Tài liệu Giới thiệu về quang phổ

is a comprehensive resource that provides an unmatched, systematic introduction to spectra and basic theoretical concepts in spectroscopic methods that creates a practical learning resource, whether you’re an introductory student or someone who needs a reliable reference text on spectroscopy. This well-rounded introduction features updated spectra, a w .c modernized presentation of one-dimensional Nuclear Magnetic Resonance (NMR) spectroscopy, the introduction of biological molecules in mass spectrometry, and inclusion of modern tech- niques alongside DEPT, COSY, and HECTOR. Count on this book’s vn he Pavia/Lampman/Kriz/Vyvyan’s Introduction to Spectroscopy, 4e, Pavia | Lampman | Kriz | Vyvyan m 4a Introduction to Spec t ros co p y Gain an understanding of t h e l a t e s t a d va n c e s i n s p e c t r o s c o p y w i t h t h e t e x t t h a t ’s s e t t h e u n r i va l e d standard for more than 30 years. ll. Pavia Lampman Kriz Vyvyan w exceptional presentation to provide the comprehensive cover- Visit us on the Web! w age needed to truly understand today’s spectroscopic techniques. a c a d e m i c . c e n g a g e . c o m /c h e m i s t r y Fourth Edition Introduction to Spectroscopy For your course and learning solutions, visit academic.cengage.com Four th Edition Purchase any of our products at your local college store or at our preferred online store www.ichapters.com 9780495114789_cvr_se.indd 1 40 AM 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page i F O U R T H E D I T I O N Donald L. Pavia m 4a ll. vn INTRODUCTION TO SPECTROSCOPY he Gary M. Lampman George S. Kriz w .c James R. Vyvyan w w Department of Chemistry Western Washington University Bellingham, Washington Australia • Brazil • Japan • Korea • Mexico • Singapore • Spain • United Kingdom • United States 2/7/08 9:11 AM Page ii w w w .c he m 4a ll. vn 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page iii vn 14782_FM_i-xvi pp3.qxd w w w .c he m 4a ll. TO ALL OF OUR “O-SPEC” STUDENTS 2/7/08 9:11 AM Page iv Introduction to Spectroscopy, Fourth Edition Donald L. Pavia, Gary M. Lampman, George S. Kriz, and James R. Vyvyan Acquisitions Editor: Lisa Lockwood Development Editor: Brandi Kirksey Editorial Assistant: Elizabeth Woods Technology Project Manager: Lisa Weber © 2009, 2001 Brooks/Cole, Cengage Learning ALL RIGHTS RESERVED. No part of this work covered by the copyright herein may be reproduced, transmitted, stored, or used in any form or by any means graphic, electronic, or mechanical, including but not limited to photocopying, recording, scanning, digitizing, taping, Web distribution, information networks, or information storage and retrieval systems, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without the prior written permission of the publisher. vn 14782_FM_i-xvi pp3.qxd Marketing Manager: Amee Mosley For product information and technology assistance, contact us at Cengage Learning Customer & Sales Support, 1-800-354-9706 Marketing Assistant: Elizabeth Wong Marketing Communications Manager: Talia Wise ll. For permission to use material from this text or product, submit all requests online at cengage.com/permissions Further permissions questions can be e-mailed to [email protected] Creative Director: Rob Hugel Art Director: John Walker Library of Congress Control Number: 2007943966 Print Buyer: Paula Vang ISBN-13: 978-0-495-11478-9 Permissions Editor: Bob Kauser Production Service: PrePress PMG Photo Researcher: Susan Lawson Cover Image: Eddie Gerald/Alamy Compositor: PrePress PMG ISBN-10: 0-495-11478-2 Brooks/Cole 10 Davis Drive Belmont, CA 94002-3098 USA he Copy Editor: Kathleen Brown Cover Designer: Dare Porter m 4a Project Manager, Editorial Production: Michelle Cole w .c Cengage Learning is a leading provider of customized learning solutions with office locations around the globe, including Singapore, the United Kingdom, Australia, Mexico, Brazil, and Japan. Locate your local office at international.cengage.com/region w w Cengage Learning products are represented in Canada by Nelson Education, Ltd. Printed