Ethylene glycol (
IUPAC name: ethane-1,2-diol) is an
organic compound widely used as an
automotive antifreeze and a precursor to polymers. In its
pure form, it is an odorless, colorless, syrupy, sweet tasting
liquid (although it should be noted that ethylene glycol is toxic,
and ingestion can result in death).
Production
Historical aspects and natural occurrence
Ethylene
glycol was first prepared in 1859 by the French
chemist
Charles-Adolphe Wurtz from
ethylene glycol diacetate via saponification with potassium hydroxide and, in 1860, from
the hydration of ethylene oxide. There appears to have
been no commercial manufacture or application of ethylene glycol
prior to
World War I, when it was
synthesized from
ethylene
dichloride in Germany and used as a substitute for
glycerol in the
explosives industry.
In the United States, semicommercial production of ethylene glycol
via
ethylene chlorohydrin was
started in 1917. The first large-scale commercial glycol plant was
erected in 1925 at South Charleston, West Virginia,, by Carbide and
Carbon Chemicals Co. (now Union Carbide Corp.). By 1929, ethylene
glycol was being used by almost all
dynamite manufacturers.
In 1937, Carbide started up the first plant based on Lefort's
process for vapor-phase oxidation of ethylene to ethylene oxide.
Carbide maintained a monopoly on the direct oxidation process until
1953 when the Scientific Design process was commercialized and
offered for licenses.
This molecule has been observed in outer space.
Current methods
Ethylene glycol is produced from
ethylene,
via the intermediate
ethylene oxide.
Ethylene oxide reacts with
water to produce
ethylene glycol according to the
chemical equation
- C2H4O + H2O →
HOCH2CH2OH
This
reaction can be
catalyzed by either
acids or
bases, or can occur at neutral
pH under elevated temperatures. The highest
yields of ethylene glycol occur at acidic or neutral pH with a
large excess of water. Under these conditions, ethylene glycol
yields of 90% can be achieved. The major byproducts are the
ethylene glycol
oligomers diethylene glycol,
triethylene glycol, and tetraethylene
glycol. About 6.7 billion kilograms are produced annually.
Uses
Approximately 60% of ethylene glycol is consumed for antifreeze,
and the remainder is mainly used as a precursor to polymers.
Because this material is cheaply available, it finds many niche
applications.
Coolant and heat transfer agent
The major use of ethylene glycol is as a medium for
convective heat transfer in, for
example, automobiles and liquid cooled computers. Ethylene glycol
is also commonly used in chilled water
air conditioning systems that place either
the chiller or air handlers outside, or systems that must cool
below the freezing temperature of water. In
geothermal heating/cooling systems,
ethylene glycol is the
fluid that transports
heat through the use of a
geothermal heat pump. The ethylene
glycol either gains energy from the source (lake, ocean,
water well) or dissipates heat to the source,
depending if the system is being used for heating or cooling.
Due to its low freezing point and tendency to form
glasses, ethylene glycol resists freezing. The
freezing point of a mixture of 60% ethylene glycol and 40% water
freezes below -45 °C. Diethyleneglycol behaves similarly. It is
used as a
deicing fluid for
windshields and aircraft. The
antifreeze capabilities of ethylene
glycol have made it an important component of
vitrification (anti-crystallization) mixtures
for low-temperature preservation of biological tissues and
organs.
Ethylene glycol disrupts hydrogen bonding when dissolved in water.
Pure ethylene glycol freezes at about 9 deg F, but when intermixed
with water molecules, neither can form an efficient crystal
structure, and therefore the freezing point of the mixture is
depressed significantly. The minimum freezing point is observed
when the ethylene glycol percent in water is about 70%, as shown
below. This is the reason pure ethylene glycol is not used as an
antifreeze--water is a necessary component as well.
However, the boiling point for aqueous ethylene glycol increases
monotonically with increasing ethylene glycol percentage. The use
of ethylene glycol not only depresses the freezing point but also
elevates the boiling point such that the operating range for the
heat transfer fluid is broadened on both ends of the temperature
scale.
This latter quality is in accordance with
Raoult's Law, which predicts the decreased
vapour pressure of the solvent
(water) proportional to a decrease in its concentration. The
numerical value of vapour pressure is a measure of the
(temperature-specific) pressure at which there exists a
thermodynamic equilibrium between
the liquid and gaseous phases; that is, an equal rate of
vaporisation and condensation.
For example, if the vapour pressure is 100
torr
for any substance at a given temperature, that substance will
predominantly vapourise if the true pressure of the environment is
equal to or less than 100 torr. Conversely, if the
environmental pressure is higher than the vapour pressure,
condensation will be predominant.
Notwithstanding that the total vapour pressure for the system is
the sum of individual vapour pressures, at 373K (100 deg C or 212
deg F) the vapour pressure gained by the addition of 1/10ths
ethylene glycol is negligible (+4mmHg) in comparison to the loss of
1/10th water (-76mmHg) and thus does little to offset the
aforementioned.
