Chapter 5

An Overview of Organic Reactions

Kinds of Reactions

• Addition Reactions

• Elimination Reactions

• Substitution Reactions

• Rearrangement Reactions

Addition Reactions

• Addition Reactions – are when two reactants add together to form a single product with no atoms “left over”.

+ H Br

Br

H

H

H

H

H

BromoethaneEthylene

Example:

Elimination Reactions

• Elimination Reactions – are when a single reactant breaks down to form two products. This is the opposite of an addition reaction.

Example:

OH

H

H

H

H

H+ H OH

Ethylene

Acid Catalyst

Substitution Reactions

• Substitution Reactions – occurs when two reactants exchange parts to give two new products.

Example:

C

H

H

H

H Cl Cl+ C

H

Cl

H

H H Cl+Light

Rearrangement Reactions

• Rearrangement Reactions – occurs when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product.

Example:

H

H3CH2C

H

H Acid Catalyst

CH3

H

H

H3C

1-Butene 2-Butene

Reaction Mechanism

• Reaction Mechanism – is an overall description of how a reaction occurs.

• This basically describes in detail exactly what takes place at each stage of a chemical transformation

• ALL CHEMICAL REACTIONS INVOLVE BOND-BREAKING AND BOND-MAKING.

Reaction Mechanism

• There are two ways that a covalent bond can break.– Unsymmetrical (is also called heterolytic)– Symmetrical (is also called homolytic)

Unsymmetrical Bond Breaking

• Unsymmetrical bond breaking - split bonds and gives both electrons to one atom. The most electronegative atom receives the both electrons.

A B A B+ -+

Polar Reactions

• Polar reactions involve a species with an even number of electrons and thus have only electrons pairs in their orbitals. This is where the electrons are taken by the most electronegative atom in a bond.

CH

H

H-

Symmetrical Bond Breaking

• Symmetrical bond breaking - split bonds and gives an electron to both atoms. The normally occurs when the electronegativity of the bond is the same.

A B A B+

Free Radicals

• A radical (also called a free radical) is described as a neutral chemical species that contains an odd number of electrons and thus has a single unpaired electrons in its orbital.

C CH

H

H

Arrow Structure

• Notice the arrows. • In a symmetrical (free radical formation) where

one electron is moved then you have an half arrow.

• In an unsymmetrical bond breaking where two electrons are moved then you have an full arrow.

A B A B+

A B A B+ -+

Free Radical Reactions

• Initiation – Starts reaction by breaking down the molecules into two free radicals.

• Propagation – The free radicals then propagate the creation of the product in larger quantities.

• Termination – Two free radicals bind to form a stable molecule and stop the reaction.

Examples of Free Radical Reactions

• Initiation – The symmetrical breaking of a bond into 2 free radicals.

Cl Cl Cl Cl+

Cl H-CH3 Cl H CH3+ +

Cl Cl+ Cl Cl

• Propagation – The reaction of a free radical with normal atoms to create new free radicals.

• Termination – The reaction of two free radical to create a normal bond again.

Polar Reactions

• Electrophile – is a substance which is “electron loving”. Remember that the nucleus is positive so the electrophile is either partially positive or totally positive. Wants to pair up with a negative charge.

• Nucleophile – is a atom in a bond which is “nucleus loving”. Remember that the many electronegative atoms are either partially negative or totally negative which seeks to bond with a partial positive or positive charge group. Wants a to pair up with a positive charge.

Polar Molecules

• If you remember molecules can be polar which means that the overall electron density is not even throughout the molecule. It maintains both a partial positive and a partial negative part of the molecule, just like a bond. This is a factor of the electronegativity of the atoms.

Examples of Polar Molecules

C

O

H H

-

+C

O

H H

-

+

H

HC

N

H H

-

+

H

H

H

Examples of Polar Reactions

• In this polar reaction, know as an electrophilic addition reaction the strong nucleophile the ethylene reacts with the proton of hydrogen bromide (electrophile) to yield bromoethane.