in the United States of America 1 2 3 4 5 6 7 12 11 10 09 08 For your course and learning solutions, visit academic.cengage.com Purchase any of our products at your local college store or at our preferred online store www.ichapters.com 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page v T m 4a ll. vn PREFACE w w w .c he his is the fourth edition of a textbook in spectroscopy intended for students of organic chemistry. Our textbook can serve as a supplement for the typical organic chemistry lecture textbook, and it can also be used as a “stand-alone” textbook for an advanced undergraduate course in spectroscopic methods of structure determination or for a first-year graduate course in spectroscopy. This book is also a useful tool for students engaged in research. Our aim is not only to teach students to interpret spectra, but also to present basic theoretical concepts. As with the previous editions, we have tried to focus on the important aspects of each spectroscopic technique without dwelling excessively on theory or complex mathematical analyses. This book is a continuing evolution of materials that we use in our own courses, both as a supplement to our organic chemistry lecture course series and also as the principal textbook in our upper division and graduate courses in spectroscopic methods and advanced NMR techniques. Explanations and examples that we have found to be effective in our courses have been incorporated into this edition. This fourth edition of Introduction to Spectroscopy contains some important changes. The discussion of coupling constant analysis in Chapter 5 has been significantly expanded. Long-range couplings are covered in more detail, and multiple strategies for measuring coupling constants are presented. Most notably, the systematic analysis of line spacings allows students (with a little practice) to extract all of the coupling constants from even the most challenging of first-order multiplets. Chapter 5 also includes an expanded treatment of group equivalence and diastereotopic systems. Discussion of solvent effects in NMR spectroscopy is discussed more explicitly in Chapter 6, and the authors thank one of our graduate students, Ms. Natalia DeKalb, for acquiring the data in Figures 6.19 and 6.20. A new section on determining the relative and absolute stereochemical configuration with NMR has also been added to this chapter. The mass spectrometry section (Chapter 8) has been completely revised and expanded in this edition, starting with more detailed discussion of a mass spectrometer’s components. All of the common ionization methods are covered, including chemical ionization (CI), fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI), and electrospray techniques. Different types of mass analyzers are described as well. Fragmentation in mass spectrometry is discussed in greater detail, and several additional fragmentation mechanisms for common functional groups are illustrated. Numerous new mass spectra examples are also included. Problems have been added to each of the chapters. We have included some more solved problems, so that students can develop skill in solving spectroscopy problems. v 14782_FM_i-xvi pp3.qxd vi 2/7/08 9:11 AM Page vi Preface vn The authors are very grateful to Mr. Charles Wandler, without whose expert help this project could not have been accomplished. We also acknowledge numerous contributions made by our students who use the textbook and who provide us careful and thoughtful feedback. We wish to alert persons who adopt this book that answers to all of the problems are available on line from the publisher. Authorization to gain access to the web site may be obtained through the local Cengage textbook representative. Finally, once again we must thank our wives, Neva-Jean, Marian, Carolyn, and Cathy for their support and their patience. They endure a great deal in order to support us as we write, and they deserve to be part of the celebration when the textbook is completed! w w w .c he m 4a ll. Donald L. Pavia Gary M. Lampman George S. Kriz James R. Vyvyan 