EXPT. 9 DETERMINATION OF THE STRUCTURE OF AN ORGANIC COMPOUND USING UV, IR, NMR AND MASS SPECTRA

Transcription

1 EXPT. 9 DETERMINATION OF THE STRUCTURE OF AN ORGANIC COMPOUND USING UV, IR, NMR AND MASS SPECTRA Structure 9.1 Introduction Objectives 9.2 Principle 9.3 Requirements 9.4 Strategy for the Structure Elucidation of the Organic Compounds by Combined Use of UV, IR, NMR and Mass Spectral Data. Suggested Guidelines for Arriving at the Structure of Organic Compounds 9.5 Practice problems 9.6 Problems for the session 9.7 Solution to practice problems 9.1 INTRODUCTION In the previous two experiments you have learnt about the applications of IR and NMR spectroscopy respectively in the determination of functional groups in an organic molecule. In this experiment you would learn about the determination of the structure of simple organic molecules using UV, IR, NMR and Mass spectral data. You have learnt about these spectroscopic methods and the structural information from them in the MCH-003 course. In addition, you have embarked on the strategies for the interpretation of the IR and NMR spectra to obtain the structural information in the previous two experiments. You would recall from these that no spectroscopic method can give all the information about the analyte being studied; however, these can provide significant inroads into the structural details. In this experiment we would first recall and reproduce the essential information from the relevant units of the MCH-003 course and the Experiments 7 and 8 of this course for the structural elucidation of organic molecules. This will be followed by a suggested strategy and its application in solving the structural problems using the data from varied spectra. You would be required to determine the structure of a few organic compounds on the basis of the provided sets of spectra. Once equipped with the necessary interpretative skills, you can sharpen your skills by interpreting as many spectra as possible to attain a kind of mastery. Objectives After studying the contents of this experiment and solving the sample problems, you should be able to: outline the salient features of different spectrometric methods, identify the presence of different functional groups and other structural features in an organic compound on the basis of the IR, Mass and NMR spectra, formulate different tentative structures of the organic compound on the basis of the above, and establish the unequivocal structure of the organic compound whose spectra have been provided. 67

2 Spectroscopic Methods Lab 9.2 PRINCIPLE In order to determine the structure of organic molecules on the basis of their spectra we need to use all the structural information available in the spectra. The obvious first step in this direction would be to dig out the same from the different spectra. You have learnt about the structural information available in different types of spectra and the strategy to obtain that from the spectra in Unit 14 of MCH-003 course and the Experiments 7 and 8 of this course. Let us collate all this here, for a quick reference for determining the structures of organic compounds from their spectra. UV spectrum An absorption in the UV-VIS region of the spectrum, i.e., the UV-VIS spectrum of a molecule indicates the presence of certain functional groups that have characteristic n π*, π π * transitions. The λ max and the intensity of the absorption bands are indicative of the extent of conjugation in the molecule; larger the wavelength, greater the conjugation. The spectrum itself does not provide much details of the structure of the molecule. However, these transitions observed in this are so characteristic that the absence of a UV-VIS spectrum for a molecule eliminates the presence of a number of functional groups in the molecule. For example, an intense signal around 210 nm is indicative of an α, β - unsaturated ketone, a diene or a polyene. Similarly two bands of medium intensity with absorption maximum above 200 nm are suggestive of aromatic ring. IR spectrum You have learnt about the theory of IR spectroscopy in the Unit 3 of MCH-003 course and the application of the same in the determination of functional groups of organic compounds in the experiment number 7 of this course. Let us recall the strategy of using the IR spectral data for structural elucidation of organic compounds. Important sp 3 C-H stretching : 2850 to 3000 cm -1 sp 2 C-H stretching : above 3000 cm -1 sp C-H stretching : ~ 3300 cm -1. A. Determine the nature of carbon skeleton (aliphatic / aromatic) i) C H stretching : The =C H stretch in aromatics is observed at cm -1 whereas the C H stretching frequencies for saturated aliphatic hydrocarbons is below 3000 cm -1. ii) C C ring stretching vibrations: The aromatic hydrocarbons show C-C ring stretching vibrations in the regions cm -1 and cm -1 iii) Out of plane C H bending vibrations: These are observed in the region cm -1 and provide information about the substitution pattern of aromatic compounds. Thus, a weak absorption in the region cm -1 accompanied by medium absorption in the ring vibrations region indicates the presence of an aromatic ring. A signal around 1605 cm -1 is quite a good indicator of an aromatic molecule; occasionally it splits into a doublet. The out of plane bending vibrations are also very significant. A lack of strong absorption band in the cm -1 region generally indicates a non aromatic structure. B. Look for the characteristic frequencies of different functional groups. a) Molecules containing only C and H i.e., hydrocarbons Look for C H stretching whether it is in the region cm -1 or above 3000 cm -1? The absorption above 3000 cm -1 indicates a double or a triple bond. The following are other important signals to look for in a 68

