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Carbinolamines and Geminal Dials in Aqueous
Environmental Organic Chemistry
Edward T. Urbansky
Reprinted from Journal of Chemical Education, Vol. 77, pp 1644-1647, December 2000.
Copyright ©2000 by the Division of Chemical Education of the American Chemical Society.
Reprinted by permission of the copyright owner.
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Carbinolamines and Geminal Dials in Aqueous Environmental
Organic Chemistry*
Edward T. Urbansky
National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive,
Cincinnati, OH 45268-0001; Urbansky.Eclwarcl@.EPA.gov
The beginning environmental chemist or environmen-
tal scientist with a bachelor's degree most likely has taken one
or two courses of organic chemistry. In general, the reactions
and mechanisms studied will have been staged in organic
solvents: ethers, alcohols, ketones, hydrocarbons, halogenated
hydrocarbons, etc. While these solvents are sometimes en-
countered as discrete phases in environmental sites (e.g., dense
nonaqueous phase liquids or DNAPLs), most environmental
chemistry takes place in water or in its presence — very much
unlike the conditions of a Grignard reaction, for example.
The traditional organic chemistry sequence is not intended
to prepare students for careers hi environmental chemistry
and may mislead them. What is generally true in an organic
solvent may not apply at all in an aqueous solution. Both
courses and texts tend to emphasize synthetic utility, and
exercises are geared to promote synthesis reasoning. This
approach has served organic chemists well, and no curriculum
can cover every nuance. Thus, it falls to teachers of environ-
mental chemistry to treat aqueous organic chemistry.
Carbinolamines and Geminal Dials
Carbinolamines or hemiaminak form as intermediates in
the reactions of aldehydes and ketones with ammonia and
amines. For primary amines, this often leads to the forma-
tion of an imine (eq 1), the most stable of which are the SchifF
bases (which usually have at least one aryl R or Z group).
R2C(OH)(NHZ) ^ R2C=NZ
caibinolaniine imine
Carbinolamines are presented only as short-lived reaction
intermediates in the formation of imines (including oximes,
hydrazones, and semicarbazones) (1-3). In fact, Fessenden
and Fessenden refer to the carbinolamine only as "an unstable
addition product" (2).
Geminal or gem diols form from a hydrolytic hydration
reaction of aldehydes or ketones (usually with electronegative
moieties on R), as shown in eq 2,
R2C=0 + H20 ^= R2C(OH)2
gem-dial
(2)
several examples with hydrolytic hydration reaction equilibrium
constants (including the favorite example of a Mickey
Finn), but Carey is the only one to solidly cover the kinetics
of dehydration and hydration, including specific acid and base
catalyses (3).
Considerably more discussion of substitution— addition
reactions of aldehydes and ketones focuses on the products
formed when alcohols act as nucleophiles to give hemiacetals,
acetals, hemiketals, and ketals. In water, however, these
compounds are usually hydrolyzed. In fact, 2,2-dimethoxy-
propane (acetone dimethyl ketal) is specifically used as a water
scavenger (4). Other dialkyl ketals can also be used in this
fashion. While acetals and ketals may serve as protecting
groups during synthesis in organic solvents, they generally
would not survive in aqueous solution. Consequently, much
of this synthetically useful chemistry finds little applicability
in environmental science.
It is unfortunate that the solvent is often not explicitly
given in many of the reactions presented in sophomore
organic chemistry textbooks, for it would be useful for students
to see how infrequently water or water-containing solutions
are used as solvents in synthesis. Although it may be argued that
students realize dais, it cannot be argued that they necessarily
understand what would happen if water were to replace the
solvent of choice. In environmental chemistry, on the other
hand, water is the most common and most abundant solvent.
Occurrence of Carbinolamines and Geminal Diols
• H2O (1) Evidence for Carbinolamines and Geminal Diols
where R can be alkyl, aryl, or hydrogen and is often substituted,
such as -CC13. In their coverage of aldehyde and ketone
reactions, three popular sophomore-level organic chemistry
textbooks discuss these species (1-3). Geminal diols are pre-
sented as novelties of organic chemistry, exceptions to the rule
of stability of the C=O double bond. Each textbook presents
tThis paper is the work product of a United States government
employee engaged in his official duties. As such, it is in the public
domain and not subject to copyright restrictions.
Despite the oft-reported instability of Carbinolamines,
they are sufficiently stable to be observed by JH NMR
spectrometry (5—10), and they have been observed by mass
spectrometry (8, 10) as well. Furthermore, these intermediates
can build up in sufficient quantity to alter reaction kinetics
(9, 10). A. substantial body of literature exists on carbinol-
amine formation (5-23), and it has been the focus of physical
organic chemistry studies of the Hammond postulate (12,
15), a discussion of which is beyond the scope of this work.
Although Carbinolamines are generally not isolable from
solution, some, for example, 2-[(acetamido)(hydroxo)methyl]-
1-methylpyridinium iodide, have been isolated (17).
Isolability should not be regarded as a criterion of stability,
however, for many chemical species can exist only in solution.
Any aldehyde or ketone that can form a gem diol is more
hydrophilic owing to increased hydrogen bonding and is more
massive because a water molecule has been incorporated into
its constitution. Under common conditions, the aldehydes
are converted to the geminal diols: for example, 0.05 mol%
CH20, 99.95% H2C(OH)2; 43-5% CH3CHO, 56.5%
CH3CH(OH)2; <0.01% CC13CHO, >99.99% CC13CH(OH)2;
58.5% CH3CH2CHO, 41.5% CH3CH2CH(OH)2 (29). In
1644 Journal of Chemical Education • Vol.77 No. 12 December 2000 • JChemEd.chem.wisc.edu
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addition to the equilibria, the kinetics and mechanisms of
the hydrolytic hydration of methanal (24-35) and other small
aldehydes (36—38), including trichloroethanal, have been
studied extensively.
