The hydroxyl radical reaction rate constant and products of 3,5-dimethyl-1-hexyn-3-ol

Article abstract of DOI:10.1002/kin.20027

A bimolecular rate constant, ^κDHO, of (29 ± 9) × 10^-12 cm³ molecule^-1 s^-1 was measured using the relative rate technique for the reaction of the hydroxyl radical (OH) with 3,5-dimethyl-1-hexyn-3-ol (DHO, HC≡CC(OH)(CH₃)CH₂CH(CH_{3 2}) at (297 ± 3) K and 1 atm total pressure. To more clearly define DHO's indoor environment degradation mechanism, the products of the DHO + OH reaction were also investigated. The positively identified DHO/OH reaction products were acetone ((CH_{3 2}C=O), 3-butyne-2-one (3B2O, HC≡CC(=O)(CH₃)), 2-methyl-propanal (2MP, H(O=)CCH(CH₃)₂), 4-methyl-2-pentanone (MIBK, CH₃C(=O CH₂ CH(CH₃)₂), ethanedial (GLY, HC(=O C(=O)H), 2-oxopropanal (MGLY, CH₃C(=O)C(=O)H), and 2,3-butanedione (23BD, CH₃C(=O)C(=O)CH₃). The yields of 3B2O and MIBK from the DHO/OH reaction were (8.4 ± 0.3) and (26 ± 2)%, respectively. The use of derivatizing agents O-(2,3,4,5,6-pentalfluorobenzyl)hydroxylamine (PFBHA) and N,O-bis (trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction products were formed. The elucidation of these other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible DHO/OH reaction mechanisms based on previously published volatile organic compound/OH gas-phase reaction mechanisms.

Full text of DOI:10.1002/kin.20027

The Hydroxyl Radical
Reaction Rate Constant
and Products of
3,5-Dimethyl-1-hexyn-3-ol
J. R. WELLS
Exposure Assessment Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and
Health, 1095 Willowdale Road, Morgantown, WV 26505
Received 9 January 2004; accepted 14 May 2004
DOI 10.1002/kin.20027
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A bimolecular rate constant, kDHO, of (29 9) × 10−12 cm3 molecule−1 s−1
was measured using the relative rate technique for the reaction of the hydroxyl radical
(OH) with 3,5-dimethyl-1-hexyn-3-ol (DHO, HC CC(OH)(CH3)CH2CH(CH3)2) at (297 3) K
and 1 atm total pressure. To more clearly define DHO’s indoor environment degrada-
tion mechanism, the products of the DHO + OH reaction were also investigated. The pos-
itively identified DHO/OH reaction products were acetone ((CH3)2C O), 3-butyne-2-one
(3B2O, HC CC( O)(CH3)), 2-methyl-propanal (2MP, H(O )CCH(CH3)2), 4-methyl-2-pentanone
(MIBK, CH3C( O)CH2CH(CH3)2), ethanedial (GLY, HC( O)C( O)H), 2-oxopropanal (MGLY,
CH3C( O)C( O)H), and 2,3-butanedione (23BD, CH3C( O)C( O)CH3). The yields of 3B2O
and MIBK from the DHO/OH reaction were (8.4 0.3) and (26 2)%, respectively. The
use of derivatizing agents O-(2,3,4,5,6-pentalfluorobenzyl)hydroxylamine (PFBHA) and N,O-
bis(trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction prod-
ucts were formed. The elucidation of these other reaction products was facilitated by mass
spectrometry of the derivatized reaction products coupled with plausible DHO/OH reaction
mechanisms based on previously published volatile organic compound/OH gas-phase reaction
mechanisms. ꢀ 2004 Wiley Periodicals, Inc.∗ Int J Chem Kinet 36: 534–544, 2004
C
complaints have an estimated impact on worker pro-
ductivity of tens of billions of dollars annually [1,2].
Unfortunately, there is currently no direct correlation
between these complaints and a specific pollutant. In
fact, exposures to multiple pollutants in the indoor en-
vironment may be responsible [3–9].
INTRODUCTION
It is estimated that over 30 million of the total 89 mil-
lion workers in indoor environments are affected by
the work environment in the form of eye, nose, and
throat irritations, headache, and fatigue. These health
A recent review of indoor environment chemistry by
Weschler highlights several research areas important to
the field [10]. Significant observations from this review
paper are that there are still several fundamental unan-
swered questions regarding the gas-phase chemistry
Correspondence to: J. R. Wells; e-mail: rwells@cdc.gov.
2004 Wiley Periodicals, Inc. ∗This article is a US Government
c
ꢀ
work and, as such, is in the public domain of the United States of
America.

HYDROXYL REACTION RATE CONSTANT AND PRODUCTS OF 3-5-DIMETHYL-1-HEXYN-3-OL
535
of indoor environments, and the indoor environment is
EXPERIMENTAL METHODS
Apparatus and Materials
chemically more complex than was previously thought.
Experimental evidence has implicated that several ini-
tiator species such as ozone (O3), hydroxyl radical
(OH), and nitrate radical (NO3) are likely to be present
indoors and the VOC concentrations indoors are higher
by a factor of 10 or more than typically found in outdoor
environments [10,11].
Oxygenated organic compounds, such as ethers, al-
cohols, and esters, are becoming more prevalent in the
environment as they are substituted for other chemicals
in consumer products. While several hydroxyl radical
(OH) + oxygenated organic bimolecular rate constants
are well known, details pertaining to the OH + oxy-
genated organic compound reaction mechanisms are
limited [12,13]. A few recent studies of the products
from OH + oxygenated organic reactions have illus-
trated the complexity of their gas-phase reaction mech-
anisms [14–20]. These investigations are needed to
support hydroxyl radical reaction mechanism patterns
based on chemical structure reactivity relationships
[21].
