No. 2
`
`REACTIONS OF INORGANIC IODINE COM-
`
`Page
`
`By K. J. Morgan
`
`OXIDATION
`
`• 123
`
`. 147
`
`FORCES AND SOME PRO-
`
`J. S. Rawlinson .
`
`. 168
`
`STRUCTURE OF THE ERGOT ALKALOIDS
`A. L. Glenn.
`
`. 192
`
`E
`
`SOCIETY
`
`Page 1 of 23
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`Noven Ex. 1020
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`

`
`The Journal and
`Reviews is ue:s1_gnect
`worker :
`It is intended that each review article
`and not
`
`reviewed.
`The submission
`
`The Chemical
`Such
`
`Page 2 of 23
`
`Noven Ex. 1020
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`

`
`y 3
`
`OLEFIN OXIDATION
`
`By L. BATEMAN
`
`(THE BRITISH RUBBER PRODUCERS' RESEARCH ASSOCIATION,
`WELWYN GA:'.'WEN CITY, HERTS.)
`
`ft
`
`lf
`sl! THE interaction of ole:fins with molecular oxygen is not only a subject of
`widespread industrial importance, but is one of the most thoroughly. under(cid:173)
`stood chemical processes. This Review attempts to give a broad picture
` of the main mechanistic features. An earlier article 1 has reviewed the
`historical background and has given details of the method of approach and
`l of the earlier kinetic data which were largely responsible for opening up
`.
`1 this field.
`In Section ( l) we present the generally accepted chain mechanism and
`l the simpler rate expressions which are often obeyed. Certain quantita(cid:173)
`1
`
`:tive comparisons of olefinic reactivity derivable on this basis are then dis-
`, cussed. Our main concern, however, is to show how comparatively small
`changes in certain mechanistic details can give rise to substantial differences
`in the observed kinetics-so much so that a profound change in mechanism
`1
`
`I might be imagined. Section (2) deals with rate measurements under non-
`
`stationary state conditions designed to determine the propagation- and
`1
`, termination-rate constants separately, and emphasises the inherent limita(cid:173)
`'\ions to accuracy which oxidation systems present in this respect.
`Sections (3), (4), and (5) are concerned with the initiation of the oxidat.ion
`chain, and the part played in this by the hydroperoxide which is the primary
`reaction product. This behaviour of the hydroperoxide is responsible for
`the autocatalytic character of the oxidations, and the complexity and
`environmental sensitivity of its decomposition serves to complicate the
`kinetics of the oxidations as a whole. Attention is drawn to circumstances
`where the fraction of hydroperoxide undergoing decomposition is large
`~ompared with that being formed, so that the character and kinetics of the
`process are greatly altered despite the same fundamental reactions being
`involved. Section (6) describes efforts to analyse the initiation process
`quantitatively in order that the number of oxidation chains being started
`under given conditions. can be specified.
`Under mild conditions of oxidation, the chain is long and the fraction of
`,~he hydroperoxide which decomposes .to initiate fresh chains is very small.
`The overall yield of hydroperoxide should thus be nearly quantitative. In
`Section (7) serious discrepancies are interpreted in terms of the dual reac(cid:173)
`tivity of peroxy-radicals towards olefins, the consequence being that the
`
`!measured rate constants are composite quantities relating to both hydrogen
`
`extraction and double-bond addition.
`.. The allylic radicals formed on removal of an <X-methylenic hydrogen
`ratom from. an olefin are mesomeric and hence the derived product may
`1 Bolland, Quart. Reviews, 1949, 3, 1.
`147
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`Page 3 of 23
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`

