Table Of ContentOTHER TITLES IN THE SERIES IN
ORGANIC FUNCTIONAL GROUP ANALYSIS
Vol. 1. DoBiNSON, HoFMANN and STARK: The Determination of Epoxide Groups
Vol, 2. DRYHURST: Periodate Oxidation of Diol and Other Functional Groups:
Analytical and Structural Applications
Vol. 3. TIWARI and SHARMA: The Determination of Carboxylic Functional Groups
THE DETERMINATION
OF ORGANIC
PEROXIDES
BY
R. M. JOHNSON
Department of Applied Biology and Food Science,
Borough Polytechnic, London
AND
I. W. SIDDIQI
Department of Chemical Pathology,
St. Mary's Hospital, London
P E R G A M ON P R E SS
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P R E F A CE
THE purpose of this monograph is to bring together the analytical chemistry
of the organic peroxides and the corresponding analytical methods. We
are convinced that the analytical chemist should be aware of sufBcient of
the underlying theory to enable him both to select the most appropriate
method and to interpret the results: this is particularly true in the analysis
of organic peroxides where reactivity varies widely and the compounds are
determined in such diverse materials.
The arrangement of the book has been designed to give the maximum
information with the minimum of repetition, but in the interests of clarity
and utihty some facts are given more than once.
We are grateful to Mrs. R. M. Johnson for typing the manuscript and
to Mr. G. A. R. Matthews and Mr. J. W. Selby for reading parts of the
monograph and for their helpful comments. We are also indebted to
Miss M. Sundquist of the University of Uppsala (Sweden) for useful dis
cussions on certain sections.
When we were approached by Professor R. Belcher and Dr. D. M. W.
Anderson to contribute this volume in their series on Organic Functional
Group Analysis, we were already aware of the need for such a monograph.
This conviction has been confirmed as we have considered the hterature
afresh and found many potential analytical methods worthy of further
study, and some problems that remain. We trust that this monograph will
not only assist the practising analyst interested in organic peroxides, but
also stimulate the analytical research chemist to follow up some of the
more promising ideas and to develop methods for some of the outstanding
problems in peroxide analysis.
London R. M. JOHNSON
I. W. SIDDIQI
CHAPTER 1
INTRODUCTION
ORGANIC peroxides^^-*^ are of importance because of their use in industry
(e.g. as synthetic intermediates or polymerisation initiators) and their
occurrence in many industrial materials, for example in foodstuffs,
polymers, and petroleum products. Appropriate analytical procedures are
needed for these materials as well as for research in organic chemistry and
radiobiology.
1. Lipid Peroxides
Unsaturated lipids are oxidised by air to give peroxides and other
degradation products that render fatty foods unpalatable. Oxidative
rancidity is accelerated by the presence of certain catalystá (e.g. copper or
iron) and retarded by antioxidants which may be present naturally (e.g.
tocopherols in vegetable oils) or added by the food manufacturer (e.g.
butylated hydroxy toluene or n-propyl gállate). The oxidative rancidity is
believed to occur by a free-radical mechanism. The free radicals, pro
duced by the influence of heat or light, react with oxygen and a chain
mechanism occurs, giving hydroperoxides.
R* + O2 -> ROO*
ROO* + RH -> ROOH + R*
The chain propagation is terminated by combination or disproportiona-
tion of the free radicals. The hydroperoxides often undergo further reac
tions, depending on the structure of the lipid.
Oxidative rancidity also occurs in non-edible organic compounds of
importance, for example in Unseed oiV^^ and lubricating oils, and consider
able attention has been given to methods of measuring oxidative rancidity.
As the precise composition of the organic peroxides is not always known
and the iodine method is the most frequently used, the peroxide content is
1
2 THE DETERMINATION OF ORGANIC PEROXIDES
often expressed as the "peroxide value". The peroxide value is defined as
the number of millilitres of 0-002 Ν sodium thiosulphate solution required
to titrate the iodine Uberated by 1 gram of the oil or fat under the pre
scribed experimental conditions. An alternative method of specifying
peroxide content is the "peroxide number", which is defined as the milU-
equivalents of oxygen per kilogram of oil or fat. The "peroxide number",
so defined, is numerically twice the "peroxide value" and this sometimes
leads to confusion.
