A fluorinated phosphite traps alkoxy radicals photogenerated at the air/solid interface of a nanoparticle

Article abstract of DOI:10.1002/poc.4115

With interests in alkoxy radical formation on natural and artificial surfaces, a physical-organic study was carried out with a Hammett series of triaryl phosphites (p-MeO, H, p-F, and p-Cl) to trap adsorbed alkoxy radicals on silica nanoparticles. A mecha

Full text of DOI:10.1002/poc.4115

Received: 22 May 2020
DOI: 10.1002/poc.4115
Revised: 6 July 2020
Accepted: 15 July 2020
R E S E A R C H A R T I C L E
A fluorinated phosphite traps alkoxy radicals
photogenerated at the air/solid interface of a nanoparticle
Goutam Ghosh1,2 | Alexander Greer1,2
1Department of Chemistry, Brooklyn
Abstract
College, Brooklyn, New York, USA
2Ph.D. Program in Chemistry, The
With interests in alkoxy radical formation on natural and artificial surfaces, a
physical-organic study was carried out with a Hammett series of triaryl phos-
phites (p-MeO, H, p-F, and p-Cl) to trap adsorbed alkoxy radicals on silica
nanoparticles. A mechanism which involves PhC (Me)2O• and EtO• trapping
in a cumylethyl peroxide sensitized homolysis reaction is consistent with the
results. The p-F phosphite was able to indirectly monitor the alkoxy radical
formation, and 31P NMR readily enabled this exploration, but other phosphites
of the series such as the p-MeO phosphite were limited by hydrolysis reactions
catalyzed by surface silanol groups. Fluorinated silica nanoparticles helped to
suppress the hydrolysis reaction although adventitious water also plays a role
in hindering efficient capture of the alkoxy radicals by the phosphite traps.
Graduate Center of the City University of
New York, New York, New York, USA
Correspondence
Alexander Greer, Department of
Chemistry, Brooklyn College, 2900
Bedford Avenue, Brooklyn, New York, NY
11210, USA.
Email: agreer@brooklyn.cuny.edu
Funding information
National Science Foundation,
Grant/Award Number: CHE-1856765
K E Y W O R D S
alkoxy radicals, nanoparticles, phosphite traps, photosensitization, reactive intermediates
1 | INTRODUCTION
surfaces, which we carry out here with triaryl phosphites
on silica particles, where 31P NMR is used.
There are difficulties in assessing surface effects on alkoxy
radicals and knowing the difference they may display at
the air/solid interface in the absence of solvent. Some chal-
lenges have been surmounted with the use of EPR. For
example, in 1994, Forbes et al. reported on a time-resolved
EPR study of organic radicals anchored to silica surfaces
to probe their magnetic and kinetic properties at the
solution/solid interface.[1–3] A unique facet of this study
was observations of stronger spin polarizations in radical
pairs tethered closely to the surface which disappeared in
longer tethers, where radical pairs behave similarly to
those in free solution. Norrish type I cleavage was used as
a means to study the biradicals after decarbonylation and
surface phenomena on these silica particles.[2]
Here, we use p-substituted aryl phosphites as traps
with potential alkoxy radical scavenging activity, due to
their oxophilic phosphorus atoms (Figure 1). The trap-
ping of cumyloxy and ethoxy radicals generated by the
photosensitization of cumylethyl peroxide 2 was investi-
gated with four phosphites, (p-X-C6H4-O)3P [X = OMe
(3a), H (3b), F (3c), and Cl (3d)]. Our method is indirect
and uses product identification to quantitate alkoxy
radical formation at the air/silica interface. Dione 1,
cumylethyl peroxide 2, and a Hammett series of triaryl
phosphites were adsorbed on silica in N2-degassed ves-
sels, in which 1 was irradiated with (280 < λ < 700 nm)
light. This Hammett study of a series of triaryl phosphites
provide an approach to the study of alkoxy radicals on
non-porous nanoparticle surfaces. After photolysis or
heating, the compounds were desorbed from the fumed
silica surface, and 31P NMR was used to quantitate the
products formed.
Natural and artificial particles have been reported to
bear surface radicals, in some instances persistent
radicals,[4–9] where EPR is often used. Thus, there is a
need for additional trapping methods for radicals on
J Phys Org Chem. 2020;e4115.