3 hydrocarbon to ascertain its nature. i) The C=C bond usually gives rise to a moderate band in the region cm -1. ii) The C C stretch appears as a weak band from cm -1. iii) The bending vibrations of the =C H group are observed in the cm -1 region. iv) The terminal C C H stretch is observed as a strong, narrow band in the range cm -1. v) The C C H bending vibration is observed in cm -1 range. vi) If a band observed at 1380 cm -1 happens to be a doublet, it may be due to the presence of more than one methyl group on the same carbon atom. b) Molecules containing C, H and O or N Look for a strong absorption in the region, cm -1 for a C=O group. If carbonyl group is present, then we have a number of possibilities; look for the following i) Two weak absorptions near 2850 and 2750 cm 1 on the lower wave number side of the CH absorptions. These are due to O=C H stretching vibrations. The band near 2830 cm -1 usually overlaps with other C H stretching vibration bands however, the presence of a moderate band near 2720 cm -1 is very likely to be helpful in determining whether or not a compound is an aldehyde. It often appears as a shoulder-type peak. ii) Broad band in the region cm -1, centred at about 3000 cm -1. This arises due to the stretching vibration of O H group of carboxylic acids. The broad nature of the band is due to the fact that carboxylic acids usually exist as hydrogen-bonded dimers. iii) Two or more strong absorption bands in the region cm -1 These are due to the C O stretching vibrations in esters. iv) If the above three are absent then the molecule could be a ketone Similarly, if C=O absorption is absent, look for the following i) The N H stretching vibrations of amines in the region cm -1 These are observed to be weaker and sharper than those observed for the O H stretching vibrations of alcohols which appearing in the same region. The presence of two bands is suggestive of a primary amine whereas a single band is indicative of a secondary amine. ii) The corresponding C N stretching vibrations of aliphatic amines are observed as medium or weak bands in the region cm -1. The same for aromatic amines are usually observed as strong band in the cm

4 Spectroscopic Methods Lab iii) Nitrile ( C N) shows a medium, sharp absorption band in the range cm -1. The isomeric isocyanate s strong bands are observed in the range cm -1. c) Molecules containing C, H, O and N For the molecules having this elemental composition, two most common functional groups are amide and nitro. Their absorptions can be looked for in the following regions. i) Amide: The amides show a characteristic absorption band for the carbonyl group in the region, cm -1. It is also referred to as the Amide I band. In addition, the N H stretching vibrations are observed in the cm -1. The primary amides show two N H stretching bands whereas the secondary amides give rise to only one such band. ii) Nitro: In nitroalkanes the N O stretching vibrations occur in the range of cm -1 ; the band at higher value being the stronger of the two. On the other hand for the nitro group attached to an aromatic ring, the N O stretching bands are observed in the ranges of cm -1 and cm -1. NMR Spectrum Chemical Structure shift (ppm) RCH R 2 CH R 3 CH ~1.5 ArCH R 2 NCH R 2 CHOR R 2 CHCl RC(=O)CHR RCHCR=CR 2 ~1.7 RC=CH ArH RC(=O)H RCCH R 2 NH 2-4 ROH 1-6 ArOH 6-8 RCO 2 H You would recall from Unit 12 of MCH-003 course and Experiment 8 that the following features of NMR spectra and the structural information available from them play important role in the structure elucidation of an organic molecule. i) The number of different signals in the 1 H-NMR spectrum indicates about the different types of protons present in the molecule. ii) The position of the signals i.e. their chemical shift values, tells about the electronic environment of a particular proton. The chemical shifts of different types of protons are given in Fig Fig. 9.1: Range of chemical shift values for different types of protons iii) iv) The area under the peaks obtained from the integrals for the signals of various types of protons provides information about the ratio of the numbers of different types of protons present in a molecule. The spin-spin splitting pattern of a particular signal gives information about the number of neighbouring protons present around the given type of protons. The splitting pattern in accordance with the n+1 rule helps in identifying important groupings. 70