In many oils, especially vegetable oils, there is an induction period in
which the development of oxidative rancidity is very slow, whilst the
natural or added antioxidants are used up. In such cases the "shelf-life" of
the oil or fat is often more important than the actual peroxide value, as
once the induction period is over, oxidative rancidity develops rapidly.
Some estimate of "shelf-Ufe" can be obtained by accelerated rancidity
tests<*> in which, for example, oxygen may be bubbled through the heated
oil or fat, samples being withdrawn at intervals for peroxide value deter
minations.
Oils (e.g. olive oil) spread on cloths and fabrics may spontaneously
oxidise, and the heat generated may reach a dangerous level. It is im
portant to be able to measure the susceptibility of such oils to oxidation.
Peroxides are formed not only in the intact oils and fats (e.g. oil, lard,
butter) but also in foods which naturally contain them, for example meat,
fish, milk, and certain cereals (oats, soya). There is an enormous variety of
compounded foods that contain oils or fats as important constituents,
salad cream, meat products, milk products and baked goods, for example.
The estimation of peroxide rancidity requires some care in the design of
the method, often including a preliminary separation of the peroxide-
containing fraction to reduce interferences. In some cases the lipid may be
extracted by cold lipid solvents (e.g. ether or petroleum ethers), but in
other cases precipitation of proteins may be needed to obtain a complete
extraction. Care is needed to ensure that this treatment does not alter the
analytical result. A further source of peroxides in bakery products arises
from the use of monomeric and linear dimeric acetone peroxide in flour
for maturing and bleaching.
The peroxide content of essential oils^^' ®^ has been used as an index of
quality; it depends on the age of an oil and the conditions of storage and
it has also been used in the control of the process of deterpenation. The
peroxide index of essential oils is the active oxygen content in micrograms
per gram.
INTRODUCTION 3
Peroxides are liable to develop in soaps and fatty acids containing un
saturated bonds,^i*^^ and in the oils and fats used for pharmaceutical
products/"^
2. Polymers
Hydroperoxides are formed in natural rubbers^"^ and since a wide
variety of organic peroxides are used as polymerisation initiators, they may
also occur as residues in synthetic polymers, including some plastics used
for food packaging.
Organic peroxides are mainly used to initiate addition polymerisation
(e.g. benzoyl peroxide, lauroyl peroxide, di-isopropyl peroxydicarbonate,
tert'hutyl pervalate, /7-chlorobenzoyl peroxide and 2,4-dichlorobenzoyl
peroxide for vinyl chloride), and some are used to initiate cross-linking
(e.g. of polyesters of glycols and maleic anhydride with phthalic or adipic
acid by means of added monomer, such as styrene). In some cases un
desirable cross-linking may occur when peroxide residues remain in the
polymer after production.
The organic peroxides are used mainly in mass or suspension poly
merisation, but inorganic peroxides (e.g. persulphate) are preferred for
emulsion polymerisation. Ethyl hydroperoxide, teri-hutyl peroxide and
di-/er/-butyl peroxycarbonate are suitable for the high-pressure poly
merisation of ethylene and for styrene and unsaturated polyesters. Cumene
hydroperoxide is used for polystyrene, butadiene/styrene copolymers, and
polyesters. Di-isopropyl hydroperoxide and /7-menthane hydroperoxide
may be used for butadiene/styrene copolymerisation and /7-menthane
hydroperoxide for styrene. Other compounds that are used include acetone
peroxide, octanoyi peroxide, perlauric acid, /er/-butyl cumyl peroxide,
succinic acid peroxide, /er/-butyl perbenzoate and peracetate, di-tert-hntyl
dipersuccinate and diperphthalate, decanoyl peroxide, methyl-wo-butyl
ketone peroxide, 2,2-bis-(ier/-butyl peroxy)-butane and hydroxyheptyl
peroxide.
The selection of an appropriate peroxide initiator depends on its rate of
decomposition at the desired temperature of polymerisation, and the
temperature is chosen primarily to give the necessary degree of polymerisa
tion, but also to control the rate of polymerisation and possibly chain
branching. The physical properties of the monomer (e.g. boiling point)
may also limit the permissible temperature range. In some cases the con
centration of initiator may exert a major controlling influence over the
molecular weight of the product.