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© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.4115

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GHOSH AND GREER
FIGURE 1 Proposed paths (A-C) for photochemical and thermal reactions of 4,40-dimethylbenzil (dione) 1 and cumylethyl peroxide
2 at the gas/silica interface probed by triaryl phosphite trapping agents. A Hammett series of p-substituted triaryl phosphites (p-X-C6H4-O)3P
[X = OMe (3a), H (3b), F (3c), or Cl (3d)] was used to probe the formation of triaryl phosphates (p-X-C6H4-O)3P=O 4a-4d, aryl hydrogen
phosphonates (p-X-C6H4-O)P(=O)OH 5a-5d, and by-products
As we will see, the results show that the tris(4-F-phe-
nyl) phosphite yielded alkoxy radical scavenging activity.
In contrast, the hydrolysis on silica points to the more
nucleophilic phosphites undergoing protonation and
acceleration in aryl hydrogen phosphonate formation.
The implications for the competition between phosphite's
alkoxy radical scavenging and conversion to aryl hydro-
gen phosphonate as a function of surface conditioning
(i.e., removal of SiOH sites by fluorination) are discussed.
Interestingly, the phosphite hydrolysis leads to phenol
derivatives, which themselves bear antioxidant character
and will be discussed.
and fluorinated silica (Figure 2). After desorption from
the particles, the percent yields of products were deter-
mined by 31P NMR (Table 1). A control photoreaction
study with dione 1 and phosphites 3a-3d, with no per-
oxide 2 present showed that the percent phosphate
formed was 0% on native silica and 0-3% on fluori-
nated silica. The percent of phosphate formed is
dependent on light and the photosensitized generation
of alkoxy radicals. Photons from the light source
(280 < λ < 700 nm) are mainly absorbed by 4,4’-
dimethylbenzil 1 due to its ꢁ10-fold greater absorption
in the blue region at 280 nm than the phosphites. The
irradiation of dione 1, peroxide 2 and phosphites 3a-
3d, increased the percent yield of phosphate up to
10%. We attribute the increase to the interaction of
RO• and R'O• with the phosphites on the native and
fluorinated silica particles. However, the alkoxy radical
trapping by phosphites was relatively low yielding for
3c (X = F, 7-10%) and 3d (X = Cl, 1-2.5%) and yielded
no phosphate for 3a (X = OMe, 0%) and 3b (X = H,
0%). As we will show later, the absence of alkoxy radi-
cal activity of phosphites (X = OMe and H) is due to
their consumption (i.e., loss) by surface hydrolysis to
aryl hydrogen phosphonates. Another kind of loss
involves the lower molecular weight R'O•, which is
not easily detected in the headspace of the air/solid
experiment, but is detected in a homogeneous reaction
in CD3CN as downstream products CH4 and MeCHO
(Figure S6, see the Supporting Information section).
Intermolecular and intramolecular reactions[10,11] of
the higher molecular weight RO• takes place, where
we also detect cumyl alcohol, β-methylstyrene, and
acetophenone. Next, we examined the alkoxy radical
trapping in connection (and competition) with a chela-
tion reaction.
2 | RESULTS AND DISCUSSION
Native silica particles or fluorinated particles were
adsorbed with dione 1 (0.01 mmol/g silica), cumylethyl
peroxide 2 (0.1 mmol/g silica), and triaryl phosphites
(in pairs of 3a and 3b, or 3b and 3c, or 3b and 3d) (each
0.1 mmol/g silica) in a N2-degassed vessel and irradiated
with (280 < λ < 700 nm) light, or maintained at room
temperature (25ꢀC) or heated to 35ꢀC. Each reaction was
carried out for 1 h. Compounds were then desorbed from
the silica particles with a polar solvent and the product
ratios measured by31P NMR. The experimental evidence
supports the mechanism in Figure 1, which has three
paths (paths A-C), as we will discuss next.
