Organic Spectroscopy Workbook

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3 Organic Spectroscopy Workbook

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5 Organic Spectroscopy Workbook TOM FORREST JEAN-PIERRE RABINE MICHEL ROUILLARD A John Wiley & Sons, Ltd., Publication

6 This edition first published John Wiley and Sons Ltd. Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall be liable for any damages arising herefrom. Print ISBN: epdf ISBN: Typeset in 9.5/11.5pt Times Roman by Laserwords Private Limited, Chennai, India

7 Contents Preface Acknowledgements vii ix Tables of δ for CH 2 Doubly Substituted in α Position 227 Tables of δ for Aromatics 229 Coupling Constants 230 How to Use this Workbook 1 Preliminary Observations 2 Centres of Unsaturation 8 Exercises Infrared Data 210 IR Frequencies: Display of Infrared Database by Frequency 210 IR Groups: Display of Infrared Database by Functional Group H NMR Data 221 General Tables of Functional Groups 221 Comparison of δ of Alkyl Groups Bearing a Substituent in α or β Position 225 Alkyl Groups with an α Substituent 226 Alkyl Groups with a β Substituent C NMR Data 233 General Ranges for CH 3, CH 2, >CH Chemical Shift Ranges for Functional Groups 234 Chemical Shift Estimations 234 Mass Spectrometry Data 240 Main Fragments Observed 240 Atomic Masses 242 Isotopic Abundances in % 243 Internet Resources 254 Glossary of Terms Used in the Exercises 255 Answers 257 Index 261

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9 Preface This workbook arose out of a long standing project for the development of computer based programs to assist students in learning how to deduce the structures of organic compounds from spectroscopic data. The ability to elucidate chemical structures is built largely on implicit heuristics and informal experience accumulated by solving many examples. The accumulation of this experience is greatly facilitated by these computer based programs, However, the knowledge that is so arduously obtained at the computer may evaporate rapidly unless some reinforcement mechanism is in place. This workbook was originally conceived as a way to provide the computer users with a vehicle for making a lasting record of their learning experience. However, we recognized that a similar situation occurs for students who prefer to undertake their study without using computer-based exercises. They could also benefit from a workbook where they could get assistance in solving spectral problems and have a record that could be used for review and reinforcement. The resulting workbook was designed so that it can be used with or without the computer based program. The computer exercises, which are freely available online to any user, provide more detailed explanations and supplementary graphical material for the structural determinations. For each problem in the workbook we provide four spectra together with a detailed analysis of the spectra and an explanation of the derivation of the structure of the compound. The level of detail provided in the solution decreases gradually, until near the end of the set of exercises, very little or no analysis is given. The solutions to all of the exercises are provided, but no structural formulae are shown on the work area of the exercise, so that the student may follow along the analysis and draw the structural components as they are revealed. There are many advanced techniques available in modern spectrometers that may be employed to assist in the solution of more difficult structural problems. In these exercises we have chosen to give low resolution electron impact mass spectra because these spectra are readily available, and are rich in structural information. Modern mass spectrometry employs a multitude of other ionization techniques that can be used in particular circumstances to provide specific information, but these are beyond the scope of an introductory spectral exercise book. In the field of NMR spectroscopy there are numerous sophisticated pulse sequences available that can generate various multidimensional spectra which are used for more difficult structural problems. These techniques are particularly useful for large complex structures, but they are aids that still require the user to have an understanding of the fundamentals of spectroscopic interpretation, which is the goal of these exercises. We hope that students will find that using this workbook will prove to be a pleasant and rewarding experience.

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12 Acknowledgements We have used the resources of our university libraries extensively in preparing this workbook, but have found that we made even more extensive use of the resources available on the internet. We are indebted to the many people who have taken the time to place their data at the disposal of the scientific community in this manner. This easy access to spectral data has been of immense assistance to us in the preparation of this workbook. The availability of such a large amount of primary data allowed us to find very good model compounds which gave more specific support for the assignments than possible from the general correlation tables of secondary publications. It is impossible to list all of the internet sites that we visited during the preparation of the manuscript; however, we would like to specifically acknowledge the providers of those sites that we have found we were consistently using. These are for the most part sites provided by institutions, universities and chemical suppliers: Sigma-Aldrich Corporation, NMR and IR spectra; Acros Organics, IR spectra; University of Wisconsin, Chemistry Department, H. J. Reich, compilation of NMR chemical shifts and coupling constants. We are grateful for the support of: Université de Nice Sophia-Antipolis; Université Numérique Thématique:UNISCIEL (Université des Sciences en Ligne) Dalhousie University NIST, National Institute of Standards and Technology, Chemistry Webbook; National Institute of Advanced Industrial Science and Technology (AIST, Japan), Spectral Database for Organic Compounds;