`
`148
`
`QUARTERLY REVIE\VS
`
`consist of ally lie isomerides. The behaviom: of mono-olefins and I: 4.!
`l
`diolefins in this respect is discussed in Section (8).
`
`1. General Kinetic Behaviour
`The following reaction scheme, where RH represents the olefin with an1
`e<.-methylenic hydrogen atom H, ri is the rate of chain initiation, and thel
`lc's are the velocity coefficients of the reactions indicated, accounts for the~
`reaction characteristics with remarkable comprehensiveness.
`
`;
`
`1
`
`I
`
`Initiation :
`Propagation :
`
`Termination :
`
`1
`
`r 1
`Production of R· or R0 2• radicals
`R· + 0 2 ~ R0 2•
`k 2
`R0 2• + RH ~ R0 2H + R·
`k 3
`2R· ~ }Non-initiati~1g or k 4
`R· + R0 2• ~ -propagatmg
`k;;
`2R0 2• ~ products
`k 6
`The more obvious of these are: (i) high yields of the hydroperoxide)
`R02H (cf. p. 162); (ii) catalysis by light and by free-radical producing!
`substances, indicating the free-radical nature of the reaction ; 2 (iii) quantu111
`yields greater than one and a proportionality between rate and the square!
`root of the light intensity in photo-oxidations, indicating a chain reaction!
`with mutual ·destruction of two· chain carriers in the termination step; a
`(iv) a parallelism between oxidisability and the relative ease of rupture of
`the C-H bond in RH, indicating the importance of a hydrogen-exchange!
`reaction such as (3) (cf. p. 149) ; (v) the formaJtion of conjugated-dienel
`hydroperoxides from I : 4-dienes, in agreement with the generation o1
`mesomeric R· radicals as in (2) (cf. p. 164) ; and (vi) the marked retardation~
`in rate produced by phenolic compounds (among others), which interfere(
`with the propagation process by providing an alternative and easier reaction\
`for the R02 • radicals that does not liberate a radical equivalent to R·.4 1
`
`It being assumed that k 5 2 = k 4 k6 , the above mechanism yields the rate
`equation (for long chains) 5
`r = r.tk k -i[RH]
`k2k6 -i[02]
`k 3k4 -![RH] + k2k6 -![02]
`3 6
`where r is the overall rate of oxidation and the square brackets
`concentration terms.
`Oxidisability at "High" Oxygen Pressures.-When reaction (2) is so
`much faster than (3) that [R] ~ [R02], termination can be assumed to occur
`entirely by reaction (6),, and equation (1) simplifies to
`r C() = riik3k 6 -![RH]
`Equation (2) accurately expresses the observed kinetics for most olefins atl
`2 Bateman and Bolland, Proc. XIth International Congress of Pure and Applied(
`Chern., 1947.
`3 Bateman and Gee, Proc. Roy. Soc., 1948, A, 195, 376.
`4 Bolland and ten Have, Trans. Faraday Soc., 1947, 43, 201; Discuss. Farada'!k
`Soc., 1947, 2, 252.
`(
`5 Bolland, Proc. Roy. Soc., 1946, A, 186, 218.
`
`1 1
`
`
`
`(15;
`I
`signify/,
`
`I
`
`\
`
`L
`
`Page 4 of 23
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`Noven Ex. 1020
`
`

`
`BATEMAN : OLEFIN OXIDATION
`
`(a)
`
`149
`0xygen pressures greater than 100 mm. (the "high" pressure region)-an
`interesting exception being discussed later. At constant rP estimates of
`k3k6 -i are thus obtained from r co/[RH] and these measm·e the relative
`reactivities of different olefu113 in reaction (3), since k6 is not very sensitive
`to changes in R (seep. 153). Bolland 6 has in this way developed a correla(cid:173)
`tion between olefinic structure and oxidisability. Referring to propene,
`OH3·CH:CH2 , at 45°, he concludes that:
`(c)
`(b)
`(i) Replacement of one or two hydrogen atoms at (a) andjor (c) by
`alkyl groups increases k3 by 3·3n, where n is the total number of substituents;
`similar replacement at (b) is without effect.
`
`k3 23-fold.
`(iii) Replacement of a hydrogen atom at (a) by an alk-1-enyl group
`1
`
`I.
`
` (ii) Replacement of a hydrogen atom at (a) by a phenyl group increases
`~ increases k3 107 -fold.
`
`i
`(iv) The value of k3 appropriate to an tl-methylenic group contained in a
`cyclic struct.ure is 1·7 times that of the group contained in an analogous
`~
`iUllJ acyclic structure.
`tar
`These rules relate to broad variations, as implied by the assumed
`Q equivalency of different alkyl groups; The assumptions that k6 is invariable
`~; and that benzoyl peroxide (used as a standard initiator) initiates throughout
`e 0 with equal efficiency also . introduce second-order uncertainties. More
`mg serious discrepancies occur in special cases. Thus 2 : 4-dimethylpent-2-ene
`ien is at least 10 times less reactive than would be predicted, 7 presumably
`1 0 because of steric hindrance at 0 3-behaviour simulated in a saturated
`tio
`:fe
`:tio
`:t·.t
`rat
`
`(r
`
`is
`JCC
`
`Olefin, RH a
`
`•
`
`*
`CH 2:CH·CH3
`*
`CH 2:CH·CH 2Alk .
`*
`CHAlk:CH·CH 2Alk
`*
`CAlk 2:CH·CH 2Alk
`*
`CH 2:CH·CH 2Ph
`*
`CHAlk:CH·CH 2·CH:CHAlk .
`*
`CH:CH·CH 2
`I
`I.
`.Alk-Alk
`
`TABLE 1
`
`Ea b
`
`13· 5
`
`AH 3 = E 3/0·4
`
`A(AH 3 )
`
`34
`
`0
`
`ll·s
`
`10· 5
`
`9
`
`10
`
`6
`
`9·s
`
`29
`
`26
`
`23
`
`25
`
`15
`
`24
`
`5
`
`8
`
`ll
`
`9
`
`19
`
`lO
`
`All values are in kcal.jmole
`Reactive ct-methylenic group
`indicated by an asterisk.
`b Calculated from
`!E1 + tE 6 , where E 0 designates the overall activation energy of oxidations
`Eia = E 0 -
`catalysed by benzoyl peroxide. E 6 is taken as zero and E 1 as 30 kcal.jmole (Bolland's
`blished values are based on Ei = 31 kcal.jmole).
`
`6 Bolland, Trans. Faraday Soc., 1950, 46, 358.
`
`7 Morris, unpublished result.
`
`Page 5 of 23
`
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`