2.1 | Path A. Interaction of RO• with aryl
phosphites at the air/particle interface
We carried out the reaction of dione 1 and cumylethyl
peroxide 2 in the presence of aryl phosphites on native

GHOSH AND GREER
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FIGURE 2 Reactions of PhC(Me)2O•
and EtO• radicals with triaryl phosphite
traps and other conversions to products
(path A)
TABLE 1 Triaryl phosphate 4a-4d percent yield measurements in photosensitized or heated reactions of dione 1, cumylethyl peroxide
2, and (p-X-C6H4-O)3P 3a-3d on native and fluorinated silica nanoparticlesa,b
Phosphate 4a-4d formed
Phosphate 4a-4d formed Phosphate 4a-4d formed
upon irradiation
[path A (%)]c,d
by irradiation
and heat
[paths A + B (%)]c,d
by heating to 35ꢀC
[path B (%)]c,d
Dione 1:
Phosphite
Peroxide 2:
Entry (p-X-C6H4-O)3P Phosphite ratio
SiO2
0
F-SiO2
SiO2
2
F-SiO2
9.5
7
SiO2
2
F-SiO2
9.5
1
2
3
4
X = OMe 3a
X = H 3b
X = F 3c
1:10:10
1:10:10
1:10:10
1:10:10
0
0
0
6.5
10
6.5
20
7
10
1
7
10
17
X = Cl 3d
2.5
10
10
11
12.5
aDione 1 (0.01 mmol/g silica), 2 (0.1 mmol/g silica), and 3a-3d (in pairs with each 0.1 mmol/g silica) were adsorbed on native silica (SiO2) or
fluorinated silica (F-SiO2) (0.2 g) and irradiated or heated.
bSamples were irradiated with (280 < λ < 700 nm) light or heated to 35ꢀC.
cThe triaryl phosphates 4a-4d were detected by 31P NMR. The data are the average of two runs with 10-15% error.
dThe percent yields of triaryl phosphates 4a-4d were based on their integrated peak areas without the use of an external standard.
2.2 | Path B. Competing alkoxy radical
trapping and chelation of phosphites at a
silica surface: More than one way to the
phosphate products
The data are rationalized by suggesting the interac-
tion of phosphite and dione 1 leads to a 2,2,2-triaryloxy-
1,3,2λ5-dioxaphosphole intermediate (Figure 3). The
chelation is a straightforward to the formation of the
dioxaphosphole, and is expected to react further by
an intramolecular migration of the p-tolyl group to
reach the ketene [2,2-bis(p-tolyl)ethen-1-one] and triaryl
phosphate [(p-X-C6H4-O)3P=O]. The source of triaryl
phosphate was examined, in which the chelation of the
phosphite relies on the conversion of the trans-dione 1 to
the reactive syn- or syn-skewed-dione 1 conformer.[12]
The results in path B supplement others[13–16] and our[12]
previous work on phosphite chelation of diones and the
NMR detection of the transient dioxaphosphole. Conceiv-
able by-products are also α-hydroxyvinyl phosphates,
but were not detected by NMR. While this experimental
The reaction of dione 1 (an α-diketone) and phosphites
3a-3e was followed in the formation of phosphates 4a-4e
on the native and fluorinated silica particles (Table 1,
path B). After 1 h at 25ꢀC (in subdued room light), the
percent yields of phosphates 4a-4e on the fluorinated sil-
ica ranged from 0 to 3%. The percent phosphate formed
was found to be dependent on heating of the sample.
After 1 h at 35ꢀC, the yields of phosphates 4a-4e on the
fluorinated surface increased and ranged from 7 to 10%.
The yield in the latter can be rationalized by the encoun-
ter of dione 1 and phosphite in equimolar amounts.

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GHOSH AND GREER
FIGURE 3 Chelation of
phosphites with dione 1 on the
silica surface (path B)
method led to the production of ketene, it was not
exploited for studies of ketenes at the air/solid surface.
through the diaryl phosphinic acid and cease at aryl
hydrogen phosphonate yielding two equivalents of the
p-substituted phenol. The reaction does not continue
further to produce phosphoric acid and a third mole of
p-substituted phenol. The importance of electronic inter-
actions of the (p-X-C6H4-O)3P compounds is also
observed by a Hammett plot of log kX/kH vs σp to give a
reaction constant ρ of −1.25 on native silica and ρ of
−2.34 on fluorinated silica (Figure 5). Even though the ρ
value on fluorinated silica is greater, the hydrolytic reac-
tivity of the aryl phosphites is significantly higher on
native silica. In the case of fluorinated silica, the
Hammett series of triaryl phosphites function as nucleo-
philes, but with surface-adsorbed water as the proton
source instead of silanols, as the silanols have been rep-
laced with fluorosilanes.
