FRAGMENTATION OF IMPORTANT FUNCTIONAL

GROUPS –ALKANES

AND CARBONYL COMPOUNDS

PREPARED BY

PRAVIN Y. PATIL FIRST YEAR M. PHARM

(PHARMACEUTICS)

MODERN ANALYTICAL TECHNIQUES

PARUL INSTITUTE OF PHARMACY,LIMDA. 1

FRAGMENTATION OF IMPORTANT FUNCTIONAL GROUPS –ALKANES

AND CARBONYL COMPOUNDS

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TABLE OF CONTENT Introduction.

Fragmentation process.

Fragmentation patterns and fragmentation characteristic

relation to parent structure and functional groups.

Alkane and it’s example.

Carbonyl compounds and it’s example.

References.

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INTRODUCTION

An extremely useful result of Electron Impact ionization

in particular is a phenomenon known as fragmentation.

The radical cation that is produced when an electron is

knocked out of a neutral closed-shell molecule in

Electron Impact MS initially possesses a lots of energy.

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CONTI…

Energy sufficient to break chemical bonds, radical cation

will usually break into a neutral radical and a cation. It

is also possible for a neutral closed shell fragment (such

as water) to fall off.

Peaks with smaller m/z values called fragment ion peaks

represent positively charged fragments of the molecule.

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BACKGROUND

Mass spectrometry uses high energy electrons to

break a molecule into fragments.

Separation and analysis of the fragments provides

information about: Molecular weight.

Structure.

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CONTI……… The impact of a stream of high energy electrons

causes the molecule to lose an electron forming a radical cation. A species with a positive charge and one unpaired

electron.

+ e-C HH

HH H

HH

HC + 2 e-

Molecular ion (M+) m/z = 16 7

CONTI……. The impact of the stream of high energy electrons

can also break the molecule or the radical cation into fragments.

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FRAGMENTATION PROCESSBombardment of molecules by an electron beam with

energy between 10-15ev usually results in the ionization of molecules by removal of one electron (Molecular ion formation).

When the energy of electron beam is increased between 50-70ev, these molecular ions acquire a high excitation resulting in their break down into various fragments. This process is called “Fragmentation process”.

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FRAGMENTATION PATTERNS AND

FRAGMENTATION CHARACTERISTICS RELATION TO

PARENT STRUCTURE AND

FUNCTIONAL GROUPS

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FRAGMENTATION PATTERNS

The impact of the stream of high energy electrons

often breaks the molecule into fragments,

commonly a cation and a radical. Bonds break to give the most stable cation.

Stability of the radical is less important.

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SATURATED HYDROCARBONS

STRAIGHT CHAIN COMPOUNDS: Following are the features of the mass spectra of Alkanes.

The relative height of the parent peak decreases as the molecular

mass increases in the homologous series.

The Molecular ion peak (although weak) is normally present.

The spectra generally consist of clusters of peaks separated by 14

mass units corresponding to differences of CH2 groups.12

CONTI….…

The largest peak in each cluster represents CnH2n+1

fragment. This is accompanied by CnH2n and CnH2n-1

fragment corresponding to the loss of one and two H

atoms respectively.

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CONTI…….

BRANCHED CHAIN HYDROCARBONS:

Greater the branching in Alkanes less is the appearance of

the molecular ion and if it appears, intensity will be low.

Bond cleavage takes place preferably at the site of

branching. Due to such cleavage, more stable secondary

or tertiary carbonium ions results.

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CONTI……. Generally, largest substituent at a branch is eliminated

readily as a radical. The radical achieves stability by the

de-localization of lone electron.

Greater number of fragments results from the branched

chain compound compared to the straight compound.

This is due to greater pathways available for cleavage.

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ALKANESThe general rules for alkane are as follows:

The relative height of the Molecular ion peak is greatest for

straight chain molecules and decreases as the branching

increases.

The relative height of the Molecular ion peak decreases with

chain length for a homologous series.

Cleavage is favoured at alkyl-substituted carbons, with the

probability of cleavage increasing as the substitution increases.16

CONTI…

Fragmentation often splits off simple alkyl groups: Loss of methyl M+ - 15

Loss of ethyl M+ - 29

Loss of propyl M+ - 43

Loss of butyl M+ - 57

Branched alkanes tend to fragment forming the most stable

carbocations.

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EXAMPLESOF

ALKANES

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THE MASS SPECTRUM OF 2-CHLOROPROPANE

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CONTI…….

The compound contains a chlorine, because M+2

peak is 1/3 the height of the molecular ion peak.

The base peak at m/z = 43 results from heterolytic

cleavage of the C–Cl bond.