`
`I
`
`150
`
`QUARTERLY REVIEWS
`
`In genera.II (
`hydrocarbon by the inertness of 2: 2 : 4-trimethylpentane. 8
`however, Bolland's rules rationalise the behaviour of different olefins anJf J
`of different allylic systems in the same olefin. For example, in the isopreniel
`unit ·OH2·C(CH3):CH·CH2·, the relative a-methylenic activity at the three! (
`(x)
`(z)
`. ·~
`(y)
`positions, x : y : z, is approximately l : 3 : IL
`( ·
`The numerical factors given for 45° become smaller at higher tempera.!
`tures, since increased reactivity partly reflects a lower activation energy (E)! ·
`for reaction ( 3). 6 Average values of E 3 for the systems considered in! '
`(i)-(iv) above are given in Table l. E 3 may be related to the corresponding! ·
`heat of rea,ction, !.lH3 , 6 whose variation, ~(!.lH 3 ), from olefin to olefin e:x:.!
`presses differences in resonance energy and other stabilising influences in/
`the different allylic radicals.
`I
`Oxygen-pressure Dependence.-Decreasing the oxygen pressure reduces!
`the overall rate of oxidation only when reaction (2) is not incomparably/
`faster than reaction (3), i.e., when [R·] is not negligible compared with\
`[R0 2·]. The pressure at which this condition prevails depends on the
`reactivity of the olefin-the lower the reactivity, the slower is reaction (3),1,
`and the lower the value of [0 2] necessary to reduce the rate of reaction (2)1'
`accordingly. This effect may be enhanced by the reactivity of R in reaction
`I
`I
`
`(a)
`
`8
`~ ,,:'
`
`400
`200
`Oxygen pressure (mm.)
`FIG. 1
`Variation of the rates of oxidation of (a) hexadec-1-ene (45°), (b) ethyl linolenate (45°)
`and (c) 2: 6-dimethylhepta-2: 5-diene (25°) with oxygen pressure.
`
`600
`
`I
`
`(2) being qualitatively the inverse of that of RH in reaction (3), although:"'
`the quasi radical-radical nature of (2) renders it far less responsive than (3)r
`to changes in R. Some rate-pressure dependences are illustrated in Fig. 1.,
`Hexadec-1-ene shows no dependence above I mm., but with increasing\
`olefin reactivity the pressure at whic.h the rate becomes insensitive also['
`increases. With the intensely reactive 2 : 6-dimethylhepta-2 : 5-diene, the
`rate at atmospheric pressure is well below r w· As only reaction (3) of
`
`the propagation and termination steps has an appreciable temperature k l
`
`8 Wibaut and Strang, Proc. K. Ned. Akad. Wet., 1951, ·54, B, 229.
`
`(
`
`(
`
`Page 6 of 23
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`

`
`BATEMAN : OLEFIN OXIDATION
`
`151
`coefficient, the dependence of rate on oxygen pressure extends to higher
`pressures at higher temperatures, as exemplified in Fig. 2.
`Generalised Rate
`some olefins, equation (1) expresses
`exactly the observed kinetics over the whole range of oxygen pressures
`In
`where accurate measurements are possible (down to about 1 mm.).
`general, however, deviations are found which arise from departures from
`2 = k4k6 (p. 148), and which vary from olefin to
`( the assumption that k5
` olefin in an intelligible manner-in extreme cases being sufficient to modify
`the oxidation kinetics at pressures near atmospheric.
`r
`
`[.
`
`l )
`
`07-----~·~----~----~~----~----~
`w
`0
`~
`~
`~
`~
`Oxygen pressure (mm.)
`FIG. 2
`Influence of temperature on the oxygen-pressure dependence of ethyl linolenate at (a) 25°,
`(b) 35°, and (c) 45°.
`
`•
`
`The completely general form of the rate equation (for long chains) is: 9
`r- 2 = ri-1(k2 - 2k4[02]- 2 + 2k2 - 17c3 - 1k5[RH]-1[02]-1 + lc3 - 2k6[RH]- 2)
`(3)
`or, alternatively, by Qombination of (3) and (2) :
`(r crjr) 2 = 1 + 2cf>k2 - 1lc4lk3k6 -l[RH] [02]-l + k2 - 2k4k3
`2k6- 1[RH]2[02]- 2 • (4)
`where 4> = k 4 -kk5k6 -k and roo is defined by equation (2). Equation (4)
`requires a plot of (r 00/r) against [02]-1 to be linear if 4> = 1 [as assumed in
`deriving (1)], concave to the latter axis if 4> < 1, and convex if 1> > L
`Examples of all three conditions are known. 11 From the slope and ordinate
`intercept of the plot of {(r 00/r) 2 - 1}[02] against [02]-1 (see equation 4),
`1> and the composite coefficients k2k 4 -! and k 3k6 -t can be determined. The
`data listed in Table 2 show that the large variations in k3k6 -i with olefinic
`structure are not paralleled by any of comparable magnitude in lc2k 4 -t.
`9 Bateman, Gee, Morris, and vVatson, Discuss. Faraday Soc., 1951, 10, 250.
`
`Page 7 of 23
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`
`