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page vii CHAPTER 1 m 4a ll. vn CONTENTS MOLECULAR FORMULAS AND WHAT CAN BE LEARNED FROM THEM 1 he Elemental Analysis and Calculations 1 Determination of Molecular Mass 5 Molecular Formulas 5 Index of Hydrogen Deficiency 6 The Rule of Thirteen 9 A Quick Look Ahead to Simple Uses of Mass Spectra Problems 13 References 14 w .c 1.1 1.2 1.3 1.4 1.5 1.6 12 CHAPTER 2 INFRARED SPECTROSCOPY The Infrared Absorption Process 16 Uses of the Infrared Spectrum 17 The Modes of Stretching and Bending 18 Bond Properties and Absorption Trends 20 The Infrared Spectrometer 23 A. Dispersive Infrared Spectrometers 23 B. Fourier Transform Spectrometers 25 Preparation of Samples for Infrared Spectroscopy 26 What to Look for When Examining Infrared Spectra 26 Correlation Charts and Tables 28 How to Approach the Analysis of a Spectrum (Or What You Can Tell at a Glance) w w 2.1 2.2 2.3 2.4 2.5 15 2.6 2.7 2.8 2.9 30 vii 14782_FM_i-xvi pp3.qxd 9:11 AM Page viii Contents 2.10 54 he w .c 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 m 4a ll. 2.11 2.12 2.13 2.14 Hydrocarbons: Alkanes, Alkenes, and Alkynes 31 A. Alkanes 31 B. Alkenes 33 C. Alkynes 35 Aromatic Rings 43 Alcohols and Phenols 47 Ethers 50 Carbonyl Compounds 52 A. Factors that Influence the CJO Stretching Vibration B. Aldehydes 56 C. Ketones 58 D. Carboxylic Acids 62 E. Esters 64 F. Amides 70 G. Acid Chlorides 72 H. Anhydrides 73 Amines 74 Nitriles, Isocyanates, Isothiocyanates, and Imines 77 Nitro Compounds 79 Carboxylate Salts, Amine Salts, and Amino Acids 80 Sulfur Compounds 81 Phosphorus Compounds 84 Alkyl and Aryl Halides 84 The Background Spectrum 86 Problems 88 References 104 vn viii 2/7/08 CHAPTER 3 w NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY PART ONE: BASIC CONCEPTS w 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 105 Nuclear Spin States 105 Nuclear Magnetic Moments 106 Absorption of Energy 107 The Mechanism of Absorption (Resonance) 109 Population Densities of Nuclear Spin States 111 The Chemical Shift and Shielding 112 The Nuclear Magnetic Resonance Spectrometer 114 A. The Continuous-Wave (CW) Instrument 114 B. The Pulsed Fourier Transform (FT) Instrument 116 Chemical Equivalence—A Brief Overview 120 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page ix Contents vn ll. w w w .c he 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 Integrals and Integration 121 Chemical Environment and Chemical Shift 123 Local Diamagnetic Shielding 124 A. Electronegativity Effects 124 B. Hybridization Effects 126 C. Acidic and Exchangeable Protons; Hydrogen Bonding 127 Magnetic Anisotropy 128 Spin–Spin Splitting (n + 1) Rule 131 The Origin of Spin–Spin Splitting 134 The Ethyl Group (CH3CH2I) 136 Pascal’s Triangle 137 The Coupling Constant 138 A Comparison of NMR Spectra at Low- and High-Field Strengths 141 1 Survey of Typical H NMR Absorptions by Type of Compound 142 A. Alkanes 142 B. Alkenes 144 C. Aromatic Compounds 145 D. Alkynes 146 E. Alkyl Halides 148 F. Alcohols 149 G. Ethers 151 H. Amines 152 I. Nitriles 153 J. Aldehydes 154 K. Ketones 156 L. Esters 157 M. Carboxylic Acids 158 N. Amides 159 O. Nitroalkanes 160 Problems 161 References 176 m 4a 3.9 3.10 3.11 ix CHAPTER 4 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY PART TWO: CARBON-13 SPECTRA, INCLUDING HETERONUCLEAR COUPLING WITH OTHER NUCLEI 177 4.1 4.2 4.3 The Carbon-13 Nucleus 177 Carbon-13 Chemical Shifts 178 A. Correlation Charts 178 B. Calculation of 13C Chemical Shifts 180 13 Proton-Coupled C Spectra—Spin–Spin Splitting of Carbon-13 Signals 181 14782_FM_i-xvi pp3.qxd 9:11 AM Page x Contents vn Proton-Decoupled 13C Spectra 183 Nuclear Overhauser Enhancement (NOE) 184 Cross-Polarization: Origin of the Nuclear Overhauser Effect 186 13 Problems with Integration in C Spectra 189 Molecular Relaxation Processes 190 Off-Resonance Decoupling 192 A Quick Dip into DEPT 192 Some Sample Spectra—Equivalent Carbons 195 Compounds with Aromatic Rings 197 Carbon-13 NMR Solvents—Heteronuclear Coupling of Carbon to Deuterium Heteronuclear Coupling of Carbon-13 to Fluorine-19 203 Heteronuclear Coupling of Carbon-13 to Phosphorus-31 204 Carbon and Proton NMR: How to Solve a Structure Problem 206 Problems 210 References 231 ll. 