C C H Br+

nucleophile electrophileEthylene Hydrogen Bromide

C C

H

H

H Br

H

H

Bromoethane

Polar Reaction Diagram

CH3-O- H Br+ CH3-OH + Br-

nucleophile

Rules to Using Curved Arrows in Polar Reactions Mechanisms

• Rule 1 – Electrons move from a nucleophilic source to an electrophilic sink.

• Rule 2 – The nucleophilie can be either negatively charged or neutral.

• Rule 3 – The electrophile can be either positively charged or neutral.

• Rule 4 – The octet must be followed.

Polarizability

• The ability of the surroundings (solvent or other polar molecules) to make a molecules polar. It is like making something magnetic by pass a magnet over it.

• It starts out neutral but when the right atom or molecule comes close it becomes partial positive or partial negative.

Carbocations

• Are carbon atom which have given an extra electron to a more electronegative atom and has become positively charged. These become reactive and are electrophiles.

CH

H

H+

Carboanions

• Are carbon atom which have taken an extra electron and has become negatively charged. These are reactive and become nucleophiles.

CH

H

H-

Equilibrium, Rates, and Energy Changes

• Reactions can go forward or in a reverse direction. Either reactions can proceed to product or can stay as reactant due to which is favored.

• The Keq describes whether the reaction proceeds forward or reverse.

Keq

• To calculate the Keq all you do is setup an equilibrium equation with products over reactants. A,B,C,D are molar amount of the reactants and products.

aA + bB <=> cC + dDReactants Products

a,b,c,d = molar equiv

Keq = [C]c + [D]d

[A]a + [B]b

Gibbs Free Energy Equation 1

G = RT ln Keq

Gibbs Free Energy can be determined by the equilibrium constant of the reactions. This determines if a reaction is favorable, will occur, or is unfavorable, not likely to occur.

Gibbs Free Energy Equation 2

G = H – TS

Gibbs Free Energy can also be determined by the enthalphy, temperature, and the entropy of a systems to determine if it is favorable.

Gibbs Free-Energy Change (G)

• Gibbs free energy is the energy change that occurs during a chemical reaction.– When G is negative then the reaction is

favorable. This is also exergonic which means that it releases heat to the vessel. Warm to the touch.

– When G is positive then the reaction is not favorable. This is also called endergonic which means that the vessel becomes cold because it absorbs energy from its surroundings. Cool to the touch.

Enthalpy Change (H)

• Enthalpy change H (also called heat of reaction) is a measure of the change of the total bonding energy during a reaction.– When H is negative then the reaction is

favorable. This is also exothermic which means that it releases heat to the vessel.

– When H is positive then the reaction is not favorable. This is also called endothermic which means that the vessel becomes cold because it absorbs energy from its surroundings.

Entropy Change S

• Entropy is a measure of the total disorderness in product formation.– When S is negative then randomness

decreases.– When S is positive then randomness increases.

Transitional State• The transitional state represents the

highest energy structure in this step of the reaction.

Energy of Activation

• Energy of activation describes the amount of energy needed to initiation the reaction and allow it to proceed to product formation.

Bond Dissociation Energy (D)

• Bond Dissociation Energy (also bond strength) is how much energy it takes to break a specific bond into two radical fragments when the molecules is in the gas phase at 25oC.

• Remember that before many reactions can start a reactant has to break a bond. This describes how much energy it takes to break this initial bond.

Energy Diagrams

Energy Diagrams

Energy Diagrams

Energy Diagram

Catalyst

• Catalyst are materials which lower the activation energy thereby making the reaction occur faster.

Catalyst Energy Difference

Take Home Message

• Types of Reactions• Reaction Mechanisms (Free Radical, Polar)• Free Radical an Polar Reactions• Polarizability• Equilibrium Constant• Gibbs Free Energy and its calculation• Energy of Activation (Energy Diagrams)• Bond Dissociation Energy• Catalyst and Energy Diagrams