Chapter 6

Understanding Organic Reactions

Major Items Associated with Most Org. Rxns

1. Substrate .. organic reactant being modified by the reagent

2. Reagent … inorganic or organic reactant that modifies the substrate

3. Solvent …… medium that dissolves the reactants

3. Catalyst ….. substance that increases rxn rate but retains its original structure

4. Condition….. The physical or physicochemical quantity used in the rxn.

A Single Step Chemical Equation

Example of a Single Step Chemical Eqn.

This is the solvent

This is symbol is used when heat is required.

This is symbol is used when light is required.

Example of a Multi-step Chemical Eqn

Types of Reactions

(S, E, and A = SEA )

Substitution Reactions

( Y will replace Z at a carbon atom)

Substitution Reactions

Elimination Reactions

( X and Y are detached from two different carbon atoms that are vicinal to each other)

Elimination Reactions

Addition Reactions

( X and Y add to two different atoms in a molecule that has one or more bonds)

Addition Reactions

Elimination is the opposite of addition .

OHBr

Practice ExercisesClassify the following rxns as substitution, elimination, or addition.

OH

OH

Practice ExercisesClassify the following rxns as substitution, elimination, or addition.

Changes in Bonding During a Chemical Rxn

(Homolysis and Heterolysis)

Heterolysis: an unequal division of a bonding electron pair

Homolysis: an equal division of a bonding electron pair

Two possible ways a bond can break:

H H

This bond can break with both e- going toone H or one e- goingto each H.

Think about a simple example like H2.

Review of Using Curved Arrows in Organic Chemistry

Bond breaking forms particles called reaction intermediates.

Common Reaction Intermediates Formed by Breaking a Covalent Bond

Br

H

O

H

Practice Exercise: Draw the structure of theproducts for each set of reactants.

OH2

Br

H

O

H

Practice Exercise: Draw the structure of theproducts for each set of reactants.

OH2

Bond Making

Thermodynamics and Bonding

Bond Dissociation Energy

Bond Dissociation Energy

Stronger bonds have a higher ΔHº

Calculating ΔHº Using Bond Dissociation Energy

Thermodynamics and Equilibrium

Relationship Between ΔGº and Keq

Relationship Between ΔGº and Keq

Free Energy, Enthalpy, and Entropy

Free Energy, Enthalpy, and Entropy

Understanding Organic Reactions

Enthalpy and Entropy

..when the total moles of products are the same as the total moles of reactants

Using Energy Diagrams

Energy Diagrams

Drawing the Structure of the Transition State

Types of Energy Diagrams

Understanding Organic Reactions

Energy Diagrams

Energy Diagrams for Two-step Reactions

Energy Diagram for a Two-Step Reaction

Summary Notes on the Energy Diagram Ea and G

Kinetics and Rate Equations

Catalysts

Catalysts

1.Changes the speed of a rxn

2.Does not appear in the product

3. Many types of catalyst can easily be recovered and used again

Identify the catalyst in each reaction

O

H2

Pt

OH

O

O

HNO3, H2SO4

O

O

NO2

OHH2O

H2SO4

How do catalysts change the speed of a rxn

Example of an Enzyme Catalyst

Understanding Organic Reactions

• Homolysis generates two uncharged species with unpaired electrons.

• A reactive intermediate with a single unpaired electron is called a radical.

• Radicals are highly unstable because they contain an atom that does not have an octet of electrons.

• Heterolysis generates a carbocation or a carbanion.• Both carbocations and carbanions are unstable

intermediates. A carbocation contains a carbon surrounded by only six electrons, and a carbanion has a negative charge on carbon, which is not a very electronegative atom.

Bond Making and Bond Breaking

Understanding Organic Reactions

• Equations for organic reactions are usually drawn with a single reaction arrow () between the starting material and product.

• The reagent, the chemical substance with which an organic compound reacts, is sometimes drawn on the left side of the equation with the other reactants. At other times, the reagent is drawn above the arrow itself.

• Although the solvent is often omitted from the equation, keep in mind that most organic reactions take place in liquid solvent.

• The solvent and temperature of the reaction may be added above or below the arrow.

• The symbols “h” and “” are used for reactions that require light and heat respectively.

Writing Equations for Organic Reactions

Using Arrows in Equations and Rxn Mechanisms