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 CHAPTER 5 m 4a x 2/7/08 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY PART THREE: SPIN–SPIN COUPLING he Coupling Constants: Symbols 233 Coupling Constants: The Mechanism of Coupling 234 1 A. One-Bond Couplings ( J) 235 B. Two-Bond Couplings (2J) 236 3 C. Three-Bond Couplings ( J) 239 4 n D. Long-Range Couplings ( J– J) 244 Magnetic Equivalence 247 Spectra of Diastereotopic Systems 252 A. Diastereotopic Methyl Groups: 4-Methyl-2-pentanol 252 B. Diastereotopic Hydrogens: 4-Methyl-2-pentanol 254 Nonequivalence within a Group—The Use of Tree Diagrams when the n + 1 Rule Fails 257 Measuring Coupling Constants from First-Order Spectra 260 A. Simple Multiplets—One Value of J (One Coupling) 260 B. Is the n + 1 Rule Ever Really Obeyed? 262 C. More Complex Multiplets—More Than One Value of J 264 Second-Order Spectra—Strong Coupling 268 A. First-Order and Second-Order Spectra 268 B. Spin System Notation 269 270 C. The A2, AB, and AX Spin Systems D. The AB2 . . . AX2 and A2B2 . . . A2X2 Spin Systems 270 w .c 5.1 5.2 w 5.3 5.4 5.5 w 5.6 5.7 233 199 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page xi Contents vn 5.11 281 ll. 5.8 5.9 5.10 E. Simulation of Spectra 272 F. The Absence of Second-Order Effects at Higher Field 272 G. Deceptively Simple Spectra 273 Alkenes 277 Measuring Coupling Constants—Analysis of an Allylic System Aromatic Compounds—Substituted Benzene Rings 285 A. Monosubstituted Rings 286 B. para-Disubstituted Rings 288 C. Other Substitution 291 Coupling in Heteroaromatic Systems 293 Problems 296 References 328 m 4a CHAPTER 6 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY PART FOUR: OTHER TOPICS IN ONE-DIMENSIONAL NMR w w .c 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 Protons on Oxygen: Alcohols 329 Exchange in Water and D2O 332 A. Acid/Water and Alcohol/Water Mixtures 332 B. Deuterium Exchange 333 C. Peak Broadening Due to Exchange 337 Other Types of Exchange: Tautomerism 338 Protons on Nitrogen: Amines 340 Protons on Nitrogen: Quadrupole Broadening and Decoupling Amides 345 The Effect of Solvent on Chemical Shift 347 Chemical Shift Reagents 351 Chiral Resolving Agents 354 Determining Absolute and Relative Configuration via NMR A. Determining Absolute Configuration 356 B. Determining Relative Configuration 358 Nuclear Overhauser Effect Difference Spectra 359 Problems 362 References 380 he 6.1 6.2 w 6.11 CHAPTER 7 ULTRAVIOLET SPECTROSCOPY 7.1 7.2 7.3 381 The Nature of Electronic Excitations 381 The Origin of UV Band Structure 383 Principles of Absorption Spectroscopy 383 329 342 356 xi 14782_FM_i-xvi pp3.qxd 9:11 AM Page xii Contents Instrumentation 384 Presentation of Spectra 385 Solvents 386 What Is a Chromophore? 387 The Effect of Conjugation 390 The Effect of Conjugation on Alkenes 391 The Woodward–Fieser Rules for Dienes 394 Carbonyl Compounds; Enones 397 Woodward’s Rules for Enones 400 a,b-Unsaturated Aldehydes, Acids, and Esters 402 Aromatic Compounds 402 A. Substituents with Unshared Electrons 404 B. Substituents Capable of p-Conjugation 406 C. Electron-Releasing and Electron-Withdrawing Effects 406 D. Disubstituted Benzene Derivatives 406 E. Polynuclear Aromatic Hydrocarbons and Heterocyclic Compounds Model Compound Studies 411 Visible Spectra: Color in Compounds 412 What to Look for in an Ultraviolet Spectrum: A Practical Guide 413 Problems 415 References 417 CHAPTER 8 he 7.15 7.16 7.17 m 4a ll. 