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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

EXPT. 9 DETERMINATION OF THE STRUCTURE

OF AN ORGANIC COMPOUND USING UV,

IR, NMR, AND MASS SPECTRA

Structure

9.1 Introduction

Objectives9.2 Principle

9.3 Requirements9.4 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 compounds9.5 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 organicmolecule. In this experiment you would learn about the determination of the structure ofsimple 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 theinterpretation 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 providesignificant 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 experiment 7 and 8 of this course forthe 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 variedspectra. You would be required to determine the structure of a few organic compoundson 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 inan 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.

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SpectroscopicMethods

Lab.9.2 PRINCIPLE

In order to determine the structure of organic molecules on the basis of their spectra weneed 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 havelearnt 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 theexperiments 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 transitions. The 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 theabsence 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 isindicative of an,- 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 courseand 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.

A. Determine the nature of carbon skeleton (aliphatic / aromatic)

i) CH stretching : The =CH stretch in aromatics is observed at 3100-3000

cm-1

whereas the CH stretching frequencies for saturated aliphatic

hydrocarbons is below 3000 cm-1

ii) CC ring stretching vibrations: The aromatic hydrocarbons show C-C

ring stretching vibrations in the regions 1600-1585 cm-1

and 1500-1400cm

-1

iii) Out of plane CH bending vibrations: these are observed in the region900-675 cm-1 and provide information about the substitution pattern ofaromatic compounds.

Thus, a weak absorption in the region 3080-3030 cm-1

accompanied by medium

absorption in the ring vibrations region indicates the presence of an aromatic ring.A signal around 1605 cm

-1is 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 900-650 cm-1

regiongenerally 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 CH stretching whether it is in the region 30002850 cm-1 orabove 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

Important

sp3

C-H stretching :

2850 to 3000 cm-1

sp

2C-H stretching :

above 3000 cm

-1

sp C-H stretching :~ 3300 cm-1.

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

hydrocarbon to ascertain its nature.

i) The C=C bond usually gives rise to a moderate band in the region1680-1640 cm

-1.

ii) The CC stretch appears as a weak band from 2260-2100 cm-1

.

iii) The bending vibrations of the =CH group are observed in the 1000-

650 cm-1

region

iv) The terminal CC H stretch is observed as a strong, narrow bandin the range 3330-3270 cm-1.

v) The CC H bending vibration is observed in 700-600 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, 1820-1660 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 cm1

on the lower wave

number side of the CH absorptions.

These are due to O=CH stretching vibrations. The band near 2830

cm-1

usually overlaps with other CH stretching vibration bandshowever, the presence of a moderate band near 2720 cm

-1is very

likely to be helpful in determining whether or not a compound is analdehyde. It often appears as a shoulder-type peak.

ii) Broad band in the region 3300-2500 cm-1

, centred at about 3000 cm-1

.

This arises due to the stretching vibration of OH group of

carboxylic acids. The broad nature of the band is due to the fact thatcarboxylic acids usually exist as hydrogen-bonded dimers.

iii) Two or more strong absorption bands in the region 1300-1000 cm-1

These are due to the CO 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 NH stretching vibrations ofamines in the region 3300-3000

cm-1

These are observed to be weaker and sharper than those observed forthe OH 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 CN stretching vibrations of aliphatic amines areobserved as medium or weak bands in the region 1250-1020 cm

-1.

The same for aromatic amines are usually observed as strong band in

the 1335-1250 cm-1

.

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SpectroscopicMethods

Lab.iii) Nitrile ( CN) shows a medium, sharp absorption band in the range

2260-2220 cm-1. The isomeric isocyanates strong bands areobserved in the range 2275-2240 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 forin the following regions.

i) Amide: The amides show a characteristic absorption band for the

carbonyl group in the region, 1700-1640 cm-1

.It is also referred to asthe Amide I band.

In addition, the NH stretching vibrations are observed in the 3500-

3100 cm-1

. The primary amides show two NH stretching bandswhereas the secondary amides give rise to only one such band.

ii) Nitro: In nitroalkanes the NO stretching vibrations occur in the

range of 1550 - 1365 cm-1

; the band at higher value being the

stronger of the two. On the other hand for the nitro group attached toan aromatic ring, the NO stretching bands are observed in the ranges

of 1550-1475 cm-1

and 1360-1290 cm-1

.

NMR Spectrum

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 1H-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 theelectronic environment of a particular proton. The chemical shifts of different

types of protons are given in Fig. 9.1.

Fig. 9.1: Range of Chemical shift values for different types of protons

iii) 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.

iv) 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.