`
`152
`
`QUARTERLY REVIE\VS
`
`TABLE 2*
`
`Olefin
`
`103k 3k 6 -!(45°)
`lo-ak2k4 -~(45°)
`(mole-t 1.! sec.-?!) (mole-! 1.! sec .. -t)
`
`Methyl oleate
`Phytene
`Digeranyl.
`Ethyllinoleate
`.
`.
`.
`Ethyllinolenate .
`2: 6-Dimethylhepta-2: 5-diene
`
`1·53
`1·07
`3·82
`20·7
`41·4
`130 (25°)
`
`0·5
`0·4
`0·9
`1·6
`1·3
`1·2 (25°)
`
`i
`
`~
`
`"'
`0·3
`1·0
`3·1
`2·5
`3·3
`6·5
`
`* Absolute comparison requires the composite and individual rate constants in this
`and the following Tables to be multiplied by factors of eiB.P and e.s.p, respectively
`where eB.P denotes the initiating efficiency of benzoyl peroxide (see p. 161). As thi;
`quantity has been determined only for a few olefins and is variable, it is preferable
`here to base all the data on the value, eB.P = l.
`cp Values.-Two points concerning the values of cp may be noted: (i) they:
`are all Tather small compared with some of the large values found for thel
`equivalent quantity for cross-termination in copolymerisations; and (li)l
`they increase with the reactivity of RH. These features probably have a!
`common link in reflecting a large diminution in the resonance ~nd polarity!
`properties of the grou_p R on relay through the 0-0 bond of the R02·;
`In copolymerisations, the analogous cp values relate to the inter.\
`radical.
`play of structural effects in substituted alkyl radicals only ; in the oxidations, [
`the R0 2• radical is essentially a common factor from system to system and
`tends to depress in reaction (5) any variation in reactivity in R· which mf
`I
`fully manifest in reaction (4).
`Influence of¢ on the Kinetic Form.-As the reactivity of RH increases,
`two factors enhance the kinetic importance of the R· radicals : (i) the lessen.J
`ing of the difference between k2 and k3 ; and (ii) the increase in cp. The
`practical repercussions are strikingly illustrated by comparing the variation![
`in the relative importance of reactions (4), (5), and (6) at different oxygen
`pressures for different olefins (Fig. 3).10 The displacement, broadening, and·
`intensification of the R· R0 2• curve on passing from phytene to(
`2 : 6-dimethylhepta-2: 5-diene leads to such marked kinetic differences asl
`to suggest that. the oxidation mechanisms a. re fundamentally different./1
`Even at pressures near atmospheric, equation (2) does not apply even
`approximately to the heptadiene; the rate is neither directly proportional/
`to [RH] nor independent of [0 2]. At pressures higher than 100 mm.,,
`termination by reaction (4) is negligible and the dependence of rate on('
`[RH] is then given by equation (3) without the term k 2 - 2k4[0 2]- 2.
`The ability of the oxidation mechanism to account in so detailed and/
`rational a manner for the kinetic behaviour of olefins of widely varying
`reactivity establishes its formal correctness. As discussed later (p. 162),1
`it is sometimes necessary to modify or supplement the scheme given on 1
`p. 148 in order to obtain consistency with product data.
`
`10 Bateman and Morris, Trans. Faraday Soc., 1953, 49, 1026.
`
`_j
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`Page 8 of 23
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`