2.3 | Path C. Surface effects and
substituent effects on the hydrolysis of
triaryl phosphites
Next, we sought to determine whether surface effects and
the tris(4-X-phenyl) electronic effects are relevant in the
hydrolysis of the triaryl phosphites. As is shown in
Table 2, both play a role. Upon heating the sample
to 35ꢀC, the formation of aryl hydrogen phosphonates
5a-5d was observed by 31P NMR and detected in varying
percent yields. On native silica, the aryl hydrogen pho-
sphonate was the major product ranging from a high of
97% for 5a to a low of 38% for 5d. This percent yield
decreased on fluorinated silica, ranging from a high of
30% for 5a to a low of 0% for 5d. Path C in Figure 4 shows
a proposed mechanism for a surface acid catalyzed
hydrolysis of the phosphites to reach aryl hydrogen pho-
sphonates and p-substituted phenols. The proposed
mechanism is similar to previous reports on phosphite
hydrolysis.[17,18] The hydrolysis is thought to proceed
3 | MECHANISM
We took advantage of a photosensitized homolysis of
PhC(Me2)COOEt to reach RO• and R'O• radicals [R = C
(Me2)Ph; R' = Et] and probe their reactivity at the air/-
solid interface with phosphite trapping agents (3a-3d).
The trapping of the alkoxy radicals afforded phosphates,
although it was not determined whether the RO• and
R'O• are in competition with the (p-X-C6H4-O)3P traps as
some R'O• may volatilize away from the surface and
escape capture under our conditions. However, in a
homogeneous experiment, the detection of trace CH4 and
MeCHO point to the intermediacy of CH3•, EtO•, and Ph
(Me2)CO•, and the idea that these radicals undergo H-
atom transfer reactions.[19–21]
The relatively small amount of phosphate formed in
the photolysis of dione 1, peroxide 2, and (p-X-C6H4-
O)3P makes a mechanistic study of surface-bound alkoxy
radicals challenging. The p-F phosphite 3c revealed the
highest trapping yield of alkoxy radicals with 10% p-F
phosphate 4c. Our results are not contrary to the
expected electrophilicity of alkoxy radicals[22] due to
their ease in forming an alkoxide anion upon accepting
an electron rather than oxylium ion (RO+) by electron
loss. Instead, the surface SiOH groups accounts for the
ability of the p-F phosphite 3c to monitor the alkoxy
radical formation, but not other phosphites in the series
such as the p-MeO phosphite 3a, in which surface effects
are less important for H-atom transfer compared to
Brønsted acidity.
TABLE 2 Aryl hydrogen phosphonate 5a-5d percent yield
measurements in photosensitized or heated reactions of dione 1,
cumylethyl peroxide 2, and (p-X-C6H4-O)3P 3a-3d on native and
fluorinated silica nanoparticlesa,b
Phosphonate
5a-5d yield
[path C (%)]c,d
Dione 1:
Peroxide 2:
Phosphite
Entry (p-X-C6H4-O)3P Phosphite ratio SiO2
F-SiO2
1
2
3
4
X = OMe 3a
X = H 3b
X = F 3c
1:10:10
1:10:10
1:10:10
1:10:10
97
76
43
38
30
6
5.5
0
X = Cl 3d
aDione 1 (0.01 mmol/g silica), 2 (0.1 mmol/g silica), and 3a-3d (in
pairs with each 0.1 mmol/g silica) were adsorbed on native silica
(SiO2) or fluorinated silica (F-SiO2) (0.2 g) and irradiated or heated.
bSamples were heated to 35ꢀC.
cThe aryl hydrogen phosphonates 5a-5d were detected by 31P NMR.
The data are the average of two runs with 10-15% error.
dThe percent yields of aryl hydrogen phosphonates 5a-5d were
based on their integrated peak areas without the use of an external
standard.

