Z Isomer: Priority groups on the same side of the double bond.

• E Isomer: Priority groups on opposite sides.

Assignment Guidelines

1. Highest atomic number gets the highest priority.

2. Ties resolved by considering the next atoms in the substituent chain.

3. Ties involving isotopes rank based on atomic mass.

4. Multiple bonds treated as equivalent to single bonds.

Optical Isomerism

• Optical Activity: Ability of certain isomers to rotate plane-polarized light, termed as optical activity.

• Dextrorotatory: Rotates light clockwise (indicated by +).

• Levorotatory: Rotates light counterclockwise (indicated by -).

Chirality and Chiral Molecules

• Chirality: A property of objects (molecules) that are non-superimposable on their mirror images.

• Chirality Center: A carbon atom bonded to four different groups. Molecules with one chirality center are termed chiral and can exist as enantiomers.

R/S Naming System

• Cahn-Ingold-Prelog System: A method for prefixing enantiomers as R (clockwise) or S (counter-clockwise) based on priority rules.

• Assignment Rules:

  1. Rank atoms directly attached to chirality center.

  2. For isotopes, higher atomic mass gets higher priority.

  3. If first atom ranks tie, move to subsequent atoms until a difference is found.

  4. Treat multiple bonds as equivalent to single bonds.

D and L Notation

• D and L Configuration: Utilized primarily in sugars and amino acids for simplification over the R/S system.

• Natural Configuration: Most amino acids (except Glycine) exhibit L configuration; majority of sugars exhibit D configuration.

Ionic Reactions and Markovnikov's Rule

• Markovnikov's Rule: In the addition of HX to unsymmetrical alkenes:

  • Hydrogen (H) adds to the carbon with more hydrogens.

  • Halide (X) adds to the carbon with more alkyl substituents.

Radical Reactions

• Anti-Markovnikov's Products: Formed under radical conditions where bromine adds to the carbon with more hydrogen atoms instead.

Reaction Mechanisms

• Concerted Reactions: All bond breaking and making occurs in a single step, exemplified by Diels-Alder reactions.

• Transition State Theory: Describes the state of reactants transitioning through an unstable intermediate to products, with reaction rates dependent on the stability and concentration of this transition state.

Thermodynamics vs. Kinetics

• Thermodynamics: Focuses on whether a reaction can occur and if sufficient driving force is present.

• Kinetics: Concentrates on the rates of reactions and how transitions occur from reactants to products.

• Controlled Reactions: The type of product (kinetic vs. thermodynamic) depends on temperature and activation energy.