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Analytical Chemistry of Polycyclic Aromatic Compounds PDF

467 Pages·1981·6.29 MB·English
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Analytical Chemistry of P o l y c y c l ic A r o m a t ic C o m p o u n ds M I L T ON L. L EE Department of Chemistry Brigham Young University Provo, Utah M I L OS V. N O V O T NY Department of Chemistry Indiana University Bloomington, Indiana K E I TH D. B A R T LE Department of Physical Chemistry University of Leeds Leeds, United Kingdom 1981 ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich, Publishers New York London Toronto Sydney San Francisco Copyright © 1981, by Academic Press, Inc. all rights reserved. no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. ACADEMIC PRESS, INC. Ill Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NW1 7DX Library of Congress Cataloging in Publication Data Lee, Milton L. Analytical chemistry of polycyclic aromatic compounds. Includes bibliographical references and index. 1. Polycyclic compounds—Analysis. 2. Aromatic compounds—Analysis. I. Novotny, Milos, Date. II. Bartle, Keith D. III. Title. Q0335.L48 547».5 80-68559 ISBN 0-12-440840-0 PRINTED IN THE UNITED STATES OF AMERICA 81 82 83 84 9 8 7 6 5 4 3 2 1 Preface ΤΓ HE RAPID GROWTH in industry over the last century and particularly within the last several decades has required more critical evaluations of environmental health hazards associated with combustion products and effluents from these industries. The recent increase in the utiliza­ tion of coal and other synthetic fuels to meet changing world energy demands places even greater emphasis on these evaluations. Fortu­ nately, developments in analytical methodology and instrumentation have largely paralleled industrial growth, and many of the chemical factors relating to the environment have been identified. Among these are the polycyclic aromatic compounds, the largest class of chemical carcinogens known today. The carcinogenic and mutagenic properties of numerous polycyclic compounds have been documented, and many others are presently under investigation. The modern age of chemical instrumentation has already had a significant impact in areas related to this class of com­ pounds: environmental toxicology, experimental carcinogenesis, en­ vironmental chemistry, chemical and fuels engineering, etc. The known health hazards associated with the increasing emission of polycyclic aromatic compounds into our environment, along with the develop­ ing societal environmental awareness dictate the need for both further structural identification and more accurate and precise quantitative measurement of these substances. Although many excellent books have been written on various aspects of carcinogenesis, and although both biological and chemical proper­ ties of polycyclic aromatic compounds have been discussed at many international symposia, it is our belief that such important subjects ix χ Preface as the chemical separation, the structural identification, and the quan­ titative measurement of these compounds had not been drawn together and treated in a comprehensive manner. This book is mainly devoted to the discussion and critical evaluation of various chromatographic (Chapters 5-7) and spectroscopic (Chap­ ters 8-10) methods. It is shown how gas chromatography and high- performance liquid chromatography can be both competitive and complementary analytical methods. Ancillary techniques of both are emphasized for the structural elucidation of individual polycyclic aromatic compounds in complex mixtures. The merits of spectroscopic methods in both structural work and quantitation are compared. New directions for these analytical techniques are also discussed. Chapters 1-3 are descriptive in nature and provide the reader with background information concerning the chemistry, occurrence, and toxicology of polycyclic aromatic compounds. These chapters were included to give the reader basic information pertinent to the under­ standing and appreciation of the analytical chemistry of these com­ pounds. The isolation of polycyclic aromatic compounds from a wide variety of materials and matrices necessitates a number of different ap­ proaches; and therefore sample collection, extraction, separation, and purification are discussed in Chapter 4. Numerous methods and examples which cover most applications are discussed. An important point stressed in this book is that no one analytical technique is sufficient to solve all analytical problems associated with this class of compounds; and that therefore, various multi-technique approaches are required. This is demonstrated in Chapter 11 with two practical examples representative of the widely differing problems that may be encountered. This book does not pretend to have the universal analytical solution to all studies involving polycyclic aromatic com­ pounds. Instead, the different