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 vn xii 2/7/08 418 w .c MASS SPECTROMETRY The Mass Spectrometer: Overview 418 Sample Introduction 419 Ionization Methods 420 A. Electron Ionization (EI) 420 B. Chemical Ionization (CI) 421 C. Desorption Ionization Techniques (SIMS, FAB, and MALDI) D. Electrospray Ionization (ESI) 426 Mass Analysis 429 A. The Magnetic Sector Mass Analyzer 429 B. Double-Focusing Mass Analyzers 430 C. Quadrupole Mass Analyzers 430 D. Time-of-Flight Mass Analyzers 432 Detection and Quantitation: The Mass Spectrum 435 Determination of Molecular Weight 438 Determination of Molecular Formulas 441 w 8.1 8.2 8.3 w 8.4 8.5 8.6 8.7 425 409 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page xiii Contents w .c he m 4a ll. vn 8.8 A. Precise Mass Determination 441 B. Isotope Ratio Data 441 Structural Analysis and Fragmentation Patterns 445 A. Stevenson’s Rule 446 B. The Initial Ionization Event 447 C. Radical-site Initiated Cleavage: a-Cleavage 448 D. Charge-site Initiated Cleavage: Inductive Cleavage 448 E. Two-Bond Cleavage 449 F. Retro Diels-Adler Cleavage 450 G. McLafferty Rearrangements 450 H. Other Cleavage Types 451 I. Alkanes 451 J. Cycloalkanes 454 K. Alkenes 455 L. Alkynes 459 M. Aromatic Hydrocarbons 459 N. Alcohols and Phenols 464 O. Ethers 470 P. Aldehydes 472 Q. Ketones 473 R. Esters 477 S. Carboxylic Acids 482 T. Amines 484 U. Selected Nitrogen and Sulfur Compounds 488 V. Alkyl Chlorides and Alkyl Bromides 492 Strategic Approach to Analyzing Mass Spectra and Solving Problems Computerized Matching of Spectra with Spectral Libraries 497 Problems 498 References 519 w 8.9 8.10 CHAPTER 9 w COMBINED STRUCTURE PROBLEMS Example 1 522 Example 2 524 Example 3 526 Example 4 529 Problems 531 Sources of Additional Problems 586 520 496 xiii 14782_FM_i-xvi pp3.qxd xiv 2/7/08 9:11 AM Page xiv Contents CHAPTER 10 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY PART FIVE: ADVANCED NMR TECHNIQUES Pulse Sequences 587 Pulse Widths, Spins, and Magnetization Vectors 589 Pulsed Field Gradients 593 The DEPT Experiment 595 Determining the Number of Attached Hydrogens 598 A. Methine Carbons (CH) 598 B. Methylene Carbons (CH2) 599 C. Methyl Carbons (CH3) 601 D. Quaternary Carbons (C) 601 E. The Final Result 602 10.6 Introduction to Two-Dimensional Spectroscopic Methods 602 10.7 The COSY Technique 602 A. An Overview of the COSY Experiment 603 B. How to Read COSY Spectra 604 10.8 The HETCOR Technique 608 A. An Overview of the HETCOR Experiment 608 B. How to Read HETCOR Spectra 609 10.9 Inverse Detection Methods 612 10.10 The NOESY Experiment 613 10.11 Magnetic Resonance Imaging 614 10.12 Solving a Structural Problem Using Combined 1-D and 2-D Techniques A. Index of Hydrogen Deficiency and Infrared Spectrum 616 B. Carbon-13 NMR Spectrum 617 C. DEPT Spectrum 617 D. Proton NMR Spectrum 619 E. COSY NMR Spectrum 621 F. HETCOR (HSQC) NMR Spectrum 622 Problems 623 References 657 w w w .c he m 4a ll. vn 10.1 10.2 10.3 10.4 10.5 587 ANSWERS TO SELECTED PROBLEMS ANS-1 APPENDICES Appendix 1 Appendix 2 Appendix 3 Infrared Absorption Frequencies of Functional Groups A-1 1 Approximate H Chemical Shift Ranges (ppm) for Selected Types of Protons A-8 Some Representative 1H Chemical Shift Values for Various Types of Protons A-9 616 14782_FM_i-xvi pp3.qxd 2/7/08 9:11 AM Page xv Contents Appendix 12 Appendix 13 Appendix 14 I-1 w w w .c he INDEX vn Appendix 8 Appendix 9 Appendix 10 Appendix 11 H Chemical Shifts of Selected Heterocyclic and Polycyclic Aromatic Compounds A-12 Typical Proton Coupling Constants A-13 1 Calculation of Proton ( H) Chemical Shifts A-17 13 Approximate C Chemical-Shift Values (ppm) for Selected Types of Carbon A-21 13 Calculation of C Chemical Shifts A-22 13 C Coupling Constants A-32 1 H and 13C Chemical Shifts for Common NMR Solvents A-33 Tables of Precise Masses and Isotopic Abundance Ratios for Molecular Ions under Mass 100 Containing Carbon, Hydrogen, Nitrogen, and Oxygen A-34 Common Fragment Ions under Mass 105 A-40 A Handy-Dandy Guide to Mass Spectral Fragmentation Patterns A-43 Index of Spectra A-46 ll. Appendix 5 Appendix 6 Appendix 7 1 m 4a Appendix 4 xv 2/7/08 9:11 AM Page xvi w w w .c he m 4a ll. vn 14782_FM_i-xvi pp3.qxd 14782_01_Ch1_p001-014.pp3.qxd 1/25/08 10:11 AM C H A P T E R Page 1 1 vn MOLECULAR FORMULAS AND WHAT CAN BE LEARNED FROM THEM B m 4a ll. efore attempting to deduce the structure of an unknown organic compound from an examination of its spectra, we can simplify the problem somewhat by examining the molecular formula of the substance. The purpose of this chapter is to describe how the molecular formula of a compound is determined and how structural information may be obtained from that formula. The chapter reviews both the modern and classical quantitative methods of determining the molecular formula. While use of the mass spectrometer (Section 1.6 and Chapter 8) can supplant many of these quantitative analytical methods, they are still in use. Many journals still require that a satisfactory quantitative elemental analysis (Section 1.1) be obtained prior to the publication of research results. 