Epichlorohydrin (ECH), also known by its IUPAC name 2-(chloromethyl)-oxirane, is a chlorinated organic intermediate. It is an epoxide, a colorless liquid with a pungent odor. Globally, epichlorohydrin is produced by epoxidation of propylene. However, its two-step synthesis from glycerol is currently receiving more attention.
Due to the presence of an epoxy ring which is susceptible to generating free radicals that may cause cancer. Therefore, it is highly recommended to strictly follow the material safety data sheet (MS-DS) while using it in laboratories and chemical plants.
Furthermore, It is used as a monomer to produce a wide range of polymers, including epoxy resins, which are used in the construction, automotive, and aerospace industries. It is used as a solvent and as a reactant in the synthesis of pharmaceuticals and other chemicals. Moreover, It is also used to manufacture a variety of other products such as plastics, resins, elastomers, adhesives, coatings, water treatment chemicals, and, medicines.
Based upon the high demand, epichlorohydrin manufacturing is increasing exponentially worldwide. Its market volume during the last few years is almost 2 million metric tons.
In 2021, its annual consumption by the region is given in the pie chart below:
Epichlorohydrin is manufactured on a commercial scale by the epoxidation of propylene in the presence of an acid catalyst. To produce an allyl chloride intermediate, chlorine is introduced into a reaction chamber containing propylene and a catalyst in the first step. Then steam is passed through it, causing chlorine to react with it to form hypohalous chloride, which then reacts with ally chloride to form halohydrin. Finally, to close the epoxy ring, a strong alkali such as sodium hydroxide (NaOH) is added. The chemical reactions taking place in this method are given below:
Feeding/charging of reactants
Propylene and chlorine are fed into a reaction vessel, which is equipped with an agitator (stirrer) for the thorough mixing of the reactants. The chlorine is used to generate hypochlorous acid, which catalyzes the epoxidation reaction.
The epoxidation reaction is allowed to proceed for a certain amount of time, after which the epichlorohydrin is separated from the reaction mixture by distillation. In this distillation process, the mixture is heated to a certain temperature at which the epichlorohydrin vaporizes. After that, these vapors passed through a condenser, thus they are liquified in another container at a low temperature.
The reactor is heated to the reaction temperature, which is typically around 50-80°C. The epoxidation reaction is exothermic, so cooling may be needed to control the temperature.
Purification of epichlorohydrin
The epichlorohydrin is then purified by further distillation to remove any impurities or by-products. The purified epichlorohydrin is then cooled and collected for further processing or use in the manufacture of various products
A suitable, sustainable synthesis method to prepare epichlorohydrin is glycerol to epichlorohydrin. As glycerol is the by-product of biodiesel production, thus easily available as the starting material.
However, this procedure is difficult to apply at the industrial level because of the high cost of glycerol.
In this reaction, the glycerol is initially converted into Monochlorohydrin and consequently into Dichlorohydrin, since the unreacted monochlorohydrin is recycled to convert it into the required Dichlorohydrin. Moreover, The most effective catalyst for this reaction is a carboxylic acid because it does not allow the formation of Trichlorohydrin which makes the reaction less effective.
Finally, sodium hydroxide is introduced to close the ring of the epoxide.
The purity of epichlorohydrin can be determined by IR spectroscopy as well as 1H NMR spectroscopy. Its reference spectra are given below:
Epichlorohydrin: toxicity and precautions
It is important to note that epichlorohydrin is a potential carcinogenic compound, classified as a hazardous air pollutant by the United States Environmental Protection Agency (EPA). Its exposure limit is permissible exposure limit (PEL) for epichlorohydrin of 0.1 ppm (parts per million) in the workplace in a working week.
Furthermore, it can have harmful effects on human health if ingested, inhaled, or absorbed through the skin. Short-term exposure to high levels of epichlorohydrin can irritate the eyes, skin, and respiratory tract, as well as nausea, vomiting, and diarrhea. Long-term exposure to epichlorohydrin may cause liver and kidney damage, as well as an increased risk of cancer.
Is epichlorohydrin used in the manufacturing of tea bags?
Tea bags used to be coated with chemicals like epichlorohydrin and dioxin, which prevented them from breaking when they came into contact with water. The food and drug administration U.S (FDA) approved this chemical harmful to be directly used in food products, based on information about its toxicity.
Therefore, famous tea brands such as Buddha teas® stated in their description that the tea bags are epichlorohydrin free.
Manufacturing of epoxy resins
Bisphenol-A and epichlorohydrin are the main constituents to synthesize prepolymers for epoxy resins and adhesives. For this purpose, firstly, bisphenol A is treated with a base and converted into its anion, which acts as a nucleophile in an SN2 reaction with epichlorohydrin.
It is to be noted that each epichlorohydrin molecule can react with two molecules of bisphenol A. Once by nucleophilic substitution displacement of chloride and the other by the nucleophilic opening of the epoxide ring. These two Bisphenol-A molecules react with two epichlorohydrin molecules simultaneously to form long chain prepolymer. Both ends of the prepolymer contain an epoxy ring.
In medicinal chemistry
Epichlorohydrin is an important starting material for the 1,2,3-substituted compounds. This is due to the presence of more electrophilic carbon than C-Cl. For example, propranolol, a beta-receptor blocker is synthesized using enantiomerically pure substance. During synthesis, 1-naphthol is reacted with epichlorohydrin. Note that, propranolol is used orally to lower blood pressure.
What is epichlorohydrin used for?
It has a wide range of applications. Certainly, it is used to produce 90% of the net epoxy resins worldwide. some other uses are listed below:
- Water treatment chemicals
What is FDA-approved epoxy?
FDA-approved epoxies are those which does contain any harmful material to the environment.
Can breathing epoxy fumes make you sick?
Yes, epoxy fumes contain prepolymer which may cause lung cancer. hence proper personnel protective equipment should be used while dealing with it.
What are the raw materials of epoxy adhesive?
Bisphenol A, epichlorohydrin, and a curing agent such as triethyl diamine are the raw materials of epoxy adhesives.