Writing Reaction Mechanisms in Organic Chemistry (eBook)

Kenneth A. Savin (Autor)

eBook Download: PDF | EPUB

2014 | 3. Auflage
510 Seiten
Elsevier Science (Verlag)
978-0-12-410509-6 (ISBN)

Infos

Writing Reaction Mechanisms in Organic Chemistry, Third Edition, is a guide to understanding the movements of atoms and electrons in the reactions of organic molecules. Expanding on the successful book by Miller and Solomon, this new edition further enhances your understanding of reaction mechanisms in organic chemistry and shows that writing mechanisms is a practical method of applying knowledge of previously encountered reactions and reaction conditions to new reactions. The book has been extensively revised with new material including a completely new chapter on oxidation and reduction reactions including stereochemical reactions. It is also now illustrated with hundreds of colorful chemical structures to help you understand reaction processes more easily. The book also features new and extended problem sets and answers to help you understand the general principles and how to apply these to real applications. In addition, there are new information boxes throughout the text to provide useful background to reactions and the people behind the discovery of a reaction. This new edition will be of interest to students and research chemists who want to learn how to organize what may seem an overwhelming quantity of information into a set of simple general principles and guidelines for determining and describing organic reaction mechanisms. - Extensively rewritten and reorganized with a completely new chapter on oxidation and reduction reactions including stereochemical reactions - Essential for those who need to have mechanisms explained in greater detail than most organic chemistry textbooks provide - Now illustrated with hundreds of colorful chemical structures to help you understand reaction processes more easily - New and extended problem sets and answers to help you understand the general principles and how to apply this to real applications - New information boxes throughout the text to provide useful background to reactions and the people behind the discovery of a reaction

Inhalt

Front Cover 1
Writing Reaction Mechanisms in Organic Chemistry 4
Copyright 5
Contents 6
Acknowledgments for the Third Edition 8
Chapter 1 - Introduction—Molecular Structure and Reactivity 10
1 HOW TO WRITE LEWIS STRUCTURES AND CALCULATE FORMAL CHARGES 10
2 REPRESENTATIONS OF ORGANIC COMPOUNDS 20
3 GEOMETRY AND HYBRIDIZATION 21
4 ELECTRONEGATIVITIES AND DIPOLES 23
5 RESONANCE STRUCTURES 25
6 AROMATICITY AND ANTIAROMATICITY 31
7 TAUTOMERS AND EQUILIBRIUM 34
8 ACIDITY AND BASICITY 37
9 NUCLEOPHILES AND ELECTROPHILES 41
ANSWERS TO PROBLEMS 45
References 62
Chapter 2 - General Principles for Writing Reaction Mechanisms 64
1 BALANCING EQUATIONS 65
2 USING ARROWS TO SHOW MOVING ELECTRONS 67
3 MECHANISMS IN ACIDIC AND BASIC MEDIA 70
4 ELECTRON-RICH SPECIES: BASES OR NUCLEOPHILES? 76
5 TRIMOLECULAR STEPS 78
6 STABILITY OF INTERMEDIATES 79
7 DRIVING FORCES FOR REACTIONS 82
8. STRUCTURAL RELATIONSHIPS BETWEEN STARTING MATERIALS AND PRODUCTS 84
9 SOLVENT EFFECTS 86
10 A LAST WORD 87
ANSWERS TO PROBLEMS 90
References 101
Chapter 3 - Reactions of Nucleophiles and Bases 102
1 NUCLEOPHILIC SUBSTITUTION 103
2 ELIMINATIONS AT SATURATED CARBON 114
3 NUCLEOPHILIC ADDITION TO CARBONYL COMPOUNDS 116
4 BASE-PROMOTED REARRANGEMENTS 130
5 ADDITIONAL MECHANISMS IN BASIC MEDIA 132
ANSWERS TO PROBLEMS 138
References 167
Chapter 4 - Reactions Involving Acids and Other Electrophiles 170
1 STABILITY OF CARBOCATIONS 170
2 FORMATION OF CARBOCATIONS 171
3 THE FATE OF CARBOCATIONS 173
4 REARRANGEMENT OF CARBOCATIONS 174
5 ELECTROPHILIC ADDITION 181
6 ACID-CATALYZED REACTIONS OF CARBONYL COMPOUNDS 185
7 ELECTROPHILIC AROMATIC SUBSTITUTION 191
8 CARBENES 195
9 ELECTROPHILIC HETEROATOMS 202
ANSWERS TO PROBLEMS 209
References 243
Chapter 5 - Radicals and Radical Anions 246
I INTRODUCTION 246
2 FORMATION OF RADICALS 246
3 RADICAL CHAIN PROCESSES 249
4 RADICAL INHIBITORS 252
5 DETERMINING THE THERMODYNAMIC FEASIBILITY OF RADICAL REACTIONS 253
6 ADDITION OF RADICALS 256
7 FRAGMENTATION REACTIONS 261
8 REARRANGEMENT OF RADICALS 265
9 THE SRN1 REACTION 268
10 THE BIRCH REDUCTION 271
11 A RADICAL MECHANISM FOR THE REARRANGEMENT OF SOME ANIONS 273
ANSWERS TO PROBLEMS 277
References 300
Chapter 6 - Pericyclic Reactions 302
1 INTRODUCTION 302
2 ELECTROCYCLIC REACTIONS 304
3 CYCLOADDITIONS 311
4 SIGMATROPIC REARRANGEMENTS 321
5 THE ENE REACTION 328
6 A MOLECULAR ORBITAL VIEW OF PERICYCLIC PROCESSES 332
ANSWERS TO PROBLEMS 346
References 361
Chapter 7 - Oxidations and Reductions 364
1 DEFINITION OF OXIDATION AND REDUCTION 364
2 OXIDATIONS 369
3 REDUCTIONS 394
ANSWERS TO PROBLEMS 425
References 438
Chapter 8 - Additional Problems 442
ANSWERS TO PROBLEMS 451
References 489
Appendix A - Lewis Structures of Common Functional Groups 490
Appendix B - Symbols and Abbreviations Used in Chemical Notation 492
Appendix C - Relative Acidities of Common Organic and Inorganic Substancesa 494
Index 502