1.1 ELEMENTAL ANALYSIS AND CALCULATIONS The classical procedure for determining the molecular formula of a substance involves three steps: he 1. A qualitative elemental analysis to find out what types of atoms are present . . . C, H, N, O, S, Cl, and so on. w .c 2. A quantitative elemental analysis (or microanalysis) to find out the relative numbers (percentages) of each distinct type of atom in the molecule. 3. A molecular mass (or molecular weight) determination. w w The first two steps establish an empirical formula for the compound. When the results of the third procedure are known, a molecular formula is found. Virtually all organic compounds contain carbon and hydrogen. In most cases, it is not necessary to determine whether these elements are present in a sample: their presence is assumed. However, if it should be necessary to demonstrate that either carbon or hydrogen is present in a compound, that substance may be burned in the presence of excess oxygen. If the combustion produces carbon dioxide, carbon must be present; if combustion produces water, hydrogen atoms must be present. Today, the carbon dioxide and water can be detected by gas chromatographic methods. Sulfur atoms are converted to sulfur dioxide; nitrogen atoms are often chemically reduced to nitrogen gas following their combustion to nitrogen oxides. Oxygen can be detected by the ignition of the compound in an atmosphere of hydrogen gas; the product is water. Currently, all such analyses are performed by gas chromatography, a method that can also determine the relative amounts of each of these gases. If the amount of the original sample is known, it can be entered, and the computer can calculate the percentage composition of the sample. Unless you work in a large company or in one of the larger universities, it is quite rare to find a research laboratory in which elemental analyses are performed on site. It requires too much time to set up the apparatus and keep it operating within the limits of suitable accuracy and precision. Usually, samples are sent to a commercial microanalytical laboratory that is prepared to do this work routinely and to vouch for the accuracy of the results. 1 14782_01_Ch1_p001-014.pp3.qxd 2 1/25/08 10:11 AM Page 2 Molecular Formulas and What Can Be Learned from Them m 4a ll. vn Before the advent of modern instrumentation, the combustion of the precisely weighed sample was carried out in a cylindrical glass tube, contained within a furnace. A stream of oxygen was passed through the heated tube on its way to two other sequential, unheated tubes that contained chemical substances that would absorb first the water (MgClO4) and then the carbon dioxide (NaOH/silica). These preweighed absorption tubes were detachable and were removed and reweighed to determine the amounts of water and carbon dioxide formed. The percentages of carbon and hydrogen in the original sample were calculated by simple stoichiometry. Table 1.1 shows a sample calculation. Notice in this calculation that the amount of oxygen was determined by difference, a common practice. In a sample containing only C, H, and O, one needs to determine the percentages of only C and H; oxygen is assumed to be the unaccounted-for portion. You may also apply this practice in situations involving elements other than oxygen; if all but one of the elements is determined, the last one can be determined by difference. Today, most calculations are carried out automatically by the computerized instrumentation. Nevertheless, it is often useful for a chemist