Production of Free Radicals by a Photo-Excited Electron and Transfer Reaction

162 Pages·44.133 MB·English

by  Bates, H. G. C

of free ra d ic a ls by a photo-excited
electro n tra n sfe r react!on .
A th e sis presented in fu lfilm en t of the
requirem ents for the degree of Doctor of
Philosophy in the V icto ria U niversity »
M anchester by H.G.C*Bates •
October , 1951*
ProQuest Number: 13916734
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The author w ishes to thank Prof«M«Q«Evans ,F .R .8.
and D r.N .U rl fo r th e ir guidance and advice , also
h is colleagues D r.i.Santappa 9 'e s s rs . G .J.B realey,
B.Copestake *nd J*Simons fo r th e ir encouragement
and suggestions • He is indebted to the I.C .I .,
D yestuffs D ivision fo r a grant during the course
of th is work •
CONTENTS
Page.
I. INTRODUCTION .
§a. Free ra d ic a ls - HOg , OH and other
ra d ic a ls • 1
§b. E lectron tra n sfe r spectra and io n -p air
complexes • 15
§c. E lectron tra n sfe r reactio n s in aqueous
so lution •
(i) Photopolym erisation • 23
fii) Photo-oxidation of ftater • 25
5d. O xidation of organic su b strates in
aqueous solutio n •
(I) *h o to -in itia te d oxidations • 26
(ii) R eactions w ith F enton's reagen t • 29
$e. R adical su b stitu tio n in the benzene
nucleus . 32
I I . Purpose of p resent in v estig atio n • 57
III.A pparatus , Reagents and E stim ations • 39
IV. Experim ental re s u lts • 55
V . DISCUSSION
A. K inetic schemes . 96
B. In te rp re ta tio n of re s u lts • 103
114
VI. Summary •
References •
Sa.FRB;: RADICALS,
Th6 im portance of the ro le of u n stab le Interm ediates
in the mechanism of reac tio n s in both gas and liq u id
phases has bean re a lise d but com paritatively recently*
These interm ed iates are free n eu tra l atoms o r ra d ic a ls
or ionic species.S ince Hey and W aters1 reviewed th is
su bject in 1937 th ere has been an extrem ely rap id
2
development as exem plified by Weiss whloh has led to
the elu cid a tio n of mechanisms fo r re a c tio n s such as
polym erisation and autoxidation.Som e of these in te r ­
m ediates and th e ir reac tio n s are discussed below*
The perhvdroxyl ra d ic a l - HQo
H .S.Taylor^ was the f i r s t to p o stu late the ex isten ce
of th is ra d ic a l whloh is assumed to play an e s se n tia l
p a rt in the reac tio n between tydrogen and oxygen.It has
been suggested by Haber th a t the ra d ic a l is formed
»
when tydrogen atom s,form ed in a discharge tube, re a c t
w ith m olecular oxygen in the presence of a th ird body.
Working a t low tem peratures,G eib and H artek-5 obtained
from th is reactio n an un stable product having an atom ic
ra tio of hydrogen to oxygen of one to one.Evidence which
strongly supports the assum ption of ra d ic a ls comes
from stu d ie s of gas phase reactio n s above
E ltenton used a mass spectrom eter to examine low pressure
flam es of oxygen and methane and propane and the re s u lts
2-
in d lc ate the form ation of H02 ra d ic a ls during the course
of hhe re a c tio n s.
C ertain photochem ical reac tio n s ocourrlng in the gas
phase have been explained as Involving the occurrence of
the perhvdroxyl rad ical.T he photochem ical form ation of
hydrogen peroxide from hydrogen and oxygen, both in the
presence and absence of ex cited mercury atom s, has been
in te rp re te d as due to the form ation and in te ra c tio n of
HOg ra d ic a ls^ . S im ilarly the production of these ra d ic a ls
is suggested as the cause of the in h ib itio n by m olecular
oxygen of the photochem ical reactio n between hydrogen