As a side benefit, understanding the compounds’
gas-phase mechanisms in detail can provide a basis
for chemical selection based on structure. There is
the possibility of synthesizing new compounds that
incorporate environmentally and technically benefi-
cial molecular structures. The information gained from
the type of research presented here can lead to more
beneficial use of these and similar compounds in the
future.
3,5-Dimethyl-1-hexyn-3-ol (DHO, HC CC(OH)
(CH3)CH2CH(CH3)2) is a volatile oxygenated organic
compound formed in the indoor environment as a result
of cleavage of 2,4,7,9-tetramethyl-5-dicyne-4,7-ol
(T4MDD, (CH3)2HCH2C(CH3)(HO)(CC CC(OH)
(CH3)CH2CH(CH3)2), a wetting and defoaming
agent in many water-based lacquer systems [22,23].
DHO was observed in experimental chambers as
an emission from finished wood furniture as well
as from pure T4MDD [23]. Most investigations of
alkyne/OH reactions have been on smaller unsub-
stituted alkynes such as acetylene, [12,13,24–28],
propyne [12,25], and 1-butyne [12,25]; however,
DHO’s structure has the unique combination of a
carbon–carbon triple bond with a neighboring alcohol
group.
Experiments to measure the gas-phase rate con-
stant of the OH + 3,5-dimethyl-1-hexyn-3-ol (DHO,
HC CC(OH)(CH3)CH2CH(CH3)2) reaction were
conducted with a previously described apparatus
[18,29–31]. A brief description is provided here.
Reactants were introduced and samples were with-
drawn through a 6.4-mm Swagelok fitting attached to
a 60–100 L TeflonꢀR film chamber. Dry compressed
air was added as a diluent to the reaction chambers
and measured with a 0–100 L min−1 mass flow
controller (MKS, Andover, MA). The filler system
was equipped with a syringe injection port facilitating
the introduction of both liquid and gaseous reactants
into the chambers by a flowing airstream. All reactant
mixtures and calibration standards were generated by
this system. Irradiations were carried out in a light-
tight chamber using 5-mL FEP TeflonꢀR film bags
(60–100 L), which were surrounded by the following
mix of lamps: six Philips TL40W/03, one GE F40BL,
two QPANEL UV351, and seven QPANEL UV340.
This lamp mixture approximates solar radiation from
300 to 450 nm.
All reaction kinetic samples were quantitatively
monitored using an Agilent (Palo Alto, CA) 6890
gas chromatograph with a 5973 mass selective detec-
tor (GC/MS) and Agilent series Chem Station soft-
ware. Gas samples were cryogenically collected em-
ploying an Entech (Simi Valley, CA) 7100 sampling
system utilizing the following trap and temperature
parameters: 50 mL of chamber contents were col-
lected onto Trap 1 (packed with Tenax TA) at −150◦C.
After sample collection, Trap 1 was heated to 40◦C
and the sample transferred under a flow of ultra high
purity helium (UHP He) onto Trap 2 (packed with
Tenax TA) cooled to −30◦C. Trap 2 was then heated
to 180◦C and the sample transferred under a UHP
He flow onto Trap 3, a silanized 0.53-mm i.d. tube
cooled to −160◦C, which was subsequently heated
to 220◦C to inject the sample onto the GC column
[Restek (Bellefonte, PA) FAMEWAX (0.25-mm i.d.,
30 m long, 0.25 ꢀm film thickness)]. These series of
cryogenic trap manipulations reduced the background
water level, ensured consistency of replicate samples,
and improved the chromatograph peak shapes. The
GC temperature program used had an initial temper-
ature of 45◦C held for 8 min after sample injection,
which was increased 10◦C/min to 220◦C and held
for 4 min. The Agilent 5973 mass selective detector
was tuned using perfluorotributylamine (FC-43). Full
scan electron impact ionization spectra were collected
In the work presented here, the rate constant of the
OH radical with DHO was measured by the relative
rate method [29]. The products of the OH + DHO
reaction are also reported and used to derive DHO’s
indoor environment reaction mechanism. Neither the
OH rate constant nor the reaction mechanism for DHO
has been reported previously.

536
WELLS
from m/z 35 to 220. Preliminary compound identifi-
cations from the Agilent 6890/5973 GC/MS data sets
were made by searching the NIST 98 Mass Spectral
Library.
All compounds were used as received and had the
followingpurities:fromSigma-Aldrich:hexane(99%),
decane (99%), 3-butyne-2-one (95%), 3,5-dimethyl-
1-hexyn-3-ol (99%), 4-methyl-2-pentanone (99.5%),
acetonitrile (99.93%), N,O-bis-(trimethyl silyl) tri-
fluoroacetamide (BSTFA) (>99%), O-(2,3,4,5,6-
pentafluorobenzyl)hydroxylamine hydrochloride (PF-
BHA) (>98%); from Ultrascientific: pentanal (98%),
2-methyl propanal (98%); from Fisher Scien-
tific: methanol (99%); from Acros: 2,3-butanedione
(99%); from Spectrum Analytical: methylene chloride
(99.5%). Water was distilled, deionized to a resistiv-
ity of 18 M ꢁ cm, and filtered using a Milli-QꢀR fil-
ter system. Nitric oxide (>99% pure) was obtained
as a 4942-ppm mixture in nitrogen from Butler Gases
(Morrisville, PA). Helium (UHP grade), the carrier gas,
was supplied by Amerigas (Sabraton, WV) and used as
received. Experiments were carried out at (297 3) K
and 1-atm pressure.
Reaction product identification experiments were
performed utilizing two methods: direct gas-phase
sampling and Agilent 6890/5973 GC/MS analysis of
the reaction chamber contents as described above or
collection and derivatization of gas-phase aldehydes
and ketones using O-(2,3,4,5,6-pentafluorobenzyl)-
hydroxylamine (PFBHA) only, and PFBHA with
additional derivatization of alcohols or carboxylic
acidsusing N, O-bis(trimethylsilyl)trifluoroacetamide
(BSTFA) followed by analysis using a Varian (Palo
Alto, CA) 3800/Saturn 2000 GC/MS system [32]. The
combination of PFBHA/BSTFA derivatizations was
used previously to identify the reaction products and
propose reaction mechanisms for the ozonolysis of
ꢁ-pinene and ꢀ3-carene [32,33]. Experimental meth-
ods for reaction product identification were similar to
methods used for kinetic experiments, except that the
reference compound was excluded from the reaction
mixture.