`
`BATEMAN : OLEFIN OXIDATION
`
`153
`
`(b)
`
`(a)
`
`~
`
`/ I I
`
`'"' ~
`'--
`~ f
`~80
`·~
`~
`~60
`
`~20
`r{:
`
`0
`
`80
`
`60
`
`40
`
`o~~~~~~,--J-J-~~~m~~~~~
`Oxygen pressure (mm.)
`FIG. 3
`rermination characteristics of the oxidation of (a) phytene (45°), (b) ethyllinoleate (45°),
`and (c) 2 : 6-dimethylhepta-2 : 5-diene (25°) at various oxygen pre88U1'e8.
`
`2. Individual Rate Coefficients of the Propagation and Termination
`Reactions
`For all chain reactions, measurements under stationary-state conditions
`rermit only composite velocity coefficients (such as k3k6 -!) to be determined.
`
`Page 9 of 23
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`

`
`154
`
`QUARTERLY REVIEWS
`
`1
`
`Non-stationarv-state measurements with oxidising olefins have been made I
`by the rotati~g-sector technique n, 12 and by following directly the photo. I
`chemical pre- and after-effects, 9 • 12 • 13 • 14 and absolute values of the several!
`propagation ~nd termination c.on~tants derived .. H.owever, .severe limits to~
`accuracy are nnposed by certam mherent complicatwns, which are common (
`to similar measurements in all gas-liquid systems ~nd deserve to be more 1
`I
`widely known.
`The principle of the photochemical pre- and after-effects is expressed/,
`If the oxidation is followed in the dark (rate= rn) and then
`in Fig. 4.
`the light is switched on, a time interval elapses before the uniform!
`
`r
`!
`
`I."~
`
`I
`
`/ /
`Dark Light
`
`,--'
`
`Time
`
`(b)
`
`~ ...
`~
`~
`t's
`.:::
`~
`~ <:::;)
`~de
`
`(a)
`
`Light Dark
`
`~ Time
`
`FIG. 4
`Definition of (a) the rate decay interce1Jt, lct, and (b) the rate growth intercept, lg.
`
`light rate (rL) is established, i.e., while the increased concentration of,,
`chain carriers builds up. The inverse occurs when the light is switched off.'
`The intercepts Ig and Id represent amounts of oxygen absorbed during
`the non-stationary state conditions, and can be shown to be defined by
`Id =a ln{(rL + rn)/2rn} and Ig = a ln {2rL/(rL·+ rn)}, where a is a
`complex quantity containing the propagation and termination constants in
`different ratios from those in the stationary rate equations (under "high"
`pressure conditions, a reduces to k3k6 - 1[RH]). The important complicating"ld
`factor is that the oxygen concentration in the solution does not remain
`constant during the change from rn to rL. As oxygen is continually being
`removed by reaction, the actual value of [02] is always lower than the satura- I
`tion value. The extent of this difference depends on the speed by which!
`
`11 Bateman and Gee, Proc. Roy. Soc., 1948, A, 195, 391.
`1 2 Bamford and Dewar, ibid., 1949, A, 198, 252.
`13 Bateman and Gee, Trans. Famday Soc., 1951, 47, 155.
`14 Bateman, Bolland, and Gee, ibid., p. 27 4.
`
`~
`(
`
`Page 10 of 23
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`

`
`BATEMAN : OLEFIN OXIDATION
`
`155
`the oxygen can be replenished from the gas phase by agitation. The inter(cid:173)
`cepts actually measured are not in fact Ig and Jd but are given by 13
`I'a = Jd + (rL- ?'n)/ks and I'g = Ig + (rL- rn)/k8 , where k8 represents
`the shaking efficiency.
`In principle, therefore, the change in oxygen
`concentration can be compensated for automatically by evaluating
`Ig = a ln{(rL + rn) 2/4rLrn} ).
`I' a- I' g ( = Jd -
`In practice, the term
`(rL- rn)/k8 , while often negligible compared with [02], is large compared
`with Jd and Ig. For example, under favourable experimental conditions
`with ethyllinoleate at 15°, an oxygen pressure of 550 mm., and a shaking
`frequency of 650 per minute, the values of 10 61' d' 10 61' g' and 10 6(rL -
`rn)/ks
`were 27, 19, and 16 molejl., respectively.
`In the "low" pressure region, where rL and rn themselves vary with
`changes in [0 2], an exceedingly complicated situation exists,14 and deriva(cid:173)
`tions of the relevant constants are subject to much greater uncertainty.
`Fairly reliable estimates of the several constants for ethyl linoleate and
`digeranyl are given in Table 3. The values of k3 and k 6 are believed to be
`numerically significant, those of k2, k4, and k5 express the order of magnitude.
`
`TABLE 3. Velocity coefficients at 25° (mole-1 l. sec.-1)
`
`Ethyllinoleate
`Digeranyl
`
`10
`1
`
`50
`3
`
`20
`1
`
`50
`10
`
`20
`10
`
`involves measurements in circum(cid:173)
`technique
`rotating-sector
`The
`stances where changes from rL to rD to rL occur in rapid succession. The
`(rL- rn)/ks terms thus cancel out automatically. Even under high
`pressure conditions (as above, a complex situation prevails at "low"
`pressures), the advantage which this confers has not been realised owing
`to a lack of sensitivity in other respects, but practical improvements to
`remedy this appear feasible and worth developing.
`
`0
`[
`
`,
`
`3. Autocatalysis and Hydroperoxide Decomposition
`Benzoyl peroxide and azoisobutyronitrile undergo unimolecular thermal
`l
`' dissociation into free radicals and catalyse the oxidation of olefins propor(cid:173)
`u tionally to the square root of their concentration. This affords critical
`1 evidence, in conjunction with photocatalysis (p. 148), for the form of ri in
`'equatwn (3) and 1ts s1mph:fied verswns.
`,n Bimolecular Hydroperoxide Decomposition.-The autocatalytic character
`n of the oxidation is illustrated in Fig. 5. The overall rate is proportional
`:a to the hydroperoxide produced during the earlier stages of the reaction,
`.c and thus from equation (3) ri ct:. [R02H]2, i.e., chain initiation ensues from
`a bimolecular decomposition of the hydroperoxide. This result was unex(cid:173)
`pected when first encountered because saturated and arylated hydroperoxides
`had previously been said to undergo a unimolecular primary scission. The
`self-consistency of the kinetic data on oxidation catalysis and a direct· study
`L
`
`Page 11 of 23
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`