analytical techniques and approaches are treated in some detail in separate sections and their advantages and disadvantages explained. It is hoped that sufficient information is pro­ vided concerning the chemical and physical properties of the polycyclic aromatic compounds as well as the principles behind the various analytical techniques so that the reader can approach any particular analytical problem with sufficient understanding. The correct nomen­ clature of the polycyclic aromatic compounds is contained in the appendices, and should provide a valuable resource for those new to this class of compounds. Preface xi This book is recommended to all scientists involved with the study of polycyclic aromatic compounds, to analysts who need to acquire routine data, as well as to individuals charged with formulating en­ vironmental policies and drafting regulations. In addition, this book may also be appreciated by engineers concerned with emission-control and energy-related industries. The timely completion of this book was aided by fellow scientists who gave permission to reproduce their data and who often provided as yet unpublished manuscripts and valuable criticisms. We would also like to thank Mary Fencl, Paul Peaden, Dan Vassilaros, Cherylyn Willey, and Bob Wright, for valuable help in assembling and proof­ reading the text, compiling the information in the appendices, and preparing the figures for reproduction. The untiring assistance of Ms. Peggy Gore is gratefully acknowledged for typing and retyping the text, and for invaluable assistance in the preparation of the manu­ script. Finally, we are grateful to our families who gave the necessary encouragement and provided an atmosphere conducive for us to initiate and complete this book. Milton L. Lee Milos Novotny Keith D. Bartle 1 Physical and Chemical Properties I. NOMENCLATURE Polycyclic aromatic compounds (PAC) have been studied for well over a century, and during this time many compounds have been named unsys- tematically. Some names reflect the initial isolation of compounds from coal tar (naphthalene, pyrene, etc.); some reflect their color (fluoranthene and chrysene—the latter erroneously, because of contamination with naph- thacene which is orange); and some reflect the shape of their molecules (coronene, ovalene). Such names passed into general use, and it proved impractical to change them when systematic nomenclature was introduced. Thus, many important PAC systems are named nonsystematically. Recently, IUPAC (International Union of Pure and Applied Chemistry) attempted to systematize PAC nomenclature, prefixing to the name of a parent ring system the names of other component parts (7). Appendices 1 through 4 list the names and structures of many of the hydrocarbons and their heterocyclic analogs discussed in the remainder of the book. An exhaustive list of PAC is contained in "The Ring Index" (2). All the ring systems used in organic chemistry are classified according to the number and identity of the atoms in each ring and are named and numbered. The numbering and names of the compounds in Appendices 1-4 are based on the following rules: 1. Rings are drawn with two sides vertical wherever possible. 2. Irrespective of their size, as many rings as possible are drawn in a horizontal line. 3. As much as possible of the rest of the structure is arranged in the top right quadrant and as little as possible in the bottom left quadrant (the middle of the first row is taken as the center of the circle). ι 2 1. Physical and Chemical Properties 4. Starting with the first carbon atom not engaged in ring fusion in the right-hand ring of the top row, numbering proceeds clockwise around the molecule (anthracene and phenanthrene are exceptions). 5. Atoms engaged in ring fusion are given the letters a, b, c, etc., after the number of the preceding atom, e.g., triphenylene (1): 2 (I) 6. Certain trivial names are retained. Otherwise the name of a fused-ring system is made up of a prefix of the fixed part (benzo, cyclopenta, or a group of rings such as indeno—see abbreviations listed in Appendix 1) followed by an italic letter or letters denoting the bond or bonds of the base (which has as many rings as possible) at which fusion occurs, a refers to the 1,2-bond, and all bonds are then lettered sequentially whether or not they carry hy­ drogen atoms; the name of the parent compound follows. Examples are benz[#]anthracene (2) and 4i/-cyclopenta[ife/]phenanthrene (3). (2) (3) If more than one ring is fused, the italic letters are separated by a comma, e.g., dibenz[<z,c]anthracene (4). (4) The fusion position on the first ring system is shown, if necessary, by the appropriate numeral which precedes the italic letters, e.g., indeno[l,2,3-cd]- pyrene (5). II. Physical Properties 3 (5) 7. A component CH of a ring is indicated by the italic Η preceded by 2 the appropriate numeral, except where its position is