and ch lo rin e 0 and the decom position of hydrogen iodide11.
12
Mlnkoff ' has summarised the evidence fo r the existence
of the H02 ra d ic a l in the gas phase.
When oxygen is passed over molten potassium , an orange*
1 3
red compound is formed ,a sim ila r compound being produced
14
by the In te ra c tio n ofozone and so lid potassium hydroxide •
The form ula of the compound in the l a t t e r case was
assumed to be » however Neumann1^ showed th a t the
anion 0^ would in a l l p ro b ab ility be co lo u rless and th a t
th erefo re the most lik e ly form would be an 0^ ion and
hence the formula K02 (which may be regarded as a s a lt
Of HOg ") .
V
During in v e stig a tio n s of the perhydroxyl ra d ic a l in
so lu tio n , W eiss1^ compared the spectrum of ozone in
aqueous so lu tio n w ith th a t obtained in aqueous potassium
hydroxide solutions the re s u lts showed th e form ation of
the ion Og ,from which one may presume the occurrence of
the reac tio n
o3 + oir —> 05 + ho2
This accounts fo r the sp eo tral and chem ioal p ro p erties
of aqueous ozone so lu tio n s and was fu rth e r used by 3 elss
to explain q u a n tita tiv e ly the decom position of ozone in
aqueous so lu tio n and the reac tio n s between ozone and
hydrogen peroxide. I t is suggested th a t the re s u lts show
th a t decom position takes place through the chain reaotio n
0 j + OH —► 02 + H02
0 HOg —> 202 + OH
A ccording to Taube and Bray1^ the i n i t i a l step is
HgOg + 0^ —* HO -f H02 + 02
the subsequent re a c tio n s being
HO + HgOg —> H^>0 + HOg
HO + 0 3 — > H02 + 02
ho2 + ho2 —> h2o<: + o2
Volman^6 who studied the photochem ical reactio n
between ozone and hydrogen peroxide regarded the ln ltla tfrg
step of the chain reac tio n as
0 ♦ fi202 ----* H02 + OH ,
the oxygen atom being formed from the ozone in the
process of lig h t ab so rp tio n ,
Taube and Bray1? had in v estig ated the c a ta ly s is by
v ario u s ca tio n s of the ozone-peroxide reac tio n and th is
was followed up by H ill1^ who studied the klnetlw e of the
decom position by oobaltous ion of ozone in the absence of
hydrogen peroxide and proposed the follow ing reac tio n
scheme,
Co**V 03 4 H20 — > CoOH*** Og 4 HO
HO 4* O3 — ¥ HOg + 02
+2 +2
H02 4 CoOH * Co + HgO 4 02
Au toxic) a t ion reactio n a in so lu tio n .
On the b asis of co n sid eratio n s Introduced by Franck,
20 21 22
Haber e t a l 9 9 a sim ple mechanism explaining
au to x id atio n reactio n s was developed by W e iss^ • He
sta te d th a t th ere is an electro n tra n s fe r reactio n from
the reducing agent to oxygen forming the m olecular oxygen
ion which is in equilibrium in aqueous so lu tio n w ith the
perhydroxyl ra d ic a l
02 4 H3O* HOg 4 HgO
Thus the form ation of hydrogen peroxide*found in many of
these re a c tio n s, may be explained.
•5'
0^5 *♦* 9 y Og
HOg + e — ► HOg
HOg + H* 5=± HgOg
24
S im ilarly the tra c e s of ozone sometimes found oan be
explained,
2HOg — * HgO ♦ 0 3
One may take as an example the oxidation of ledoo-dyes
and of m etal ions in solution.A ccording to Baxendale and
Lewin , the ra te determ lng step fo r the oxidation of
leuco-phenol indophenola is a hydrogen or electro n tra n sfe r
re a c tio n .
Primary io n isa tio n QHg ;?=£ QH* + H*
Rate deternjfing step Og + QH" — ► Q* +• HOg or
Og + QH" * QH + Og
The seoond a lte rn a tiv e had been previously assumed by
Weiss*' , who had suggested as the mechanism fo r the
subsequent re a c tio n s,
0“ + h* ;= £ HOg
QH" + HOg — ► QH + HO 5
2 QH — * Q + QH2
QH +■ 0” — ► Q ♦ HOg
HOg 4 li ^ ” HgOg •