The peaks at m/z = 63 and m/z = 65 have a 3:1

ratio, indicating the presence of a chlorine atom.20

CONTI……. A cleavage results from the homolytic cleavage of a C–C bond at

the α carbon.

21

PENTANE The base peak of 43 in the mass spectrum of pentane indicates the

preference for C-2 to C-3 fragmentation.

22

CONTI……. To identify fragment ions in a spectrum, determine the

difference between the m/z value of a given fragment ion and that of the molecular ion.

Carbocations can undergo further fragmentation.

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CONTI……. 2-methylbutane has the same m/z as pentane.

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2-METHYLPENTANE Mass spectrum of 2-methylpentane(M.W.-86)

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CARBONYL COMPOUNDS

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McLAFFERTY REARRANGEMENT

Fragmentation due to rearrangement of Molecular or Parent ion: Here cleavage of bonds in Molecular ion is due to the intramolecular atomic rearrangement.

This leads to fragmentation whose origin cannot be described by simple cleavage of bonds.

When fragments are accompanied by bond formation as well as bond for breaking, a rearrangement process is said to have occurred.

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CONTI… Such rearrangement involves the transfer of hydrogen

from one part of the molecular ion to another via

preferably, a six-membered cyclic transition state.

This process is favoured energetically because as many

bonds are formed as are broken.

Compounds containing hydrogen atom at position

gamma to carbonyl group have been found to a relative

intense peak. 28

CONTI… This is probably due to rearrangement and fragmentation

is accompanied by the loss of neutral molecule. This rearrangement is known as McLafferty rearrangement.

The rearrangement results in the formation of charged enols and a neutral olefins.

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CONTI…

To undergo McLafferty rearrangement, a molecule must

posses.

1. An appropriately located heteroatom (Ex. Oxygen).

2. A double bond.

3. An abstractable Hydrogen atom which is γ (gamma)

to C=O system.30

ALDEHYDES The Molecular ion peak of aliphatic Aldehydes is weak.

Aromatic Aldehydes shows moderate intense peak.

R-CH=O+ R-C≡O+ + H+

Characteristic feature of Aldehyde (especially Aromatic Aldehyde)

is loss of α hydrogen.

Second common feature of Aliphatic Aldehyde is β cleavage.

For ex, Aldehyde with CH2-CHO end group gives rise to

characteristic M-43 peaks. 31

EXAMPLE Mass spectra of Valeraldehyde.

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KETONES

Molecular ion peaks are more intense than Aldehyde.

Most of the abundant ions in the mass spectra of

Ketones can be accounted by α cleavage and

McLafferty rearrangement.

Strong molecular ion peak.

Formation of acylium ion (RCO+).33

EXAMPLE Mass spectrum of 2-butanone.

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ESTERS Weak Molecular ion peak but generally observable. Loss of alkyl group attached to the carbonyl group by α

cleavage. Formation of acylium ion (RCO+). McLafferty rearrangement. Acyl portion of ester is OR+. Methyl esters: M-31 due to loss of OCH3.

Highly esters: M-32, M-45, M-46, M-59, M-60, M-73 etc.

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EXAMPLE Mass spectra of Methyl butyrate.

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CARBOXYLIC ACIDS

ALIPHATIC CARBOXYLIC ACID:

Weak Molecular ion peak but observable.

A cleavage on either side of C=O.

M-17 due to loss of –OH.

M-45 due to loss of –COOH.

McLafferty rearrangement gives base weak.

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EXAMPLE Mass spectra of Butyric acid.

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CONTI…….

AROMATIC CARBOXYLIC ACIDS:

Strong Molecular ion peak.

A cleavage on either side of C=O.

M-17 due to loss of –OH.

M-18 due to loss of HOH.

M-45 due to loss of –COOH.

McLafferty rearrangement gives base peak. 39

EXAMPLE Mass spectra of para-Anisic acid.

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REFERENCES

Silverstein R.M., Webster F.X., Kiemle D.J., Spectrometric Identification of

Organic Compounds, 7th edition, John Wiley and Sons, Inc.; 2005, 19-70.

Skoog D.A., West D.M., Principle of Instrumental Analysis, 2nd edition,

Sounders Golden Sunburst series, 1980; 476-496.

Sharma Y.R., Elementary Organic Spectroscopy Principles and Chemical

Applications, 1st edition, New Delhi, S. Chand and Company Ltd.; 2008,

291-345 .

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CONTI…

Pavia D. L., Lampman G.M., Kriz G.S., Introduction to

Spectroscopy, 3rd edition, India Edition, Cengage Learning

Publication; 2001, 404-441.

Patel K.G., Shah P.A., Raval H.G., Shah D.A., Baldania S.L.,

Laboratory Handbook in Instrumental Analysis, 1st edition,

Nirav Prakashan, 2005; 327-350.

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