`
`156
`
`QUARTERLY REVIEWS
`
`3
`
`/
`
`5
`
`20
`15
`10
`[02]absorbed (mole/l.)
`
`2
`
`10
`
`25
`
`FIG. 5
`Autoxidation of cyclohexene at 45° and 728 mm.
`
`of the decomposition of an olefinic hydroperoxide,l 5 however, combine to
`establish its validity. As mentioned later, differences concerning the order
`of peroxide decomposition may not be antagonistic.
`For the above catalytic form in the "high" pressure region, we have,
`r = {ek"[R02H] 2}tk3k6 -l[RH]
`.
`.
`.
`where k" is the bimolecular velocity coefficient for the hyd.roperoxide
`
`(5) I
`decomposition and e represents the efficiency with which the liberated r
`radicals produce R· or R02• radicals. From benzoyl-catalysed oxidations,\
`I
`
`TABLE 4
`
`ll
`
`Olefin
`
`Hydroperoxide-type, %
`
`prim.
`
`sec.
`
`tert.
`
`l04ks7c 6 -:! *
`(mole- i I.! sec.
`
`106ek" *
`(moie- 1 L sec.- 1)
`
`')
`
`Allylbenzene
`Oct-1-ene
`
`Methyl oleate .
`cycloHexene
`Ethyllinoleate
`
`4-Methylhept-3-ene
`1-Methylcyclohexene .
`
`1 : 3: 5-TrimethylcycZohexene
`DicycZohex-2-enyl .
`
`Squalene
`Digeranyl
`
`100
`70
`
`30
`
`100
`100
`190
`
`70
`70
`
`15
`25
`
`30
`30
`
`85
`75
`
`[.......,.
`r-.;
`
`*At 55°.
`
`14·4
`3·6
`
`21·5
`37·0
`278·
`
`32·4
`65·1
`
`150
`200
`
`39·7
`49·4
`
`0·28
`0·29
`
`0·46
`0·54
`0·47
`
`1·72
`1·14
`
`3·25
`2·48
`
`2·97
`2·62
`
`w
`T
`
`16 Bateman and (Mrs.) Hughes, .J., 1952, 4594.
`
`):;
`- - - - - - - - - - - - - - r'
`
`Page 12 of 23
`
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`
`