5 Some characteristic features to look for in the NMR spectrum A combination of two proton quartet and a three proton triplet are suggestive of an ethyl group; similarly, a six proton doublet and a one proton septet (or multiplet) is characteristic of an isopropyl group. A signal in the range of 9-10 ppm is characteristic of an aldehyde group as no other type of hydrogen appears in this region; similarly, a signal in the range of ppm is characteristic of a carboxylic acid; the hydrogen is highly deshielded by the oxygen and is acidic in nature. The broadened singles in the spectrum indicate towards the presence of OH or NH protons. The absorptions in the range of 7 8 ppm suggest the presence of an aromatic ring; benzene absorbs at 7.27 ppm. The aromatic absorptions are farther downfield than δ = 7. 27, indicate the presence of electron-withdrawing substituents. The absorptions in the region of δ = 2. 1 to δ = 2. 5 are indicative of the protons adjacent to a carbonyl group or an aromatic ring. Mass spectrum + i) The m/z value of the molecular ion, M gives the molecular mass and can also be used for generating the molecular formula (subsec ). + ii) The relative intensities of [M + 1] and [M + 2] peaks can be related to the number and nature of hetero atoms present in a molecule. You would recall from Unit 13 that a typical pattern of M + 1 and M + 2 peaks is observed if a chlorine or bromine atom is present in the molecule. iii) The odd molecular mass is indicative of the presence of a nitrogen atom in the molecule. However, this has to be further confirmed by other means or by analysing the fragmentation patterns for the typical nitrogen containing functional groups. iv) The characteristic peaks arising from typical fragmentation patterns of various classes of functional groups such as α cleavage, loss of small molecules such as H 2 O, C 2 H 4, etc. are quite useful. v) Certain peaks which may be attributed to the rearrangement of the molecular ion or its fragments ions also give significant structural leads. Table 13.2 of Unit 13 of MCH-003 course containing commonly lost fragments and stable fragment ions observed in the mass spectrum is being reproduced here so as to facilitate you in the interpretation of the mass spectra of the examples being taken up in the next section. Commonly lost fragments Fragment lost Peak obtained Fragment lost Peak obtained. +.. CH M 15 OCH + 3 M OH M CN M H 2 C CH 2 +. M 28. Cl. CH 3 C + O + M M OCH 2 CH + 3 M

6 Spectroscopic Methods Lab. CH 2 CH 3 M M CH 2 m/z values m/z = 43 Common stable ions Ion + CH 3 C O m/z = CH m/z = M - 1 R O. + CH R C O Having learnt about the information available from different types of spectral data, we can now take up how to use it to arrive at the structure of a given compound. 9.3 REQUIREMENTS The main objective of this experiment is to highlight the structure-spectrum relationships of organic molecules and interpret the spectra of some simple organic molecules. We intend to inculcate elementary interpretative skills in you so that you can take up the interpretation of the different spectra of some simple molecules to determine their structure. Accordingly, we need the spectra of some simple molecules to be used as examples and some others to be used as study problems. 9.4 STRATEGY FOR THE STRUCTURE ELUCIDATION OF THE ORGANIC COMPOUNDS BY COMBINED USE OF UV, IR, NMR AND MASS SPECTRAL DATA You would have realised from your study of the MCH-003 course on Spectroscopic Methods of Analysis and the previous two experiments of this course that there is no unique methodology to decipher the structural aspects of the molecule on the basis of their spectra. You have learnt about interpreting the IR and NMR spectra in the previous two experiments while in Unit 14 of the MCH-003 course you learnt about using Mass spectrum. You are advised to have a relook at the above referred content though some part of that will be reproduced in the next section. Based on the experience in solving structural problems, we suggest a comprehensive scheme / strategy of arriving at the structure of the molecule from its various spectra. Let us reiterate that this is not a unique approach, you may follow any other strategy available elsewhere with an objective of being able to decipher the structure of the organic molecule from its spectral data Suggested Guidelines for Arriving at the Structure of Organic Compounds When you first look at a spectrum, consider the major features before getting down to the minor details. The following are a few of the important characteristics you might look for: 1. If the molecular formula is known, compute the index of hydrogen deficiency (IHD). This would help ascertain the number of elements of unsaturation in the compound. This in turn suggest about the presence of rings, double bonds, or triple bonds. 72