The derivatized aldehydes, ketones, and alcohols
were analyzed with a Varian 3800 gas chromatograph
and a Varian Saturn 2000 ion trap mass spectrome-
ter operated in the EI mode. Compound separation
was achieved by a J&W Scientific DB-5MS (0.32 mm
i.d., 30 m long, 1 ꢀm film thickness) column and the
following GC oven parameters: 60◦C for 1 min, then
10◦C/min to 280◦C and held for 10 min. Samples were
injected in the splitless mode, and the GC injector was
returned to split mode 1 min after sample injection,
with the following injector temperature parameters:
60◦C for 1 min, then 180◦C/min to 250◦C and held
to the end of the chromatographic run [32]. The Saturn
2000 ion trap mass spectrometer was tuned using per-
fluorotributylamine (FC-43). Full scan electron im-
pact ionization spectra were collected from m/z 40 to
650.
Experimental Procedures
The experimental procedures for determining the
DHO + OH reaction kinetics were similar to those
described previously [19,20].
kDHO
DHO + OH −−−→ Products
(4)
(5)
kR
Reference + OH −→ Products
The rate equations for reactions (4) and (5) are
combined and integrated, resulting in the following
equation:
ꢀ
ꢁ
ꢀ
ꢁ
[DHO]0
[DHO]t
kDHO
[R]0
[R]t
ln
=
ln
(6)
kR
If reaction with OH is the only removal mechanism
for DHO and reference, a plot of ln([DHO]0/[DHO]t )
versus ln([R]0/[R]t ) yields a straight line with an in-
tercept of zero. Multiplying the slope of this linear plot
by kR yields kDHO (Fig. 1). The OH rate constant ex-
periments for DHO employed the use of two refer-
ence compounds, decane and pentanal. The use of two
different reference compounds with different OH rate
constants more definitively assured the accuracy of the
DHO/OH rate constant and demonstrated that other re-
actions were not removing DHO.
For the DHO/OH kinetic experiments, the typi-
cal concentrations of the pertinent species in the 60–
100 L TeflonꢀR chamber were 0.3–1-ppm DHO, 0.7–
8-ppm reference, 10–100-ppm CH3ONO, and 0.6-ppm
NO in air. These mixtures were allowed to mix
for 30–60 min before initial species concentration
OH radicals, one of the primary oxidizing radicals
in the indoor environment [11,34–36], were generated
from the photolysis of methyl nitrite (CH3ONO) in the
presence of nitric oxide (NO) in air [29].
CH3ONO + hꢂ → CH3O + NO
CH3O + O2 → CH2O + HO2
H2O + NO → OH + NO2
(1)
(2)
(3)
CH3ONO was prepared in gram quantities using the
method of Taylor et al. [37] and stored in a lecture bottle
atroomtemperature. TheCH3ONOpurity(>95%)was
verified by GC/MS.

HYDROXYL REACTION RATE CONSTANT AND PRODUCTS OF 3-5-DIMETHYL-1-HEXYN-3-OL
537
Figure 2 Plot of 4-methyl-2-pentanone (MIBK) formed,
corrected for MIBK/OH reaction, versus DHO reacted. The
slope of the linear least squares analysis with 95% confidence
interval is 0.26 0.2.
Figure 1 3,5-Dimethyl-1-hexyn-3-ol relative rate plot with
decane (ꢀ) and pentanal ( ) as reference compounds. The
ꢀ
OH + DHO rate constant, kDHO, measured is (28.8 1.2) ×
10−12 cm3 molecule−1 s−1
.
([X]0) samples were collected. Typically, four pho-
tolysis intervals of 2 to 10 s each were used on
the reaction mixture for a combined total photoly-
sis time of approximately 8–40 s. The total ion chro-
matogram (TIC) from the Agilent 5973 mass selective
detector was used to determine DHO and reference
concentrations.
The experimental methods and parameters for ob-
servation of DHO/OH reaction products were similar
to those for reaction rate experiments, except that the
reference compound was excluded from the reaction
mixture and the DHO concentration was increased to
1–3 ppm. The reaction mixtures were irradiated for
2–10-s intervals followed by sample collection. Com-
mercially available samples of the identified products
were analyzed to verify matching ion spectra and chro-
matographic retention times.
When possible, the loss of the parent DHO was
plotted against the formation of products, generating
a straight line with a slope equal to the product yield
(Fig. 2). Product quantitation was performed using the
Agilent 6890/5973 GC/MS system.
Derivatization of aldehydes and ketones was initi-
ated by flowing of 15–25 L of chamber contents at
3.8 L min−1 through 3 mL of acetonitrile in an impinger
with no effort to prevent acetonitrile evaporation dur-
ing sample collection. The sample was removed from
the impinger and 200 ꢀL of 0.25-M PFBHA was added
to derivatize aldehyde and ketone reaction products to
oximes [32] over a 24–48-h time period in the dark. The
reacted solutions were gently blown to dryness with
UHP N2, reconstituted with 100 ꢀL of methanol, and
1 ꢀL of the reconstituted solution was injected into the
Varian 3800/Saturn 2000 GC/MS system. The derivati-
zation of hydroxy groups (either alcohol or carboxylic
acid) was achieved by subsequent addition of 100 ꢀL
of commercially available BSTFA to the PFBHA
oximesreconstitutedwith100ꢀLofhexane:methylene
chloride (1:1). These PFBHA/BSTFA solutions were
heated to approximately 60◦C for 45 min to complete
the silylation, and then 1 ꢀL of the solution was in-
jectedintotheVarian3800/Saturn2000GC/MSsystem
[32].