`
`157
`BATEMAN : OLEFIN OXIDATION
`and determination of the appropriate value of e for this system,16 k3k6-t can
`be determined and thus (ek") from (5).
`In Table 4,17 the olefins are grouped
`according to the structural type of hyd.roperoxide involved, as deduced
`from the relative susceptibility of non-equivalent cx.-methylenic C-H bonds
`to attack by R02 • radicals (p. 149) and from the tendency of allylic systems
`to form isomeric products (p. 166).
`No parallelism is apparent between the differences in ek" and the
`oxid.isabilities of the olefin (k 3k6 -t), but a clear correlation exists with
`hyd.roperoxide type in the sense ek"prim.: ek"sec.: ek"tert. = 1 : 2 : 14. For
`reasons unknown, this order is the reverse of the commonly recognised
`stability of analogous saturated hydroperoxides.
`
`16
`
`f'/2
`1.1
`(\.)
`.._">
`~
`~
`<:) 8
`~
`1:..
`'0
`~
`
`e
`
`2
`
`6
`4
`!Oz [ 02] absorbed (mole/ l.)
`FIG. 6
`Autoxidation of (a) tetralin at 75° and 180 mm. and (b) 1-methylcyclohexene at 65° and
`350 mm. at low extents of oxidatlon.
`
`8
`
`!0
`
`f Unimolecular Hydroperoxide Decomposition.-A curious feature of the
`; plots of r against [02]absorbed (such as in Fig. 5) is that extrapolation of
`~ the linear portion to [02]absorbed = 0 gives a small but real intercept on the
`r-axis. This was first thought to represent the rate of the direct reaction
`between the olefin and oxygen (RH + 0 2 ~). 1 In fact, the basis of
`performing the extrapolation has proved fallacious. The true behaviour
`.;;.~'is shown for two olefins in Fig. 6. The curvature towards the origin in
`( the very early stages of the reaction denotes catalysis of the form
`roc [R02H]t, instead of the commonly observed r oc [R02H]. This in turn
`(
`implies that the hyd.roperoxide at low concentrations ( < I0- 2 molejl. in
`( the temperature range studied) yields radicals by a first-order decomposition,
`l which is superseded by a second-order decomposition at a higher concen(cid:173)
`
`··, tration. This ·unique change in decomposition order wlth concentration
`[':.
`16 Bateman and Morris; Trans. Faraday Soc., 1952, 48, 1149.
`17 Morris, Ph.D. Thesis, London, 1952.
`
`(
`
`-
`
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`

`
`158
`
`QUARTERLY REVIEWS
`
`appears to be associe"ted with the state of molecular association of the
`hydroperoxide :
`
`H
`2(R0 2H) ~ (RO·Q ...... H·O 2R)
`
`unimol./
`"" bimol.
`decomp.
`~decomp.
`2(RO· + ·OH)
`RO· + H 20 + R0 2 •
`Infra-red spectroscopy provides clear-cut evidence that the intense {
`association in the neat hydroperoxide becomes progressively less with:
`In general, conditions conducive to a low degree of association I
`dilution.15
`would be expected to emphasise the first-order decomposition. Consistently, t
`the catalytic form r' oc [R02H]t persists to higher concentrations at higher r
`tempe:ratures,17 and the addition ofmore strongly bonding substances than 1
`the peroxide itself suffices to change the observed catalysis from r oc [R0 2H]
`to r oc [R02H]t,l 5 • 17 The effect of temperature is significant in providing
`a probable explanation of the differences in hydroperoxide decomposition
`reported by different workers (cf. p. 155). The inference from the oxidation
`kinetics of a bimolecular mechanism relates to temperatures lower than
`about 80°, while the direct decompositions have mostly been studied at
`above 130° where the first-order dissociation will be greatly favoured.
`
`TABLE 5
`
`107e'k' I
`T
`Olefin
`T
`107e'k'
`~----0-le_fi_n _______ e_m_p. __ <_se_c._-1_) _· l---------l--e_m_p._ (sec.-1)
`
`Allyl benzene
`cycloHexene .
`Ethyllinoleate
`1-Methylcyclohex -1-ene
`1 : 3: 5-Tri~ethyl-
`cyclohex -1-ene
`
`75°
`55
`55
`45
`65
`65
`
`2·9
`0·1 5
`0·20
`0·45
`3·9
`3·7
`
`Dicyclohex- 2 -enyl
`2-Methyl~ct-2-ene .
`2-Methyl-4-phenyl-
`but-2-ene
`Digeranyl
`
`0·7
`6·9
`0·50
`1·6
`
`45°
`65
`55
`55
`
`45
`
`....
`(
`
`ll,
`
`\
`
`.. s
`The quantity e' k', analogous to ek", can likewise be determined for the ( 1:
`first-order hydroperoxide initiation process (Table 5).17 For the limited/ 8
`data available, no well-defined correlation with hydroperoxide type as in t
`the case of ek" can be recognised, but the predominantly tertiary derivatives' iJ
`again seem to be the more reactive.
`a
`'I
`4. The Direct Reaction between an Olefin and Oxygen
`:i:lh
`A natural consequence of the free-radical character of oxygen is that it c1
`should display in some measure the reac~ivity of R02 • radicals towards ti
`It is actually so much less reactive that direct olefin-o:Arygen inter· p
`olefins.
`action (RH + 0 2 --)-) has so far proved impossible to measure. As described[ tl
`in section _(3), instead of bein~ able to define this :ate relatively easily by al
`extrapolatmg the plot of r agamst [02]absorbed (as Frg. 5) to [02]absorbed = 0~
`(e.g., 6 x 10-6 mole 1.-1 sec. - 1 for tetralin at 75°), the true value is so many['
`times smaller that it is difficult to observe (see Fig. 6). The absorption of
`I
`
`I a
`
`Page 14 of 23
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`
`