GHOSH AND GREER
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FIGURE 4 Triaryl
phosphite hydrolysis and the
formation of aryl hydrogen
phosphonate and phenol
(path C)
FIGURE 5 Hammett plots of log
kX/kH vs σp for the formation of aryl
hydrogen phosphonates 5a-5d in
the hydrolysis of 3a-3d, respectively
on native silica (left plot:
Y = −1.2465x - 0.02, R2 = 0.8769)
and on fluorinated silica (right plot:
Y = −2.3421x + 0.0564 R2 = 0.9843)
The Hammett series of phosphites are susceptible to
hydrolysis and influenced by both surface proticity and
electronic effects. Proton transfer from ionizable SiOH
groups to phosphite can explain why the hydrolysis yield
is increased with electron donating substituents, for
example, formation of 80% 5a and 14% 5d on native sil-
ica. Slowing of the hydrolysis reaction is possible by sur-
face fluorination and removal of the SiOH sites and thus
dampen the path to aryl hydrogen phosphonate. The
hydrolysis mechanism can be considered, but thought to
be improbable by invoking a SiO− anion Arbuzov-type
rearrangement as was suggested for trimethyl phosphite
adsorbed on silica surface,[23,24] or a photo-Arbuzov type
reaction as seen with phosphites and aryl halides in C–P
bond formation.[25] Also, we do not invoke chemistry
arising from the direct irradiation of the phosphites,
as has been reported for phosphines and phosphites,
for example with arenediazonium salt reductants,[26–28]
FIGURE 6 31P NMR spectra of
3a-3d, 4a-4d, and 5a-5d in
acetonitrile-d3 following their
desorption from native silica after a
photoreaction of dione 1 and
cumylethyl peroxide 2 in the
presence of triaryl phosphites 3a-3d,
respectively

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GHOSH AND GREER
TABLE 3 Proton decoupled 31P NMR chemical shifts for (p-X-C6H4-O)3P 3a-3d, (p-X-C6H4-O)3P=O 4a-4d, and (p-X-C6H4-O)P(=O)(H)
OH 5a-5d
Compd
3a
(δppm
130.0
15.3
)
(δppm
)
Compd
3b
(δppm
129.0
-17.0
1.3
)
(δppm
)
Compd
(δppm
128.8
-16.6
1.9
)
(δppm
)
Compd
3d
(δppm
128.0
-17.5
1.4
)
(δppm)
129.3a
-15.6a
NA
128.0a
-17.0a,b
1.3c-f
3c
4c
5c
127.8a
-16.6a
NA
127.0a
-17.5a
NA
4a
4b
4d
5a
2.6
5b
5d
aRef. 53.
bRef. 58.
cRef. 17.
dRef. 59.
eRef. 60.
fRef. 61.
which can undergo one-electron oxidation to radical cat-
ions and couple with oxygen, or in aryl phosphites with
P–O cleavage to reach a triplet aryl cation and hydrogen
phosphonate.[29] In our case, dione 1 is selectively irradi-
ated and sensitizes the homolyis of 2 to reach alkoxy radi-
cals in an oxygen-free environment. Our vessel was
degassed with N2 so that photooxidation chemistry is
unavailable, as was observed for phosphine–1O2 reactions
to form dioxaphosphorane intermediates[30,31] or via radi-
cal cation peroxy species with eletron-poor sensitizers,
such as 9,10-dicyanoanthracene.[32,33]
Our results suggest a give-and-take where proton-
ation enhances the conversion of (p-X-C6H4-O)3P
(a modest antioxidant) to p-X-C6H4-OH (a superior anti-
oxidant) depending on electronics and surface effects.
Along these lines, in previous studies of the oxidation of
alkyl phosphites,[12] (MeO)3P is reactive with alkoxy rad-
icals, but its hydrolysis product MeOH is modest antioxi-
dant requiring ꢁ0.5 M concentrations to intercept
reactive oxygen species.[34] Unlike alkyl alcohols, pheno-
lic compounds have high antioxidant potencies and thus
are often used in tenths or single digit millimolar con-
centrations, such as in 5-amino salicylic acid[35] and
butylated hydroxytoluene (BHT).[36] In terms of our sys-
tem for alkoxy radical photogeneration at the air/solid
interface, it bears some resemblance to surface-bound
photochemical[37–39] and sensitization reactions,[40,41]
and selective photooxidation chemistry,[42–48] although
usually in porous media unlike our current study on
nonporous nanoparticles.
phosphites 3a-3d undergo alkoxy radical oxidation, che-
lation, and hydrolysis on silica nanoparticles by different
mechanisms. These mechanisms are summarized in
Figure 1 (paths A-C).