All measurements were at least duplicated. A rela-
tive standard deviation (the data set standard deviation
divided by the data set average) of approximately 2.5%
was achieved with the described sampling method uti-
lizing the Entech 7100/Agilent 6890/5973 cryogenic
sampling GC/MS system. To determine possible chro-
matographic interferences from reference/OH reaction
products, both DHO and the reference compounds
were reacted with the OH radical in separate exper-
iments and analyzed as described above. No chro-
matographic interferences were observed. Two exper-
iments were conducted to determine the stability of
the reference and DHO coexisting in the same bag.
First the reference and DHO were injected into the
bag, analyzed, photolyzed for 6 min and reanalyzed.
Then methyl nitrite, NO, the reference, and DHO were
injected into the bag, analyzed, left several hours,
and then reanalyzed. None of these preliminary ex-
periments yielded chromatographic peak interferences
or observable reactions occurring without photoiniti-
ation. At the end of each run, the TeflonꢀR bag was
cleaned by flushing the bag 6 times with ultrapure air
(<0.1 ppm total hydrocarbon). Measurements of an
air-filled bag showed no cross contamination between
runs.

538
WELLS
derivatization or both, were acetone ((CH3)2C O),
3-butyne-2-one (3B2O, HC C( O)(CH3)), 2-methyl-
propanal (H(O )CCH(CH3)2), 4-methyl-2-pentanone
(MIBK, CH3C( O)CH2CH(CH3)2), ethanedial (GLY,
HC( O)C( O)H), and 2,3-butanedione (23BD,
CH3C( O)C( O)CH3) and are listed in Table I. Many
other DHO/OH reaction products were observed
utilizing the derivatization methods described above.
Elucidation of these other reaction products was
facilitated by mass spectrometry of the derivatized
reaction products coupled with plausible DHO/OH
reaction mechanisms based on previously published
volatile organic compound/OH gas-phase reaction
mechanisms [12,14–20,26]. These other reaction
products are listed in Table II.
The results for both the positively identified reac-
tion products and the other observed reaction products
are described below. The reported yields for MIBK
and 3B2O are based on the slopes of the DHO reacted
versus product formed plots. The reported error in the
product yield is the 95% confidence level from the ran-
dom uncertainty in the slope of these plots.
RESULTS
Hydroxyl Radical/DHO Reaction Rate
Constant
The OH rate constant for 3,5-dimethyl-1-hexyn-3-ol
(DHO, HC CC(OH)(CH3)CH2CH(CH3)2) was ob-
tained using the relative rate method described above.
Typically, five experimental runs were conducted on
each DHO/reference pair. The plot of a modified ver-
sion of Eq. (6) is shown in Fig. 1. The ln([R]0/[R]t )
term is divided by the respective reference rate constant
[decane (11.0 2.2) × 10−12 cm3 molecule−1 s−1 and
pentanal (28.5 8.6) × 10−12 cm3 molecule−1
s
−1)]
[20,37] and multiplied by 10−12 cm3 molecule−1 s−1
resulting in a unitless number. This yields a slope that is
equal to the hydroxyl radical/DHO rate constant, kDHO
,
divided by 10−12 cm3 molecule−1 −1. This modifica-
s
tion allows for a direct comparison of the two refer-
ence compound/DHO data sets. The slope of the line
shown in Fig. 1 yields a hydroxyl radical bimolecu-
lar rate constant, kDHO, of (28.8 1.2) × 10−12 cm3
molecule−1 −1. The data points at the origin are ex-
s
Because the DHO/OH reaction products could also
reactwithOH, theobservedproductconcentrationshad
to be corrected for OH + reaction product reactions.
perimental points because pre-irradiation, t = 0, data
showed no detectable loss of DHO or reference. The
error in the rate constant stated above is the 95% con-
fidence level from the random uncertainty in the slope.
Incorporating the uncertainties associated with the ref-
erence rate constants ( 30% for pentanal and 20%
for decane) used to derive the DHO/OH rate constant
yields a final value for kDHO of (29 9) × 10−12 cm3
molecule−1 s−1 [12,13,37].
Table I Molecular Structure and Confirmation Method
of Some DHO/OH Reaction Products
DHO/OH
Reaction
Product Name
Molecular
Structure
Confirmation
Method(s)
The DHO/OH rate constant, kDHO, has not been pre-
viously reported. The observed rate constant can be
compared with a kDHO = 17 × 10−12 cm3 molecule−1
Acetone
Oxime derivatization
s
−1, calculated using a structure reactivity relationship
[21]. The calculated value is lower than the measured
value reported here and may be due in part to underes-
timation of OH addition to the carbon–carbon triple
bond or OH abstraction of the alkyne hydrogen in
DHO.
4-Methyl-2-
pentanone
Direct gas-phase
detection and oxime
derivatization
3-Butyne-2-one
Direct gas-phase
detection and oxime
derivatization
DHO/OH Reaction Products
The reaction products observed from the initial
DHO/OH hydrogen abstraction or OH addition to the
triple bond are consistent with previously observed
hydroxyl radical reaction mechanisms for oxygenated
organic species [14–20]. Typically, the oxygenated
organic parent compound reacts with OH to subse-
quently generate other oxygenated organic products.
For DHO, the DHO/OH reaction products observed
and positively identified, using the pure compound
for verification either by direct gas-phase detection or
2-Methyl-
propanal
Oxime derivatization
Oxime derivatization
Oxime derivatization
Ethanedial
2-Oxopropanal
2,3-Butanedione
Oxime derivatization

HYDROXYL REACTION RATE CONSTANT AND PRODUCTS OF 3-5-DIMETHYL-1-HEXYN-3-OL
539
Table II Proposed DHO/OH Reaction Products Based
on Data from PFBHA and/or PFBHA/BSTFA
Derivatizations
DHO loss due to reaction with OH could not be de-
termined simultaneously since DHO is not derivatized
by PFBHA or PFBHA/BSTFA. Additionally, many
of the proposed reaction products are not commer-
cially available, preventing synthesis of oximes to
conclusively verify chromatographic and mass spectra
data.