Naturally Occurring Species and Disinfection
Byproducts
Several short-chain oxo-containing compounds are
formed as ozonation byproducts (OBPs) from the disinfection
of drinking water supplies (39, 40). Methanal is ubiquitous
and results from natural processes, in addition to anthropogenic
ones. Although the very low concentrations of the nonhalo-
genated compounds that occur in potable water supplies have
not been implicated in any adverse health effects, they can serve
as nutrients for microbes in the distribution system. Some
chlorinated aldehydes and ketones are found as chlorination
byproducts (CBPs), for instance, 2,2,2-trichloroethane-l,l-
diol (39). There are two important consequences of the hydro-
philicity associated with gem diol formation: lower volatility
and reduced partitioning into organic solvents. Because these
compounds are quantitated by extraction into organic solvents,
such as £-butyl methyl ether (41, 42), this behavior raises the
lower limit of detection relative to what would be expected
for the aldehydes.
Nonetheless, these compounds remain sufficiendy volatile
that air-stripping (a treatment process used to purge volatile
compounds from water) can remove them. On a laboratory
scale, monoaldehydes of 1-4 carbons can be sparged away
with a stream of argon or nitrogen gas. However, this process is
not efficient and air-stripping methanal (methanediol) takes
longer than would be predicted because of the hydrophilicity
of the diol.
When two oxo groups are present, the process has mini-
mal effect. Ethanedial (glyoxal) and 2-oxopropanal (methyl
glyoxal or pyruvaldehyde) are not removed by sparging with
an inert gas. Ethanedial actually exists as mixture of gem
diols and cyclic ethers, such as ethane-l,l,2,2-tetraol,
CH(OH)2CH(OH)2, or (45,55)-l,3-dioxa-2-dihydroxy-
methylcyclopentane-4,5-diol, 1 (43—45).
OH
XU
OH
Derivafization in Environmental Analytical Chemistry
Derivatives (hydrazones, oximes, and semicarbazones) are
framed in terms of identification. Melting points and other
properties of derivatives distinguish carbonyl compounds, and
lab manuals tabulate these (46). In environmental chemistry,
oximes and hydrazones are studied by quantitative instrumen-
tal techniques. Dinitrophenylhydrazone concentrations can be
measured by liquid chromatography with UV detection (47),
gravimetry (48, 49), or other techniques (49—51). In drinking
water, O-(2,3,4,5,6-pentafluorobenzyl)oximes of carbonyl
compounds are determined by gas chromatography with
electron-capture detection (GC-ECD) (52-68). These include
the cc-oxocarboxylates, •which occur as ozonation byproducts
(39, 63-66, 70). O-(2,3,4,5,6-Pentafluorobenzyl)oxylamine
derivatization has also been applied to determination of oxo
compounds in other matrices (69).
Carbinolamines are intermediates in the formation of the
derivatives; accordingly, their stability and lability influence
derivatization. In water, the range of pH allows for varying
protonation; consequendy, derivatization rarely progresses to
completion and perhaps cannot. Geminal diol formation, too,
can interfere in this process. It is generally accepted that die
initial nucleophilic attack of a derivatizing agent occurs at
the carbonyl carbon as opposed to an SN2 process at a
tetraligated gem diol (11—15). Tying up carbonyls as the un-
reactive gem diols therefore reduces the derivatization rate.
The kinetics can be fairly complicated, with general acid- and
base-assisted steps (10, 11—15). This may explain some of
the problems in analyzing dihydroxopropanedioate (oxo-
propanedioate) (70). Because this species exists >99.9 mol%
as the gem diol (71), the rate of derivatization should be
reduced. A rapid equilibrium step or a reversible reaction must
occur first. Either would reduce the concentration of the
reactant, the 2-oxo species.
Consequences and Conclusions
Any aqueous process whereby an aldehyde or ketone
undergoes nucleophilic substitution can be expected to have
some effects from the stability of carbinoiamines and gem
diols imparted by the water (72). As summarized above, a
significant body of literature exists on the role of these
compounds in aqueous organic chemistry. Nevertheless,
carbinoiamines are still commonly regarded as short-lived
intermediates and gem diols as exceptions by sophomore
organic chemistry textbooks (1—3), and thus students come to
view them that way. In environmental applications, however,
these species are ubiquitous and may dominate—or at least
alter—the observable chemistry. For example, carbinoiamines
play key roles in the formation of cyanogen chloride from
methanal and chloramine (10).
Even in the field of drinking water chemistry, these
species have been largely ignored. No effort has been made
to exploit carbinolamine reaction kinetics or irnine formation
in analytical chemistry, such as adding arylamines as catalysts
or adjusting pH, based on past reports (14, 22, 73, 74). Such
modifications could lead to improved analytical methods,
with advantages in convenience (shorter time for oximation)
or lower limits of detection and are worthy of exploration.
Four aldehydes dominate DBF studies: methanal,
ethanal, ethanedial, and oxopropanal. Ethanal and oxo-
propanal experience some degree of hydration, but methanal
and ethanedial are >99% hydrated in aqueous solution.
Geminal diol stability usually decreases with increasing size of
substituent and increases •with increasing electronegativity of
substituent moieties; practical consequences may be kinetic
or thermodynamic. Stable gem diols and hemiaminals are
readily found in environmental chemistry applications, but
only graduate texts delve deeply into the behavior of these
compounds (75—77). Ideally, future sophomore texts will
include environmental application notes; for now, it is hoped
that this work illuminates the significance of geminal diols and
carbinoiamines in aqueous environmental organic chemistry.
JChemEd.chem.wisc.edu • Vol.77 No. 12 December 2000 • Journal of Chemical Education 1645
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