Textauszug

Chapter 1

Introduction—Molecular Structure and Reactivity

Abstract

To understand the way atoms bond to one another and then transform into new structures it is important to have a common set of rules and definitions. Herein, we describe key aspects of bonding, equilibrium, and driving forces that influence the structure and allow us to model and predict what are otherwise very complicated events. Many of the concepts presented in this chapter are a review for the student who has already taken the basic Organic series and the General Chemistry sequence, with a few new concepts and further explanations on some specific topics. This chapter helps create a common language and concepts that will be encountered in the book.

Keywords

Acidity; Aromaticity; Basicity; Bond; Charge; Dipole; Electronegativity; Electrophile; Formal charge; Hybridization; Lewis structure; Nucleophile; Octet; Resonance; Tautomers

Reaction mechanisms offer us insights into how molecules react, enable us to manipulate the course of known reactions, aid us in predicting the course of known reactions using new substrates, and help us to develop new reactions and reagents. In order to understand and write reaction mechanisms, it is essential to have a detailed knowledge of the structures of the molecules involved and to be able to notate these structures unambiguously. In this chapter, we present a review of the fundamental principles relating to molecular structure and of the ways to convey structural information. A crucial aspect of structure from the mechanistic viewpoint is the distribution of electrons, so this chapter outlines how to analyze and depict electron distributions. Mastering the material in this chapter will provide you with the tools you need to propose reasonable mechanisms and to convey these mechanisms clearly to others.

1. How to Write Lewis Structures and Calculate Formal Charges

The ability to construct Lewis structures is fundamental to writing or understanding organic reaction mechanisms. It is particularly important because lone pairs of electrons frequently are crucial to the mechanism but often are omitted from structures appearing in the chemical literature.

There are two methods commonly used to show Lewis structures. One shows all electrons as dots. The other shows all bonds (two shared electrons) as lines and all unshared electrons as dots.

A. Determining the Number of Bonds

HINT 1.1

To facilitate the drawing of Lewis structures, estimate the number of bonds.

For a stable structure with an even number of electrons, the number of bonds is given by the equation:

ElectronDemand?ElectronSupply)/2=NumberofBonds

 

The electron demand is two for hydrogen and eight for all other atoms usually considered in organic chemistry. (The tendency of most atoms to acquire eight valence electrons is known as the octet rule.) For elements in group IIIA (e.g., B, Al, Ga), the electron demand is six. Other exceptions are noted, as they arise, in examples and problems.

For neutral molecules, the contribution of each atom to the electron supply is the number of valence electrons of the neutral atom. (This is the same as the group number of the element when the periodic table is divided into eight groups.) For ions, the electron supply is decreased by one for each positive charge of a cation and is increased by one for each negative charge of an anion.

Use the estimated number of bonds to draw the number of two-electron bonds in your structure. This may involve drawing a number of double and triple bonds (see the following section).