4 THE DETERMINATION OF ORGANIC PEROXIDES
If the polymer has a tendency to be unstable, unduly high concentra
tions of initiator tend to enhance this instabihty. Often the monomer (e.g.
styrene) is mixed with the peroxide (e.g. cyclohexanone peroxide) and an
activator (e.g. cobalt naphthenate) which causes the peroxide to decom
pose at lower temperatures. In some cases peroxides have been used as
blowing agents for foam production, and occasionally there is an induc
tion period prior to polymerisation as radical-absorbing compounds are
used up.
Analytical methods have been described for these products, for example
for the determination of benzoyl peroxide in styrene polymerisation,^^^^
and hydroperoxides in polymer latex. In some cases the polymers may
be dissolved, e.g. in benzene, prior to estimating the peroxide present, and
in other cases the peroxides may be extracted with, for example, ethanol.
3. Petrochemicals
On exposure to air petroleum products are Hable to form peroxides.^^^^
Even low concentrations of peroxides are harmful. In refined distillates
they give rise to the formation of gums and sediments, in white mineral oil
disagreeable odours are produced and in engine oil, the corrosion of
bearings is accelerated. As in the case of lipids, the primary products are
hydroperoxides rather than dialkyl peroxides. The tendency of peroxides
to initiate polymerisation is more pronounced when the hydrocarbons are
highly unsaturated. Peroxides have also been reported^^'-"^ among the
products formed when low molecular weight hydrocarbons are combusted
(e.g. isopentane), as autoxidation products in the drying oils of paints,^^^^
and in coal.^^i)
4. Irradiation
Organic peroxides are formed by the irradiation of certain organic
compounds<22' and biological materials. <24) xj^jg is of considerable im
portance in radiobiology and medicine. It is to be expected in view of the
fact that one effect of irradiation is to produce free radicals. In some in
stances cell damage has been attributed to the formation of peroxides on
irradiation.<2*· Highly speciaUsed analytical methods are needed for
meaningful studies of these effects.
5. Miscellaneous
Peracetic acid, produced commercially or in the laboratory as an oxidis
ing agent, contains the parent carboxylic acid, hydrogen peroxide and
INTRODUCTION 5
water as well as the peroxy acid/^«) Care is necessary if the analyst is
interested in the concentration of peracetic rather than the total peroxide
oxygen, as even traces of catalysts (e.g. molybdates) may rapidly estabhsh
an equilibrium between hydrogen peroxide and the peracid.
Organic peroxides are used as rocket fuels, and for bleaching wool and
paper pulp.<") jj^g transannular peroxide ascaridole, which occurs in
chenopodium oil,^^e. 29) ^q^q^ a particular analytical problem as it is
amongst the most chemically inert of the peroxy compounds.
The formation of peroxides in solvents, especially ethers, is well known.
Peroxide-containing solvents are highly explosive and reliable methods for
the detection, estimation, and removal of the peroxides are essential.
In view of the extreme reactivity of many of the organic peroxides,
precautions are necessary when handling materials that contain appreciable
concentrations of peroxide, especially in contact with inflammable
materials. General hints on the detection and removal of peroxides have
been given together with methods of handling bulk quantities,^^°> but the
instructions of the chemical supplier should be noted. Laboratory quan
tities of organic peroxides should be stored in glass or polyethylene, but
never in a glass bottle with a ground glass stopper or screw cap. A serious
explosion may occur if minute fragments of friction-sensitive peroxides
(e.g. benzoyl peroxide) are trapped in the cap or bottle thread. Ideally
organic peroxides should be stored under refrigeration and protected from
light.
Many peroxides in solid or paste form may decompose rapidly at
temperatures between 75~120°C (e.g. lauroyl peroxide, cyclohexanone
peroxide paste, and moist benzoyl peroxide) and dry benzoyl peroxide
explodes at 100-102°C. On the whole the liquid peroxides or solutions of
peroxides are less hazardous. Acetyl peroxide (25 % in dimethyl phthalate)
explodes mildly at 90-92°C and tert-hutyl peroxyacetate (75 % in benzene)
explodes at 158°C. Many sterically hindered peroxides will reflux on
heating, e.g. di-ierr-butyl peroxide (100°C) and others decompose mildly
e.g. i^r/-butyl peroxybenzoate (IITC) and 2,5-dimethyl-2,5-di(rer/-butyl-
peroxy) hexane.
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6 THE DETERMINATION OF ORGANIC PEROXIDES
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