Despite relatively few reports,[49–51] the sensitized
homolysis of an organic peroxide is used here to
provide a facile source of alkoxy radicals. In our
case, cumyloxy and ethoxy radicals are generated and
react with (p-F-C6H4-O)3P and to a lesser extent
(p-Cl-C6H4-O)3P suggesting that these alkoxy radicals do
not efficiently acquire H-atoms from surface SiOH groups
or surface-adsorbed water. However, the surface proticity
can readily catalyze the hydrolysis of more electron rich
phosphites, such as 3a, thereby releasing of p-substituted
phenols. Interestingly, this opens up the possibility for
studying increases in antioxidant power as phenols release
from their phosphites.
Future work can include an exploration of the radical
scavenging ability of phenols p-X-C6H4-OH compared to
the steric BHT-type phenolic antioxidants. Furthermore,
sensitized peroxide homolysis experiments for RO• •OR0
to reach chain-terminated (scrambled) ROOR and
R'OOR0 products could provide a means to assess the rad-
ical migratory aptitude on the nanoparticles.
5 | EXPERIMENTAL
5.1 | General
The following reagents were purchased from Sigma
Aldrich and used as received: hydrophilic fumed silica
nanoparticles (200-300 nm diameter, 200 25 m2/g sur-
face area), 4,40-dimethylbenzil 1, cumene hydroperoxide,
ethyl bromide, (PhO)3P 3b, 4-methoxyphenol, p-cresol,
4-fluorophenol, 4-chlorophenol, phosphorous trichloride,
trimethyoxy(3,3,4,4,5,5,6,6,6-nonafluorohexyl)silane,
tetra-N-butylammonium bromide (TBAB), KOH, NaOH,
MgSO4, pyridine, cyclohexane, and diethyl ether. The
4 | CONCLUSION
Surface-bound radicals can be challenging to study, and
yet are prevalent on atmospheric particles and biological
surfaces. Despite progress with EPR, 31P NMR and phos-
phite traps can offer advances to the field, as is demon-
strated here. Our results demonstrate that triaryl

GHOSH AND GREER
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following solvents were purchased from VWR and used
as received: acetonitrile, acetonitrile-d3, chloroform-d,
methanol, dichloromethane, and hexanes.
purified by silica gel column chromatography using 4:1
dichloromethane-hexanes as an eluent to afford phos-
phates 3a, 3c, and 3d, each as a colorless viscous oil.
5.2 | Synthesis of cumylethyl peroxide 2
5.4 | Silica preparation
Cumylethyl peroxide 2 is a known compound and was
synthesized using a literature procedure.[52] Briefly, KOH
(6 mmol, 0.673 g), phase-transfer catalyst (TBAB,
0.3 mmol, 0.010 g), and cumene hydroperoxide (3 mmol,
0.456 g) were stirred in cyclohexane (3.0 mL) at 50ꢀC for
30 min. Then, a solution of ethyl bromide (3 mmol,
0.222 mL, 0.327 g) in cyclohexane (2 mL) was added and
the reaction stirred at 50ꢀC for 4 h. Afterward, by-product
KBr was filtered off and the reaction mixture poured into
a separatory funnel and the cyclohexane layer separated.
The cyclohexane layer was washed with 5% aqueous
NaOH, and then washed with water three times. The
solution was dried over MgSO4, after which the cyclohex-
ane was evaporated at reduced pressure to afford 2. The
1H NMR spectrum (Figure S2) showed the presence of
impurities, in which an HPLC trace (Figure S3) pointed
to 82% purity of 2 based on its relative peak area to those
of side-products, such as cumene hydroperoxide and
cumyl alcohol. Upon weighing the sample with an ana-
lytical balance and considering the 82% purity, a 74%
yield of 2 is calculated. An external standard was not
used in calculating the percent yield. 1H NMR (400 MHz,
CDCl3) δ 7.55 – 7.48 (m, 3H), 7.38 – 7.35 (m, 2H), 4.01 (q,
J = 7.0 Hz, 2H), 1.63 (s, 6H), 1.18 (t, J = 7.0 Hz, 3H).