B. Determining the Number of Rings and/or ? Bonds (Degree of Unsaturation)

The total number of rings and/or ? bonds can be calculated from the molecular formula, bearing in mind that in an acyclic saturated hydrocarbon the number of hydrogens is 2n + 2, where n is the number of carbon atoms. Each time a ring or ? bond is formed, there will be two fewer hydrogens needed to complete the structure.

HINT 1.2

On the basis of the molecular formula, the degree of unsaturation for a hydrocarbon is calculated as (2m + 2 ? n)/2, where m is the number of carbons and n is the number of hydrogens. The number calculated is the number of rings and/or ? bonds. For molecules containing heteroatoms, the degree of unsaturation can be calculated as follows:

Nitrogen: For each nitrogen atom, subtract 1 from n.

Halogens: For each halogen atom, add 1 to n.

Oxygen: Use the formula for hydrocarbons.

This method cannot be used for molecules in which there are atoms like sulfur and phosphorus whose valence shell can expand beyond eight.

EXAMPLE 1.1

CALCULATE THE NUMBER OF RINGS AND/OR ? BONDS CORRESPONDING TO EACH OF THE FOLLOWING MOLECULAR FORMULAS

a. C2H2Cl2Br2

There are a total of four halogen atoms. Using the formula (2m + 2 ? n)/2, we calculate the degree of unsaturation to be (2(2) + 2 ? (2 + 4))/2 = 0.

b. C2H3N

There is one nitrogen atom, so the degree of unsaturation is (2(2) + 2 ? (3?1)) = 2.

C. Drawing the Lewis Structure

Start by drawing the skeleton of the molecule, using the correct number of rings or ? bonds, and then attach hydrogen atoms to satisfy the remaining valences. For organic molecules, the carbon skeleton frequently is given in an abbreviated form.

Once the atoms and bonds have been placed, add lone pairs of electrons to give each atom a total of eight valence electrons. When this process is complete, there should be two electrons for hydrogen; six for B, Al, or Ga; and eight for all other atoms. The total number of valence electrons for each element in the final representation of a molecule is obtained by counting each electron around the element as one electron, even if the electron is shared with another atom. (This should not be confused with counting electrons for charges or formal charges; see Section 1.D.) The number of valence electrons around each atom equals the electron demand. Thus, when the number of valence electrons around each element equals the electron demand, the number of bonds will be as calculated in Hint 1.1.

Atoms of higher atomic number can expand the valence shell to more than eight electrons. These atoms include sulfur, phosphorus, and the halogens (except fluorine).

HINT 1.3

When drawing Lewis structures, make use of the following common structural features.

1. Hydrogen is always on the periphery because it forms only one covalent bond.

2. Carbon, nitrogen, and oxygen exhibit characteristic bonding patterns. In the examples that follow, the R groups may be hydrogen, alkyl, or aryl groups, or any combination of these. These substituents do not change the bonding pattern depicted.

(a) Carbon in neutral molecules usually has four bonds. The four bonds may all be ? bonds, or they may be various combinations of ? and ? bonds (i.e., double and triple bonds).

    There are exceptions to the rule that carbon has four bonds. These include CO, isonitriles (RNC), and carbenes (neutral carbon species with six valence electrons; see Chapter 4).

(b) Carbon with a single positive or negative charge has three bonds.

(c) Neutral nitrogen, with the exception of nitrenes (see Chapter 4), has three bonds and a lone pair.

(d) Positively charged nitrogen has four bonds and a positive charge; exceptions are nitrenium ions (see Chapter 4).

(e) Negatively charged nitrogen has two bonds and two lone pairs of electrons.

(f) Neutral oxygen has two bonds and two lone pairs of electrons.

(g) Oxygen–oxygen bonds are uncommon; they are present only in peroxides, hydroperoxides, and diacyl peroxides (see Chapter 5). The formula, RCO2R, implies the following structure:

(h) Positive oxygen usually has three bonds and a lone pair of electrons; exceptions are the very unstable oxenium ions, which contain a single bond to oxygen and two lone pairs of electrons.

3. Sometimes a phosphorus or sulfur atom in a molecule is depicted with 10...

Produkt-Details

• Erscheint lt. Verlag: 10.7.2014
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Analytische Chemie
• Themenwelt: Naturwissenschaften ► Chemie ► Organische Chemie
• Themenwelt: Technik
• ISBN-10: 0-12-410509-2 / 0124105092
• ISBN-13: 978-0-12-410509-6 / 9780124105096