7 2. Examine each spectrum (IR, mass spectrum, 1 H NMR) in turn for obvious structural elements: 1. Examine the IR spectrum for the presence or absence of groups with diagnostic absorption bands e.g. carbonyl groups, hydroxyl groups, NH groups, C C or C N, etc. using the suggestions given in Experiment 7 and reproduced in section Explore the mass spectrum for typical fragments e.g. PhCH 2, CH 3 CO CH 3, etc. look for the characteristic fragmentation pattern as detailed in Unit 13 of the MCH-003 course; these are reproduced in section 9.3 for your reference. Do remember that the mass spectra can provide clues about the presence of halogen atoms also. 3. The number of signals in the 1 H-NMR spectrum indicates about the different types while their position indicates about the electronic environment of different protons in the molecule. Analyse the NMR spectrum for the characteristic spectral clues for the characteristic groupings like C 2 H 5, CH (CH 3 ) 2, aromatic rings, acid/ aldehyde groups etc. as detailed in Experiment 8 and reproduced in section Write down all structural elements you have identified. You may arrange them as monofunctional (i.e. CH 3, C 2 H 5, C N, PhCH 2, CH 3 CO, NO 2, etc.) bifunctional (e.g. CO, CH 2, C C, COO, etc.), or trifunctional (e.g. >CH, >N etc.). Add up the atoms of all the identified structural elements and compare it with the molecular formula to determine the unaccounted component of the molecular formula. The nature of the undetermined structural elements may be quite apparent. 4. Try to assemble the structural elements to workout a tentative structure of the molecule. You may have more than one tentative structures assembled from the fragments. 5. Revisit the spectra again to see that whether they (especially the NMR and Mass spectra) are accounted for by which of the proposed tentative structure. You may need to relook into some of your earlier assignments at this stage. These guidelines are suggestive only and seem to facilitate making educated guesses about the major structural features of a compound from its different spectra. The ingenuity lies in connecting the pieces of structural information to arrive at the structure of the molecule. The only way to acquire expertise in obtaining the structures of organic compounds from spectra is to practice, practice, more practice and even more practice. 9.5 PRACTICE PROBLEMS Having recalled what you had learnt earlier about the information available from different types of spectra and having learnt the proposed strategy for interpreting the spectra to decipher the structure of simple organic compounds you have the necessary tools in your toolkit to unravel the structure- spectral interrelationships. You are being provided with the Mass IR and NMR spectra of some known organic compounds so that you can test the proposed strategy or may be devise one of your own for determining the structure of an organic compound from its spectra. The answers to these are provided in section 9.7. You are required to solve these practice problems and then verify your answers. Refrain from looking into the answers before solving the problem. Practice problem 1: The following are the Mass, IR and NMR spectra for a simple organic compound having a molecular formula, C 4 H 8 O. Determine its structure. 73

8 Spectroscopic Methods Lab Practice problem 2: The following are the Mass, IR and NMR spectra for a simple compound having a molecular formula C 5 H 10 O 2. Determine its structure. 74

9 Practice problem 3: The following are the Mass, IR and NMR spectra for a simple compound having a molecular formula C 3 H 8 O. Determine its structure. 75

10 Spectroscopic Methods Lab Practice problem 4: The following are the Mass, IR and NMR spectra for a simple compound having a molecular formula C 9 H 10 O. Determine its structure. 76

11 Practice problem 5: The following are the Mass, IR and NMR spectra for a simple compound having a molecular formula C 3 H 5 N.. Determine its structure. 77

12 Spectroscopic Methods Lab 9.6 PROBLEMS FOR THE SESSION We are sure that on the basis of the suggested strategy and the expertise you had acquired in interpreting the IR and NMR spectra in Experiments 7 and 8 you have been able to solve the problems raised in the previous section. If not (for any problem) then you would have convinced yourself that the given spectra are consistent with the structure of the organic compound given in section 9.7. We hope that you are now equipped to interpret the spectra of simple organic molecules to determine their structure. You are provided with four sets of spectra for interpretation. You may take the spectra in any sequence and try to interpret on the basis of the knowledge gained. (Your counselor may provide you additional sets of spectra of simple organic molecules to assess your understanding.) You must pin up the spectra in the record book and submit your observations and results to your counsellor for evaluation. 78

13 Session problem 1 Session problem 2 79

14 Spectroscopic Methods Lab Session problem 3 80

15 Session problem 4 81

16 Spectroscopic Methods Lab Session problem 5 82

17 9.7 SOLUTION TO PRACTICE PROBLEMS Practice problem 1: Butanone-2 Practice problem 2: Ethylpropionate Practice problem 3: 1-propanol Practice problem 4: Propiophenone Practice problem 5: Propane nitrile 83