`
`1
`
`BATEMAN : OLEFIN OXIDATION
`
`159
`118 little as 0·02~0·05 ml. (N.T.P.) of oxygen per ml. produces a degree of
`hydroperoxide . catalysis sufficient to obscure any possible initiation by
`djrect olefin-oxygen interaction.17 , 1s The latter clearly cannot be an
`observable component of the overall oxidation reaction at moderate tem(cid:173)
`peratures. The initiation step as a whole accounts for only 1/nth of the
`total products, where n is the chain length, and hydroperoxide decomposition
`110counts for nearly all of this fraction.
`Obviously no examination of the oxidation product can hope to provide
`1 i information on any non-hydroperoxiciic initiation.
`In fact it would appear
`that the only means of obtaining critical evidence on the direct olefin-oxygen
`l.
`1
`l L reaction is to study the system in the presence of a highly efficient inhibitor
`~ (which would prevent any primary peroxy-intermediate from becoming a
`'I hydroperoxide (therefore not a phenolic-type inhibitor).
`.
`·
`.I
`Hydroperoxide initiation is likewise predominant in photochemical
`oxidations. The formation of a very small amount of hydroperoxide
`has been sho·wn quantitatively to establish R0 2H + hv-+ rather than
`RH + hv-+ as the primary activation process. 3
`
`5. Metallic-ion Catalysis
`The intense activity of certain metallic compounds (notably those of
`iron, cobalt, nickel, copper, ancl·manganese) as oxidation catalysts is a
`matter of immense technological interest. The consequences can be both
`highly undesirable and advantageous. Thus the comparatively rapid
`-~>oxidative deterioration induced in rubber or lubricating oils calls for strict
`I preventive measures; while the use of cobalt compounds as "driers" to
`i promote the rapid oxidative hardening of unsaturated esters is all-important
`! in paint technology. Although knowledge of how these compounds act
`remains obscure in many details, the general picture is fairly clear. Of
`particular interest in the present context are certain distinctive kinetic
`characteristics.
`·
`The active metals are those having two or more valency states, clearly
`suggesting that an oxidation-reduction process is involved. They function
`via their ions, as is evident from the industrial practice of using the so-called
`sequestering (complex-forming) agents (e.g. ethylenediaminetetra-acetic acid)
`to counteract metallic contamination-the metal is converted from an ionic
`into a chelated form and thereby rendered innocuous. Obtaining of a suit(cid:173)
`able homogeneous reaction system for mechanistic studies is thus a difficulty .
`. The solvent employed so .far has been acetic acid, which is a catalyst for
`~1 hydroperoxide decomposition and therefore might be expected to create
`confusion in any direct comparison of results with those obtained for oxida(cid:173)
`tion in hydrocarbon solvents. Whether this is so or not remains to be
`, proved, but fortunately the catalysis by active metal salts is so great that
`the reaction can be studied under conditions where oxidation in acetic acid
`alone is negligible.
`Working with cobaltous acetate
`
`in acetic acid, Bawn and his
`
`18 Bateman, (Mrs.) Hughes, and Morris, Discuss. Faraday Soc., 1953, 14, 190.
`
`Page 15 of 23
`
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`
`

`
`160
`
`QUARTERLY REVIEWS
`
`co-workers 19 • 20 conclude that the rate-determining initiation process is
`R0 2H + M 3+
`____,.. R0 2• + MH + H+ ... n(M)
`where M3+ represents a complex tervalent cobaltic ion. The
`ion produced is immediately reconverted into M3 + by the much
`reaction;
`
`:
`1
`
`!
`
`R0 2H + MH ----)- RO· + Ma+ + OR-
`The sum of these consecutive reactions i~ seen to 1Je exactly the bimolecular I
`decomposition pattern proposed for the hydroperoxide by itself :
`'
`2R02H ____,.. R0 2 • + RO· + H 20
`Oxidation subsequently proceeds by the ordinary mechanism, i.e., in. f
`cf. p. 158, and the truly catalytic role of the metal salt is readily apparent.,
`volving reactions (2), (3), and (6) under the conditions employed.
`In 1
`conformity with this, the reaction shows autocatalysis in the earlier stages I
`and hydroperoxide is steadily formed. However, since the catalyst pro.l1
`motes the decomposition of the hydroperoxide so strongly, ri(M) will rapidly
`increase, the chain length {r/ri(M)} will decrease correspondingly, and we
`should expect the reaction soon to lose its chain character. For these r t
`circumstances, the formation and decomposition of the hydroperoxide 1 1
`become equal. This can arise, of course, independently of the mode of 1
`initiation, and the generalised kinetic changes and their detection experi-) t
`mentally have b