Molecule Name
Molecular Structure
2-Hydroxy-2-methylbut-3-ynal
Derivatization of nonsymmetric carbonyls using
PFBHA or PFBHA/BSTFA typically resulted in mul-
tiple chromatographic peaks due to geometric iso-
mers of the oximes. Identification of multiple peaks
of the same oxime compound is relatively simple since
the mass spectra for each chromatographic peak are
almost identical. Typically, the PFBHA derivatized
oximes’ (generic structure: F5C6CH2ON C(R1)(R2))
mass spectra included an ion at m/z 181 ([CH2C6F5]+
fragment) with a large relative intensity (>40%) and a
[PFBHA oxime + 181]+ ion (due to reactions in the
ion trap mass spectrometer) [32]. In most cases the
m/z 181 ion relative intensity for the chromatographic
peaks due to DHO/OH reaction product oximes was ei-
therthelargestoroneofthelargestinthemassspectrum
and was used to generate selected ion chromatograms
[32]. The mass spectra of compounds that were addi-
tionally derivatized with BSTFA had m/z 73 ions from
the [Si(CH3)3]+ fragments [32]. The product data are
described below.
4-Hydroxy-4-methylhex-5-yne-
2-one
4-Hydroxy-2,4-dimethyl-6-
oxohex-5-enal
4-Hydroxy-2-methylpentanal
3-Hydroxy-3-methylhex-1-ene-
1,5-dione
This correction has been described in detail [16,38] and
has the following form:
[DHO]t
[DHO]0
1 −
(kDHO − kP)
F =
×
(7)
3-Butyne-2-one (3B2O, HC CC( O)CH3)
ꢂ
ꢃk /k
P
DHO
[DHO]t
[DHO]0
[DHO]t
[DHO]0
kDHO
−
3-Butyne-2-one was identified using the Agilent
6890/5973 GC/MS system and quantified using
Agilent Chem Station extracted ion analysis of the total
ion chromatogram. The 3B2O yield was determined to
be (8.4 0.3)%. Though not previously measured, a
value of 9.25 × 10−12 cm3 molecule−1 s−1 was calcu-
lated for k3B2O using structure–reactivity relationships
and was used in Eq. (7) [21]. From Eq. (7) the aver-
age [3B2O] correction was 26% (maximum correction
44%). The PFBHA-3B2O oxime had retention times
of 12.2 and 12.6 min, determined utilizing the Varian
3800/Saturn 2000 GC/MS system. Pure 3B2O and syn-
thesis of the 3B2O oxime was utilized to confirm chro-
matographic results.
F, the correction factor, was multiplied by the prod-
uct concentration data; kDHO is the OH + DHO
rate constant, and kP is the rate constant for the re-
action of OH with reaction product. The measured
value for kP was used when possible, but kP was cal-
culated using structure–reactivity relationships [21]
when no measured value was available in the lit-
erature. It should be noted that the observed reac-
tion products exhibited linear concentration profiles.
The lack of curvature strongly suggests that no un-
usual side reactions generated or removed primary
reaction products. For completeness, the kP values
are presented with the respective product. The only
quantified DHO/OH reaction products were 4-methyl-
2-pentanone (MIBK, CH3C( O)CH2CH(CH3)2) and
3-butyne-2-one (3B2O, HC CC( O)CH3) because
both DHO and these reaction products were the only
compounds observed with the cryogenic sampling/
Agilent 6890/5973 GC/MS analysis system. The many
other products observed utilizing the PFBHA or
PFBHA/BSTFA derivative/Varian 3800 Saturn 2000
GC/MS analysis system were not quantified because
4-Methyl-2-pentanone
(MIBK, CH3C( O)CH2CH(CH3)2)
4-Methyl-2-pentanone (methyl isobutyl ketone,
MIBK) was identified using the Agilent 6890/5973
GC/MS system and quantified using Agilent Chem
Station extracted ion analysis of the total ion chro-
matogram. The yield data (MIBK/OH corrected) are

540
WELLS
plotted in Fig. 2 and resulted in a (26 1.6)% product
yield. The rate constant, kMIBK, has been measured
[12] and a value of 14.5 × 10−12 cm3 molecule−1 s−1
was used in Eq. (7). From Eq. 7 the average [MIBK]
correction was 41% (maximum correction 72%).
The PFBHA-MIBK oxime had retention times of
14.0 and 14.2 min, determined utilizing the Varian
3800/Saturn 2000 GC/MS system. Pure MIBK and
synthesis of the MIBK oxime were utilized to confirm
chromatographic results.
474 (293 + 181) ion. Using the derivatized molec-
ular weight of 293 for the oxime suggests a ketone
with a molecular weight of 98. The single chromato-
graphic peak suggests an aldehyde structure or a sym-
metric ketone structure. A proposed DHO/OH reaction
product assignment of 2-hydroxy-2-methylbut-3-ynal
(H(O )CC(OH)(CH3)C CH) was made on the basis
of the observed data.
Retention Times 15.6 and 16.0 min
The following chronological chromatographic re-
tention time results and mass spectra data were
observed utilizing PFBHA or PFBHA/BSTFA deriva-
tization and the Varian 3800/Saturn 2000 GC/MS
system. The reaction products reported here had chro-
matographic peak areas proportional to initial DHO
concentration and were observed only after OH initia-
tion of DHO/methyl nitrite/NO/air mixtures. Derivati-
zation experiments performed in the absence of DHO
did not result in any of the data reported below ex-
cept for acetone, which was also observed in pre-
photoinitiated DHO/OH derivatization samples. How-
ever, the acetone oxime peak area increased with DHO/
OH reaction initiation, indicating that acetone is a
likely product of the DHO + OH reaction. Aside from
the positively identified reaction products (Table I)
with the synthesis of the derivatized oxime com-
pound, DHO/OH reaction product identification was
derived from mass spectra data and previously pub-
lished VOC/OH reaction mechanisms [12–20,26].