Fluorinated fumed silica was prepared based on a
method we previously reported,[54] which is similar
to other reports.[55,56] Hydrophilic fumed silica
nanoparticles (0.41 g) were placed into a 30 mL toluene
solution of trimethyoxy(3,3,4,4,5,5,6,6,6-nonafluorohexyl)
silane (18 mmol, 6.62 g) and refluxed for 24 h under a N2
atmosphere. The loading of the fluorosilane to the surface
by siloxane bonds was approximately 1.45 mmol/g of sil-
ica, where any fluorosilane not covalently bound to
the particles was removed by Soxhlet extraction with
methanol. Compounds were adsorbed to native and fluo-
rinated silica in a manner previously reported.[51,57] In
subdued room light, 4,40-dimethylbenzil 1 (0.01 mmol),
cumylethyl peroxide 2 (0.1 mmol), and triaryl phosphites
3a-3d (pairs of them each 0.1 mmol) were dissolved in
5 mL dichloromethane and stirred with 1.0 g of native sil-
ica or fluorinated silica particles for 1 h in a 25 mL round
bottom flask. The dichloromethane was evaporated from
the particles with a stream of N2 gas and then by apply-
ing a vacuum. The percent loading of adsorbed sensitizer
1, peroxide 2, and phosphites 3a-3d was calculated to be
0.7%, 7%, and 7%, respectively (equations 1-5). Equation 1
gives the number of silanol groups per gram of native sil-
ica as the commercial sample contains about 4 silanol
groups/nm2. Equation 2 shows the number of moles of
silanol per gram of silica by dividing number of silanol,
obtained in equation 1, with Avogadro's number
6.0221367 × 1023 mol−1 (NA). Dividing the moles of 1 or
2 or 3a-3d with moles of silanol groups in equation 2,
give the percent loading of each via equations 3, 4, and
5, respectively.
5.3 | Synthesis of tris(4-X-phenyl)
phosphites [X = MeO (3a), F (3c), and Cl
(3d)]
Phosphites 3a, 3c, and 3d are known and were synthe-
sized using a literature procedure[53] in 40%, 27%, and
64% yields, respectively. The percent yields were deter-
mined after purification by column chromatography and
weighing of the dried compounds on an analytical bal-
À
Á
Number of silanol=g of SiO2 = 200 × number of silanol=m2 ð1Þ
200 × ðnumber of silanol=m2Þ
Moles of silanol=g of native silica =
ð2Þ
ð3Þ
NA
1
ance. The H NMR spectra did not show the presence of
moles of 1
moles of silanol=g of SiO2
impurities, and thus the percent purities of phosphites
3a, 3c, and 3d were estimated to be ꢁ99%. Briefly, to a
solution of 19.6 mmol p-substituted phenol and 25 mmol
pyridine in 20 mL diethyl ether was added dropwise
5.0 mmol phosphorus trichloride at 0ꢀC under a N2 atmo-
sphere. Then the reaction mixture was stirred under N2
at 25ꢀC for 1.5 h, and subsequently quenched with water.
The diethyl ether phase was separated, washed with
water and brine, dried over MgSO4, and concentrated
under reduced pressure. The residue for each sample was
%loading of 1 =
moles of 2
moles of silanol=g of SiO2
%loading of 2 =
ð4Þ
ð5Þ
moles of 3a−3d
moles of silanol=g of SiO2
%loading of phosphite 3a−3d =
Adsorbed compounds are assumed to be dispersed
homogeneously on the silica nanoparticle surfaces.

8 of 9
GHOSH AND GREER
Expected loading is on the outer layer of the particles
as there is no pores in these fumed silica samples. No
special precautions were used to remove physisorbed
water.
ACKNOWLEDGEMENT
National Science Foundation, Grant/Award Number:
CHE-1856765. We thank Leda Lee for the graphic
arts work.
ORCID
Alexander Greer https://orcid.org/0000-0003-4444-9099
5.5 | Photolysis and heating
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Ghosh G, Greer A. A
fluorinated phosphite traps alkoxy radicals
photogenerated at the air/solid interface of a
nanoparticle. J Phys Org Chem. 2020;e4115. https://
doi.org/10.1002/poc.4115
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