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14 How to Use this Workbook The exercises give spectral charts and verbal analyses of the spectra, but the structural diagrams that would complete the picture are missing. Although annotated structures would be very helpful in the spectral assignments, they are deliberately omitted to avoid a premature revelation of the answer. You don t want to see the solution before you have had the opportunity to think about the problem. You may want to use a small sheet of paper to cover the conclusion/summary section as you try to solve each structure. There is adequate white space available on the pages to allow you to draw the appropriate structures, and to make notes on the spectral assignments. Complete structural drawings are available in the Answers section of the workbook. Each exercise has a second number in brackets which identifies the number by which the compound can be located in the Answers section, and in the computer-based exercises. The structures are not presented in the same order as the exercises, thus lessening the possibility of an unwanted exposure of the answer to the next exercise. There is no single method that provides the best way to solve these problems. There is usually an abundance of information that allows many paths to the solution. However, it is unlikely that the complete structure will be revealed intact, so the general approach is to derive the partial structures of the molecule and to connect them to form the complete structure. The order of the spectral analysis provided in each exercise is not presented as the best way to approach the problem. Many people do a preliminary scan of the spectra to look for significant features, identify the responsible structural component, then verify its presence using other spectra. For example, a carbonyl group that stands out in the IR spectrum can be verified by the peak at the appropriate chemical shift in the 13 C NMR spectrum. As one moves from spectrum to spectrum like this, it is imperative that an organized approach be maintained and that good notes are preserved. It is recommended that you write down the partial structures as you derive them. You can always erase them if you make a mistake or, better still, simply draw a mark through them, and make notes to ensure that you have them for review and can learn from your mistakes. Take the opportunity to make detailed notes before you leave each exercise. As clear as it will be in your mind at that moment, it is amazing how the details evaporate with time. All the 13 C NMR DEPT spectra are DEPT-135 spectra in which methylene carbons show a negative signal and quaternary carbons show no signal. Methyl and methine carbons show up as normal positive signals. All the mass spectral charts are low-resolution electron impact mass spectra (EIMS). These spectra frequently provide the molecular weight of the compound, and the high degree of fragmentation also offers valuable information to those with knowledge of reactivity of organic compounds. In some examples, the results of high-resolution mass spectra (HRMS) are provided which can be used to obtain the molecular formula directly. The exercises are organized somewhat in the order of complexity of the molecule and level of explanation provided. You will find very simple spectra and detailed explanations at the beginning, and eventually more complex exercises with less in the way of explanation. For the last 20 exercises, only general hints and suggestions are provided. In these exercises the 1 H NMR spectral data is provided in standard text format to show the coupling constants where appropriate. These data should be compared to the spectral pattern, so that you could write such a verbal description of the spectrum. The resource material you require to analyse the spectra is provided in tables and algorithms in separate data sections for each of the spectroscopy types. The exercises presented in this work book are also available in an on-line program, Multispectroscopy, which you may access at spectros.unice.fr. This program provides more detailed explanations, as well as elaborate diagrams and graphics, to help to explain the spectral analysis and structure determination. You may wish to take advantage of this additional resource, while you are using this workbook. Organic Spectroscopy Workbook, First Edition. Tom Forrest, Jean-Pierre Rabine, and Michel Rouillard John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

15 Preliminary Observations It is easy to get some information from a quick glance at some spectra. There are some typical shapes that give a good indication of particular structural features. It is essential that these deductions are verified by detailed inspection of the spectrum, and with data from other spectra. Infrared This indicates an alkene. This indicates an alcohol. This indicates an acid. This indicates a terminal alkyne. Organic Spectroscopy Workbook, First Edition. Tom Forrest, Jean-Pierre Rabine, and Michel Rouillard John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.