One of the largest product peaks observed, the oxime
observed with chromatographic peaks at retention
times of 15.6 and 16.0 min had ions of m/z 59, 72,
181, 253, 296, and 312. Importantly, the m/z 59 ion
had a larger relative intensity than the m/z 181 ion.
Using the derivatized molecular weight of 312 for the
oxime suggests a ketone with a molecular weight of
117. The PFBHA/BSTFA derivatization method de-
scribed above yielded a chromatographic peak with a
17.6 min retention time and a mass spectrum with a
large relative intensity m/z 131 ion and additional ions
of m/z 73, 181, 255, 310, and 368. If the large m/z
59 ion observed in the PFBHA derivatized sample is
due to the [HC(OH)(CH3)CH2]+ fragment, then addi-
tional derivatization by BSTFA on the alcohol group
would result in a fragment with m/z 131. Addition-
ally, the PFTBA/BSTFA m/z 368 ion is 72 mu larger
than the m/z 296 ion observed in the PFTBA deriva-
tization sample only. A proposed DHO/OH reaction
product assignment of 4-hydroxy-2-methylpentanal
(HC(OH)(CH3)CH2CH(CH3)C( O)H) was made on
the basis of the observed data. The molecular weight
of 4-hydroxy-2-methylpentanal is 116, and so the m/z
312 ion observed from PFTBA derivatization could be
due to a derivatized molecular weight +1 (M + 1), ion
which is a common occurrence in mass spectrometry.
Acetone ((CH3)2C O)
The acetone oxime (PFBHA C(CH3)2) was observed
at approximately 11.2 min employing the Varian
3800/Saturn 2000 GC/ion trap mass spectrometer sys-
tem described above. Acetone oxime was synthesized
to confirm this chromatographic assignment. Acetone
oxime was observed in pre-photolysis samples, but
the peak area increased upon initiation of DHO/OH
reaction indicating acetone as a DHO/OH reaction
product.
Retention Time 18.1 min
This DHO/OH reaction product was observed after
PFBHA/BSTFA derivatizations and had a retention
time of 18.1 min. The major ions were m/z 45, 73, 117,
181, 269, 285, 361, 391, and 466. Assuming that the
m/z 391 ion is the doubly derivatized (one PFBHA and
one BSTFA) reaction product, a molecular weight of
124 can be calculated. The m/z 117 ion could be due to
the [C( O)OSi(CH3)3]+ fragment, indicating that this
compoundcouldbeacarboxylicacidwithanadditional
ketone or aldehyde [32]. Incorporation of the molecu-
lar structural elements, the molecular weight, and po-
tential reaction mechanisms did not result in a plau-
sible reaction product molecular structure that would
be consistent with all the observed data. Currently, the
2-Methyl-propanal (2MP, HC( O)CH(CH3)2)
The 2MP oxime (PFBHA CHCH(CH3)2) was ob-
served at 12.2 min. 2MP oxime was synthesized to
confirm this chromatographic assignment.
Retention Time 14.3 min
The oxime observed with chromatographic peak at
retention time of 14.3 min had ions of m/z 42, 55,
112, 181, 292, 293, and a low relative intensity m/z

HYDROXYL REACTION RATE CONSTANT AND PRODUCTS OF 3-5-DIMETHYL-1-HEXYN-3-OL
541
identity of this compound is unknown and proposal of a
potential reaction product’s molecular structure would
be pure speculation.
for m/z 307 and 311. Assuming the oxime molec-
ular weight is 311 yields a reaction product molec-
ular weight of 116. Interestingly, the addition of the
BSTFA derivatization step resulted in a disappearance
of this chromatographic peak unlike any other PFBHA-
derivatized reaction product. A chromatographic peak
at 23.1 min with ions of m/z 73, 181, 255, 265, 367,
383, 549, and 564 was observed after PFBHA/BSTFA
derivatization of the reaction products. Adding the
mass of [Si(CH3)3] in the place of an alcoholic hydro-
gen to both the m/z 295 ion and the m/z 311 ion could
result in corresponding ions of m/z 367 and 383, re-
spectively, observed in the PFBHA/BSTFA derivatized
samples. The data suggests that this reaction product
contains both a carbonyl group (C O) and an alco-
hol group (C OH), but incorporation of the structural
elements, the molecular weight, and potential reac-
tion mechanisms did not result in a plausible reaction
product molecular structure that would be consistent
with all the observed data. Currently, the identity of
this compound is unknown and proposal of a potential
reaction product’s molecular structure would be pure
speculation.
Retention Time 18.8 Through 19.2 min
The oxime observed with chromatographic peaks over
the retention time span of 18.8–19.2 min had ions
at m/z 43, 98, 123, 140, 181, 252, 279, 320, and
337. Assuming a PFBHA derivatized molecular weight
of 337 for the oxime indicates a reaction product
molecular weight of 142. There are three distinct
chromatographic peaks and one broad peak suggest-
ing an unsymmetrical ketone with multiple derivatiza-
tion sites. A proposed DHO/OH reaction product as-
signment of 3-hydroxy-3-methyl-hex-1-ene-1,5-dione
(O C CHC(CH3)(OH)CH2C( O)CH3) was made on
the basis of the observed data.
Ethanedial (GLY, HC( O)C( O)H)
The chromatographic peaks for the oxime observed at
20.9 and 21.0 min had ions at m/z 117, 181, 251, 253,
267 (very low relative intensity) and 448 (relative in-
tensity 25% of the m/z 181 ion). The m/z 448 ion is
the result of a double PFBHA derivatization indicating
a reaction product with a molecular weight of 58. The
PFBHA-GLY oxime was synthesized to confirm this
chromatographic assignment.