Vapor pressure is exponentially proportional to temperature. When
temperature is reduced, vapour pressure is lowered for all
substances; water itself has little appreciable vapour pressure at
0C. Thus, it is important to note that the mechanism which
precludes boiling is NOT responsible for the anti-freeze properties
observed at low temperatures. Appreciating that the mechanisms
responsible are different will help one understand the apparently
paradoxical, dualistic effects of ethylene glycol on water
thermodynamics.
Precursor to polymers
In the
plastics industry, ethylene glycol is
important precursor to
polyester fibers
and
resins.
Polyethylene terephthalate, used
to make plastic bottles for
soft drinks,
is prepared from ethylene glycol.
Hydrate inhibition
Because of its high boiling point and affinity for water, ethylene
glycol is a useful
desiccant. Ethylene
glycol is widely used to inhibit the formation of
natural gas clathrates (hydrates) in long
multiphase pipelines that convey natural gas from remote gas fields
to an onshore processing facility. Ethylene glycol can be recovered
from the natural gas and reused as an inhibitor after purification
treatment that removes water and inorganic salts.
Natural gas is dehydrated by ethylene glycol. In this application,
ethylene glycol flows down from the top of a tower and meets a
rising mixture of water vapor and
hydrocarbon gases. Dry gas exits from the top of
the tower. The glycol and water are separated, and the glycol
recycled. Instead of removing water, ethylene glycol can also be
used to depress the temperature at which
hydrates are formed. The purity of glycol used for
hydrate suppression (mono-ethylene glycol) is typically around 80%,
whereas the purity of glycol used for dehydration (tri-ethylene
glycol) is typically 95-99+%. Moreover, the injection rate for
hydrate suppression is much lower than the circulation rate in a
glycol dehydration tower.
Niche applications
Minor uses of ethylene glycol include the manufacture of
capacitors, as a chemical intermediate in the
manufacture of
1,4-dioxane and as an
additive to prevent
corrosion in liquid
cooling systems for
personal
computers. Ethylene glycol is also used in the manufacture of
some
vaccines, but it is not itself present
in these injections. It is used as a minor (1–2%) ingredient in
shoe polish and also in some inks and
dyes. Ethylene glycol has seen some use as a rot and fungal
treatment for wood, both as a preventative and a treatment after
the fact. It has been used in a few cases to treat partially rotted
wooden objects to be displayed in museums. It is one of only a few
treatments that are successful in dealing with rot in wooden boats,
and is relatively cheap. Ethylene glycol may also be one of the
minor ingredients in screen cleaning solutions, along with the main
ingredient
isopropyl alcohol.
Ethylene glycol is commonly used as a preservative for specimens in
schools, frequently during dissection. It is said to be safer than
formaldehyde, but the safety is
questionable.
Chemical reactions
Ethylene glycol is used as a
protecting
group for
carbonyl groups in
organic synthesis. Treating a
ketone or aldehyde with ethylene glycol in the presence of an acid
catalyst (e.g.,
p-toluenesulfonic
acid;
BF3•Et2O) gives the
corresponding a 1,3-dioxolane, which is resistant to bases and
other nucleophiles. The 1,3-dioxolane protecting group can
thereafter be removed by further acid
hydrolysis. In this example,
isophorone was protected using ethylene glycol
with p-toluenesulfonic acid in moderate yield. Water was removed by
azeotropic distillation to
shift the equilibrium to the right.
Toxicity
The major danger from ethylene glycol is ingestion as it is
somewhat toxic with
LD50 = 1.4 g/kg for humans.
Due to its sweet taste, children and animals will sometimes consume
large quantities of it if given access to antifreeze. Upon
ingestion, ethylene glycol is oxidized to
glycolic acid which is, in turn, oxidized to
oxalic acid, which is toxic. It and its
toxic byproducts first affect the
central nervous system, then the
heart, and finally the kidneys. Ingestion of sufficient amounts can
be fatal.
Industrial hazards
Ethylene glycol can begin to break down at 230° – 250°F (110° –
121°C). Note that breakdown can occur when the system bulk
(average) temperature is below these limits, because surface
temperatures in heat exchangers and boilers can be locally well
above these temperatures.
The
electrolysis of ethylene glycol
solutions with a
silver anode results in an
exothermic reaction. In the
Apollo 1 fire catastrophe a coolant
consisting of ethylene glycol and water was implicated as a
possible cause via this reaction.
References
- Siegfried Rebsdat1 and Dieter Mayer "Ethylene Glycol” in
Ullmann’s Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH,
Weinheim. .
- Ethylene glycol. National Institute for
Occupational Safety and Health. Emergency Response Database. August
22, 2008. Retrieved December 31, 2008.
External links
Wikis
Quiz
Map
Search