to understand the fundamental principles of the calculations. Table 1.2 shows how to determine the empirical formula of a compound from the percentage compositions determined in an analysis. Remember that the empirical formula expresses the simplest whole-number ratios of the elements and may need to be multiplied by an integer to obtain the true molecular formula. To determine the value of the multiplier, a molecular mass is required. Determination of the molecular mass is discussed in the next section. For a totally unknown compound (unknown chemical source or history) you will have to use this type of calculation to obtain the suspected empirical formula. However, if you have prepared the compound from a known precursor by a well-known reaction, you will have an idea of the structure of the compound. In this case, you will have calculated the expected percentage composition of your TA B L E 1 . 1 he CALCULATION OF PERCENTAGE COMPOSITION FROM COMBUSTION DATA CxHyOz + excess O2 ⎯→ 23.26 mg 9.52 mg w .c 9.83 mg x CO2 + y/2 H2O 23.26 mg CO2 = 0.5285 mmoles CO2 millimoles CO2 = ᎏᎏ 44.01 mg/mmole mmoles CO2 = mmoles C in original sample w (0.5285 mmoles C)(12.01 mg/mmole C) = 6.35 mg C in original sample w 9.52 mg H2O = 0.528 mmoles H2O millimoles H2O = ᎏᎏ 18.02 mg/mmole ( ) 2 mmoles H (0.528 mmoles H2O) ᎏᎏ = 1.056 mmoles H in original sample 1 mmole H2O (1.056 mmoles H)(1.008 mg/mmole H) = 1.06 mg H in original sample 6.35 mg C % C = ᎏᎏ × 100 = 64.6% 9.83 mg sample 1.06 mg H % H = ᎏᎏ × 100 = 10.8% 9.83 mg sample % O = 100 − (64.6 + 10.8) = 24.6% 14782_01_Ch1_p001-014.pp3.qxd 1/25/08 10:11 AM Page 3 1.1 Elemental Analysis and Calculations 3 TA B L E 1 . 2 CALCULATION OF EMPIRICAL FORMULA Using a 100-g sample: 64.6% of C = 64.6 g 10.8% of H = 10.8 g 10.8 g moles H = ᎏᎏ = 10.7 moles H 1.008 g/mole giving the result C5.38H10.7O1.54 m 4a 24.6 g moles O = ᎏᎏ = 1.54 moles O 16.0 g/mole ll. 64.6 g moles C = ᎏᎏ = 5.38 moles C 12.01 g/mole vn 24.6 g 24.6% of O = ᎏᎏ 100.0 g Converting to the simplest ratio: 5.38 C⎯ ⎯ H 10.7 ⎯— O1.54 ⎯— = C3.49H6.95O1.00 1.54 1.54 1.54 he which approximates C3.50H7.00O1.00 or w .c C7H14O2 w w sample in advance (from its postulated structure) and will use the analysis to verify your hypothesis. When you perform these calculations, be sure to use the full molecular weights as given in the periodic chart and do not round off until you have completed the calculation. The final result should be good to two decimal places: four significant figures if the percentage is between 10 and 100; three figures if it is between 0 and 10. If the analytical results do not agree with the calculation, the sample may be impure, or you may have to calculate a new empirical formula to discover the identity of the unexpected structure. To be accepted for publication, most journals require the percentages found to be less than 0.4% off from the calculated value. Most microanalytical laboratories can easily obtain accuracy well below this limit provided the sample is pure. In Figure 1.1, a typical situation for the use of an analysis in research is shown. Professor Amyl Carbon, or one of his students, prepared a compound believed to be the epoxynitrile with the structure shown at the bottom of the first form. A sample of this liquid compound (25 μ L) was placed in a small vial correctly labeled with the name of the submitter and an identifying code (usually one that corresponds to an entry in the research notebook). Only a small amount of the sample is required, usually a few milligrams of a solid or a few microliters of a liquid. A Request for Analysis form must be filled out and submitted along with the sample. The sample form on the left side of the figure shows the type of information that must be submitted. In this case, the professor calculated the expected results for C, H, and N and the expected formula and molecular weight. Note that the compound also contains oxygen, but that there was no request for an oxygen analysis. Two other samples were also submitted at the same time. After a short time, typically within a week, the