Retention Time 23.8 and 24.0 min
The oxime compound at retention times 23.8 and
24.0 min had ions at m/z 69, 82, 96, 110, 181, 280,
307, and 321. Assuming 321 as the derivatized molecu-
lar weight, a reaction product molecular weight of 126
can be derived. A proposed DHO/OH reaction prod-
uct assignment of 4-hydroxy-4-methyl-hex-5-yne-2-
one (HC CC(OH)(CH3)CH2C( O)CH3) was made
on the basis of the observed data.
2-Oxopropanal (MGLY, CH3C( O)C( O)H)
The single peak for the oxime observed at 21.3 min
had ions at m/z 117, 181, 265, 432, and 462. The m/z
462 ion is the result of a double PFBHA derivatization
indicating a reaction product with a molecular weight
of 72. The PFBHA-MGLY oxime was synthesized to
confirm this chromatographic assignment.
Retention Time 24.1 Through 24.3 min
Chromatographic peaks with some of the largest peak
areas were observed for the oxime compound at re-
tention times 24.1 through 24.3 min with ions m/z
43, 57, 83, 251, 295, 333, 351, 491, 531, and 549.
Assuming 351 as the derivatized molecular weight
yields a reaction product molecular weight of 156.
The multiple chromatographic peaks suggest an un-
symmetrical ketone with possible multiple derivati-
zation sites. A proposed DHO/OH reaction product
assignment of 4-hydroxy-2,4-dimethyl-6-oxo-hex-5-
enal (O C CHC(OH)(CH3)CH2CH(CH3)C( O)H)
was made on the basis of the observed data.
2,3-Butanedione
(23BD, CH3C( O)C( O)CH3)
The single chromatographic peak for the oxime ob-
served at 21.6 min had ions at m/z 42, 117, 181, 279,
446, 456, and 476. The 476 ion is the result of a dou-
ble PFBHA derivatization indicating a compound with
a molecular weight of 86. The PFBHA-23BD oxime
was synthesized to confirm this chromatographic
assignment.
DISCUSSION
Retention Time 23.4 min
The oxime compound at retention time 23.4 min had
OH reacts with DHO by H atom abstraction or
ions at m/z 181, 295, and low relative intensities
OH addition to the carbon–carbon triple bond [12].

542
WELLS
•
HC CC(OH)(CH3)CH2C(OO )(CH3)2 + NO
The“reactivestructure”ofDHOcanbedrawnasshown
in structure 1:
•
→ NO2 + HC CC(OH)(CH3)CH2C(O )(CH3)2
(9b)
HC CC(OH)(CH3)CH2C(O )(CH3)2 → beta scission
HC C C(OH)(CH3) CH2 CH(CH3)2
I
II
III
IV
V
VI
•
1
•
→ O C(CH3)2(ACE) + HC CC(OH)(CH3)CH2
DHO is a molecule with five distinct sites for possible
hydrogen abstraction (I, III, IV, V, VI) and one site for
OH addition (II). Site I has a negligible contribution
to the DHO/OH rate constant [21], site II contributes
approximately 64%, site IV contributes 16%, and site
V contributes 14% to the structure reactivity calculated
DHO/OH rate constant of 17 × 10−12 cm3 molecule−1
s−1 [21]. These three molecular groups contribute ap-
proximately 94% to the value of the calculated rate
constant. This calculated value is in agreement with
the measured value reported here [(29 9) × 10−12
(9c)
For 3-butyne-2-one (3B2O) and 2-methyl propanal
(2MP) formation:
HC CC(OH)(CH3)CH2CH(CH3)2 + OH
•
→ HC CC(OH)(CH3)C( )HCH(CH3)2 + H2O
(10)
•
HC CC(OH)(CH3)C( )HCH(CH3)2 + O2
•
→ HC CC(OH)(CH3)C(OO )HCH(CH3)2 (11a)
cm3 molecule−1 −1), considering that OH rate con-
s
•
HC CC(OH)(CH3)C(OO )HCH(CH3)2 + NO
stants calculated using structure reactivity are typically
within a factor of 2 of the measured OH rate constant
[21]. A difference between the calculated and mea-
sured rate constant could be the group substituent fac-
tor of 1.6 used for OH addition to the carbon–carbon
triple bond, based on a C(OH)H2 group and not
on C(OH)(CH3) [21]. A larger group substituent
factor would increase the calculated DHO/OH rate
constant. To date, DHO represents one of the largest
alkynes to be investigated for an OH radical reaction
rate constant and subsequent gas-phase mechanism.
The OH radical combustion and atmospheric mech-
anisms of the alkyne acetylene (C2H2) have received
the most research attention. [12,13,24–28].
•
→ NO2 + HC CC(OH)(CH3)C(O )HCH(CH3)2
(11b)
•
HC CC(OH)(CH3)C(O )HCH(CH3)2
•
→ beta scission → HC CC(OH)(CH3)
+ O CHCH(CH3)2 (2MP)
(11c)
(11d)
•
HC CC(OH)(CH3) + O2
→ HO2 + HC CC( O)(CH3)(3B2O)
For 4-methyl-2-pentanone (MIBK):
HC CC(OH)(CH3)CH2CH(CH3)2 + OH
The experimental parameters were set to minimize
other side reactions and highlight the first OH hydrogen
abstraction and OH addition step. Nitric oxide (NO)
was added to facilitate the generation of OH and to
minimize ozone (O3) and NO3 formation, preventing
other possible radical reactions. The possible mecha-
nistic steps leading to product formation are described
below. For some reaction products there may be mul-
tiple pathways. Depending on the nature of the radical
formed in reaction (4), some of the possible mecha-
nisms for the observed reaction products are as given
below.