assumed, as for exam­ ple in the compounds trivially named indene and fluorene (Appendix 1), e.g., l/f-benz[tffe]anthracene (6). Where there is a choice, the carbon atom carrying an indicated hydrogen atom is numbered as low as possible. (6) 8. Hydrogenation is denoted by prefixes such as dihydro, etc., followed by the name of the corresponding unreduced hydrocarbon, e.g., 1,4-dihy- dronaphthalene (7): (7) An even greater variety of structure is presented by the heterocyclic analogs and derivatives of polycyclic aromatic hydrocarbons (PAH), even if only compounds containing a single heteroatom are considered (Appen­ dices 2, 3, and 4). Similar rules of nomenclature apply as outlined above, since, in general, the name is composed of a hydrocarbon ring system followed by a trivial heterocycle name,. Numbering also follows the rules for hydro­ carbons; where there is a choice of orientation, low numbers are assigned to heteroatoms. In an alternative IUPAC-approved scheme, heterocyclic systems may be named by prefixing aza (N), oxa (O), or thio (S) to the name of the corresponding hydrocarbon: thus benz[#]isoquinoline is named 2-azaanthracene. II. PHYSICAL PROPERTIES The physical and spectroscopic properties of PAC are dominated by the conjugated π-electron systems which also account for their chemical stability. 4 1. Physical and Chemical Properties All PAC, with the exception of a few hydrogenated derivatives, are solids at ambient temperatures and are the least volatile of the hydrocarbons. PAC occur in air, mainly adsorbed on particles (Chapter 2). The boiling points of PAC are markedly higher than those of the w-alkanes of the same carbon number. Nonetheless, losses from environmental PAC samples are probable without stringent precautions (3). Particulates from urban air stored in an open container, but in the dark, showed the following losses (4): pyrene 88%, benzo[tf]pyrene 32%, benzo[#A/]perylene 10%, but coronene only 1%. The availability of high-energy π-bonding orbitals and of relatively low- energy 7i*-antibonding orbitals in PAC leads to the absorption of visible or ultraviolet (UV) radiation by the transition of an electron from the the π- to 7r*-orbital which gives characteristic absorption and fluorescence spectra. The processes occurring when UV or visible light is absorbed by PAC are illustrated in Fig. 1-1. Excitation Ε takes the molecule from the ground-state singlet S to the first excited state S. In condensed systems, 0 i any excess vibrational energy is lost within a picosecond by transfer to neigh­ boring ground-state molecules. From the lowest vibrational level of S the 1 molecule may either: (a) return to the ground-state S by radiationless 0 internal conversion (IC); (b) return to S by fluorescent emission (F); or 0 (c) be converted to the excited vibrational levels of the lowest triplet state T, and then by very rapid (less than 1 ps) internal conversion (intersystem x Fig. 1-1. Electronic energy levels and transitions for PAC. II. Physical Properties 5 crossing—ISC) go to the lowest vibrational level of T. The lifetime of S 1 x is short (10-100 ns for PAH), but process (a) above is generally unimportant: for anthracene in ethanol at 20°C, about 30% of the excited singlet disappears by the S -+S fluorescence process and 70% by the S ->Τχ process. x 0 i Once in the triplet state, the radiative process ^-^So, called phos­ phorescence (P), is slow because the transition involves a change in spin multiplicity and is "forbidden." Weak phosphorescence is often observed, however, and the variation with time of the intensity of the phosphorescence emission may be used to measure the lifetime of the triplet state. The energy differences between S and S, and therefore the wavelengths 0 x of exciting radiation, depend on the separations between the various molecu­ lar orbitals. Certain features of their UV/visible spectra thus are common to all PAC and can be interpreted as follows. The strong absorption bands designated (5, 6) a, p, and /?, which occur in that order at decreasing wave­ lengths, have extinction coefficients (E ) of generally ~102, 104, and max 105 M"1 cm" respectively. The p-band is assigned (Fig. 1-2) to transitions from the highest occupied molecular orbital to the lowest unoccupied molec­ ular orbital, and the a- and /?-bands to transitions from the next highest occupied to lowest unoccupied molecular orbital, and from the highest occupied to the next higher unoccupied molecular orbital. The phenomenon named annellation by Clar may be used (5,6) to explain why the α-, ρ-, and jS-bands retain their characteristic features while shifting towards the red with ring number. Briefly, in fused-ring PAH, some rings give Unoccupied MOs bands Increasing energy e-band Occupied MOs Fig. 1-2. Electron transitions corresponding to α-, β-, and p-bands in UV/visible spectra of PAH.

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