Www.ochem4free.com Organic Chemistry

Richard F. Daley and Sally J. Daley



Organic

Chemistry

Chapter 6

Reaction Mechanisms

An Overview of Organic Chemistry

6.1 Chemical Equilibria and Rates

246

6.2 Equilibrium Thermodynamics 250

6.3 Reaction Kinetics253

6.4 Reaction Profiles and Mechanisms

255

6.5 Why Reactions Occur

260

6.6 Organic Reaction Terminology

264

6.7 Classification of Reagents in Organic Reactions

6.8 Writing Reaction Mechanisms

270

6.9 Substitution Reactions

278

6.10 Addition Reactions

281

6.11 Elimination Reactions

284

Key Ideas from Chapter 6 287

270

Organic Chemistry - Ch 6

243

Daley & Daley

Copyright 1996-2005 by Richard F. Daley & Sally J. Daley

All Rights Reserved.

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5 July 2005

Organic Chemistry - Ch 6

244

Daley & Daley

Chapter 6

Reaction Mechanisms

An Overview of Organic

Chemistry

Chapter Outline

6.1

Chemical Equilibria and Rates

A review of chemical equilibria

6.2

Equilibrium Thermodynamics

A review of the thermodynamics of equilibrium

theory

6.3

Reaction Kinetics

The rate at which reactions occur

6.4

Reaction Profiles and Mechanisms

How a reaction profile relates to the reaction

mechanism

6.5

Why Reactions Occur

A look at how polarity, electronegativity, and

acidity/basicity affect a reaction.

6.6

Organic Reaction Terminology

The terminology organic chemists use to describe

chemical reactions

6.7

Classification of Reagents in Organic

Reactions

An introduction of typical nucleophiles and

electrophiles used in organic reactions

6.8

Writing Reaction Mechanisms

The general operations used in writing reaction

mechanisms

6.9

Substitution Reactions

An overview of the various types of substitution

reactions

6.10

Addition Reactions

An overview of the various types of addition

reactions

6.11

Elimination Reactions

An overview of the various types of elimination

reactions



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Organic Chemistry - Ch 6

245

Daley & Daley

Objectives

Review the concepts of equilibrium reactions

Understand the relationship between rates of reactions and

equilibrium

Know what the reaction progress diagram tells about a reaction

mechanism

Become more proficient in using curved arrows to describe electron

flow in a reaction

Understand the relationship between a reaction profile and a

reaction mechanism

Know the descriptive terminology that applies to organic reactions

Be able to write the generalized form of the five mechanistic

operations used in writing reaction mechanisms

Recognize when a reaction is substitution, elimination, or addition

If you can think ¡°mechanism¡± with reactions, then

organic chemistry is yours!

¡ªRichard Daley

C

hemical reactions and chemical synthesis are the ¡°really

interesting stuff¡± that organic chemistry is all about. But

before you get to those things, you need to review some material from

general chemistry that is pertinent to organic reactions and synthesis

as well as learn some other basic information. The first half of this

chapter is a review; the last half briefly previews the basic reaction

types of organic chemistry. The goal for this preview is to provide you

with a framework onto which you can organize new material as you

learn it. Chapter 7 begins the detailed study of the reactions of organic

chemistry.

6.1 Chemical Equilibrium and Rates



5 July 2005

Organic Chemistry - Ch 6

The term equilibrium

refers to reversible

reactions.

The rate of a reaction is

the speed that the

concentrations of the

reactants change per

unit of time¡ªusually

stated as moles per

liter per second.

Note that equilibrium

does not imply an

equality of

concentrations of A and

B relative to C and D

but an equality of rates

for the forward and

reverse reactions.

Organic chemists view

an equilibrium as

favorable if a

reasonable amount of

product is present.

A mechanism is the

step-by-step pathway

that the reaction

follows on its way from

reactants to products.

Section 6.5, page 000,

begins the discussion of

reaction mechanisms.

Chapters 3 and 11

provide the background

for predicting reaction

rates on the basis of

structure.

246

Daley & Daley

The study of reactions is the essence of much of organic

chemistry. To understand reactions, you need to know how they occur,

and knowing how they occur involves the study of reaction

equilibrium and rate. An equilibrium equation has the following

form:

A + B

C

+ D

In this reaction, the reactants, A and B, react to form the products, C

and D; and, at the same time, the products, C and D, react to form the

reactants, A and B. Equilibrium occurs when the rates of these two

reactions are equal. An equilibrium is a state in which the forward

and reverse reactions are continuous and simultaneous. The

thermodynamics of the system determines whether the reactants or

products are favored at equilibrium. The position of equilibrium favors

the lowest energy state for the system.

A number of factors affect the probability and the rate at which

a reaction reaches a favorable equilibrium. The major factor is the

mechanism that the reaction follows on its way from the reactants to

the products. If the reaction pathway is thermodynamically favorable

and if the products have less energy than the reactants, then the

reaction can establish a favorable equilibrium. A secondary factor is

the structure of the reactants. Other secondary factors are the

reaction conditions, such as temperature and reactant concentrations.

These secondary factors affect the rate at which a favorable

equilibrium is established.

For a brief look at the effect of reaction conditions, consider the

reaction of methane with chlorine:

CH4 + Cl2

CH3Cl + HCl

Although this reaction moves toward a favorable equilibrium, its rate

is easily controlled through regulating the reaction conditions. Mixed

methane and chlorine gases, stored in the dark at room temperature,

have no appreciable reaction even for extended time periods. However,

if you either illuminate or heat the flask containing the mixture, a

very rapid reaction immediately occurs. With other reactions, the

rates at which the reactants attain a favorable equilibrium vary from

imperceptibly slow to virtually instantaneous. By understanding these

factors, not only can you control the rate of reaction, but you can also

gain a better understanding of what happens¡ªthat is, what steps a

reaction takes as it goes from reactants to products.

Writing an equilibrium equation in the form shown below

indicates the specific rates at which the forward and reverse reactions

proceed.



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