•
→ H(HO)C C( )C(OH)(CH3)CH2CH(CH3)2
(12)
•
H(HO)C C( )C(OH)(CH3)CH2CH(CH3)2 + O2
•
→ H(HO)C C(OO )C(OH)(CH3)CH2CH(CH3)2
(13a)
•
H(HO)C C(OO )C(OH)(CH3)CH2CH(CH3)2 + NO
•
→ H(HO)C C(O )C(OH)(CH3)CH2CH(CH3)2
+NO2 (13b)
For acetone (ACE) formation:
•
H(HO)C C(O )C(OH)(CH3)CH2CH(CH3)2
→ beta scission → H(HO)C C O(?)
HC CC(OH)(CH3)CH2CH(CH3)2 + OH
•
+ C(OH)(CH3)CH2CH(CH3)2
(13c)
•
→ HC CC(OH)(CH3)CH2C( )(CH3)2 + H2O
(8)
•
C(OH)(CH3)CH2CH(CH3)2 + O2
•
HC CC(OH)(CH3)CH2C( )(CH3)2 + O2
→ C( O)(CH3)CH2CH(CH3)2 (MIBK) + HO2
(13d)
•
→ HC CC(OH)(CH3)CH2C(OO )(CH3)2
(9a)

HYDROXYL REACTION RATE CONSTANT AND PRODUCTS OF 3-5-DIMETHYL-1-HEXYN-3-OL
543
The variety of molecular structures of the DHO/OH
reaction products, both identified and proposed, are
due to multiple hydrogen abstraction sites on the DHO
molecule, the carbon–carbon triple bond on one end of
the DHO molecule, and the length of the carbon chain
on the other end of the DHO molecule [39].
described above, because this reaction product is
not expected to be derivatized by the PFBHA or
PFBHA/BSTFA methods described [32].
The combination of PFBHA/BSTFA derivatizations
was used recently to identify the reaction products
and propose reaction mechanisms for the ozonoly-
sis of ꢁ-pinene and ꢀ3-carene [32,33]. The separate
use of PFBHA and PFBHA/BSTFA derivatizations
was critical for insight into the molecular structure
of the DHO/OH reaction products. Comparison of
the chromatographic and mass spectroscopic results
from the PFBHA and PFBHA/BSTFA methods high-
lighted the portions of the DHO/OH reaction prod-
uct molecules that contained alcohol or carboxylic
acid groups such as was observed for the pro-
posed reaction product 4-hydroxy-2-methylpentanal
(HC(OH)(CH3)CH2CH(CH3)(C( O)H).
Most of the previously published alkyne/OH re-
action investigations were conducted in the interest
of understanding atmospheric and combustion chem-
istry [12,13,24–28]. Only a few alkyne/OH reactions
have been investigated more than once, with acety-
lene receiving the most attention. The recommended
alkyne/OH rate constants for acetylene, propyne, and
2-butyne at atmospheric pressure and ∼298 K are 9 ×
10−13, 5.9 × 10−12, and 27.4 × 10−12 cm3 molecule−1
s
−1, respectively [12,24]. The increase in alkyne/OH
rate constant is proportional to alkyne chain length and
may be due in part to stabilization of the electroni-
cally excited alkyne-OH adduct, which could minimize
adduct decomposition to initial reactants [12,26,28].
The DHO/OH rate constant reported here [(29 9) ×
CONCLUSIONS
10−12 cm3 molecule−1 −1] is close in value to the 2-
s
The OH radical can either abstract hydrogen or add
to the carbon–carbon triple bond of DHO. A bimole-
cular rate constant, kDHO, of (29 9) × 10−12 cm3
molecule−1 s−1 was measured using the relative rate
technique.
butyne/OH rate constant, which may suggest that there
is a stabilization limit.
Previous investigations of the alkyne/OH reaction
mechanism have identified the most significant re-
action step as the addition of the OH radical into
the carbon–carbon triple bond yielding an electroni-
cally excited species (for acetylene/OH, it is proposed
to be C2H2OH∗). From previous investigations, the
fate of this excited species is dependent on pressure,
temperature, alkyne carbon chain length, and oxygen
concentration [24–28]. Hatakeyama et al. determined
the main reaction products of the acetylene/OH reac-
tion to be HCOOH [formic acid, (40 10)% yield)
and HC( O)C( O)H (GLY, (70 30)% yield] [26].
The observed DHO/OH reaction products at atmo-
spheric pressure were consistent with previously re-
ported alkyne/OH reaction products, but additional
reaction products were observed because of DHO’s
more unusual molecular structure (Tables I and II)
[12,13,24–28].
The identification and quantitation of the DHO/OH
reaction products was facilitated by the use of derivatiz-
ing agents PFBHA and BSTFA. While many DHO/OH
reaction products were simply proposed the basis
of observed experimental data and previously pub-
lished VOC/OH reaction mechanisms, several reac-
tion products such as acetone, 4-methyl-2-pentanone
(MIBK), 3-butyne-2-one (3B2O), 2-methyl-pentanal
ethanedial, 2-oxopropanal, and 2,3-butanedione were
positively identified. The structures of the identified re-
action products indicate that the carbon–carbon triple
bond and the hydrocarbon side chain play important
roles in the formation of DHO/OH reaction products
through radical stabilization, methyl group transfer,
and/or hydrogen-transfer isomerization.
Recently, PFBHA-coated solid-phase microex-
traction (SPME) fibers were used for the passive
sampling of the gas-phase reaction products of
Z-3-hexen-1-ol/OH, 2-methyl-3-buten-2-ol/OH, and
a series of diols/OH. These reactions yielded products
that supported OH addition to carbon–carbon double
bond and hydrogen abstraction from the C H bonds of
HC(OH) groups [40,41]. However, OH addition to the
DHO carbon–carbon triple bond one of the expected
reaction products, 3,5-dimethyl-1-hexyn-1,3-diol
(HOC CC(OH)(CH3)CH2CH(CH3)2), was unfortu-
nately not observed in the experimental system
The author wishes to thank the reviewers for their thorough
assessment of the manuscript prior to publication.
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