Structure

chemical

shift

(ppm)

RCH3 0.8 - 1.2

R2CH2 1.1 - 1.5

R3CH ~1.5

ArCH3 2.2 - 2.5

R2NCH3 2.2 - 2.6

R2CHOR 3.2 - 4.3

R2CHCl 3.5 - 3.7

RC(=O)CHR22.0 - 2.7

RCHCR=CR2 ~1.7

RC=CH 4.9 - 5.9

ArH 6.0 - 8.0

RC(=O)H 9.4 - 10.4

RCCH 2.3 - 2.9

R2NH 2 - 4

ROH 1 - 6

ArOH 6 - 8

RCO2H 10 - 12

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

Some characteristic features to look for in the NMR spectrum

A combination of two proton quartet and a three proton triplet are suggestive ofan ethyl group; similarly, a six proton doublet and a one proton septet (ormultiplet) 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

11-12 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 ppm87 suggest the presence of an aromatic ring;

benzene absorbs at 7.27ppm. The aromatic absorptions are farther downfieldthan 27.7= , indicate the presence of electron-withdrawing substituents.

The absorptions in the region of 1.2= to 5.2= 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. 13.4.1).

ii) The relative intensities of+

+ 1][M and

+

+ 2][M peaks can be related to the

number and nature of hetero atoms present in a molecule. You would recall fromUnit 13 that a typical pattern of M+1 and M+2 peaks is observed if a chlorine orbromine atom is present in the molecule.

iii) The odd molecular mass is indicative of the presence of a nitrogen atom in themolecule. However, this has to be further confirmed by other means or byanalysing the fragmentation patterns for the typical nitrogen containing functional

groups.

iv) The characteristic peaks arising from typical fragmentation patterns of variousclasses of functional groups such as cleavage, loss of small molecules such

as H2O, C2H4, 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 andstable 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

CH3

.

+.M 15

OCH3

.

M - 31+.

OH.

+.M 17

Cl.

M - 35+.

CN.

+.M 26

CH3C O

+.M - 43

H2C CH

2 +.M 28

OCH2CH

3

.

+.M - 45

CH2CH

3.

+.M 29 CH

2

.

+.M - 91

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SpectroscopicMethods

Lab.Common stable ions

m/z values Ion

m/z = 43CH

3 C O+

m/z = 91

CH2

.+

.+

+.m/z = M - 1

CH

O .+

R C OR

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 cantake 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 oftheir 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 contentthough some part of that will be reproduced in the next section.

Based on the experience in solving structural problems, we suggest a comprehensivescheme / 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.

9.4.1 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 thecompound. This in turn suggest about the presence of rings, double bonds, or

triple bonds.

2. Examine each spectrum (IR, mass spectrum,1H NMR) in turn for obvious

structural elements:

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

1. Examine the IR spectrum for the presence or absence of groups withdiagnostic absorption bands e.g. carbonyl groups, hydroxyl groups, NH

groups, CC or CN etc. using the suggestions given in experiment 7 and

reproduced in section 9.3.

2. Explore the mass spectrum for typical fragments e.g. PhCH2 , CH3CO

CH3 , etc. look for the characteristic fragmentation pattern as detailed in

Unit 13 of the MCH-003 course; these are reproduced in section 9.3 foryour reference. Do remember that the mass spectra can provide clues about

the presence of halogen atoms also.

3. The number of signals in the1H-NMR spectrum indicates about the

different types while their position indicates about the electronicenvironment of different protons in the molecule. Analyse the NMRspectrum for the characteristic spectral clues for the characteristic

groupings like C2H5, CH (CH3)2, aromatic rings, acid/ aldehyde groupsetc. as detailed in experiment 8 and reproduced in section 9.

3. Write down all structural elements you have identified. You may arrange them as

monofunctional (i.e. CH3, C2H5 , CN, PhCH2 , CH3CO , NO2 etc.)

bifunctional (e.g. CO , CH2 , CC, 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 componentof 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 themolecule. You may have more than one tentative structures assembled from thefragments.

5. Revisit the spectra again to see that whether they (especially the NMR and Massspectra) 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 organiccompounds 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 fromdifferent 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 beingprovided 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 tothese are provided in section 9.7. You are required to solve these practice problems andthen 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, C4H8O. Determine its structure.

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Lab.

Practice problem 2: The following are the Mass, IR and NMR spectra for a simple

compound having a molecular formula C5H10O2. Determine its structure.

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

Practice problem 3: The following are the Mass, IR and NMR spectra for a simple

compound having a molecular formula C3H8O. Determine its structure.

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Lab.

Practice problem 4: The following are the Mass, IR and NMR spectra for a simplecompound having a molecular formula C9H10O. Determine its structure.

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

Practice problem 5: The following are the Mass, IR and NMR spectra for a simple

compound having a molecular formula C3H5N. Determine its structure.

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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 experiment 7 and 8 you have been

able to solve the problems raised in the previous section. If not (for any problem) thenyou 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 moleculesto 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 spectraof simple organic molecules to assess your understanding). You must pin up the spectrain the record book and submit your observations and results to your counsellor for

evaluation.

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

Session problem 1

Session problem 2

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Session problem 3

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

Session problem 4

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Session problem 5

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Determination of The

Structure of an Organic

Compound Using UV,

IR, NMR, and Mass

Spectra

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