The Raschig–Hooker process is a
chemical process
In a scientific sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by an outside force, and involves a chemical reaction of some ...
for the production of
chlorobenzene
Chlorobenzene is an aromatic organic compound with the chemical formula C6H5Cl. This colorless, flammable liquid is a common solvent and a widely used intermediate in the manufacture of other chemicals.
Uses
Historical
The major use of chlorob ...
and
phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...
.
The Raschig–Hooker process was patented by
Friedrich Raschig, a German chemist and politician also known for the
Raschig process, the
Olin Raschig process and the
Raschig ring
Raschig rings are pieces of tube, approximately equal in length and diameter, used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic, metal or glass and provide a la ...
. He first begun to use this reaction in 1891 in order to manufacture phenol.
![RaschigHookerProcess](https://upload.wikimedia.org/wikipedia/commons/b/bb/RaschigHookerProcess.png)
The main steps in this process are the production of chlorobenzene from
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
,
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
and
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
, and the subsequent hydrolysis of chlorobenzene to phenol. The first step uses either a
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
or
iron chloride Iron chloride may refer to:
* Iron(II) chloride
Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are o ...
catalyst and exposes the materials to air at 200–250℃.
In the second step, the resulting chlorobenzene is introduced to steam at 450℃ over a silicon catalyst that hydrolyses the chlorobenzene, giving phenol and hydrogen chloride that can then be recycled back to the first step.
Due to the two step nature, the Raschig–Hooker process can be used to produce either chlorobenzene or phenol.
![Raschighookerprocess](https://upload.wikimedia.org/wikipedia/commons/b/bb/RaschigHookerProcess.png)
The Raschig–Hooker process's ability to make phenol makes it comparable to other methods, such as the
Dow and Bayer process, which also converts benzene into phenol. In fact, the ability to recycle the hydrogen chloride made the Raschig–Hooker process preferable to the Dow and Bayer process, which requires its sodium chloride product to be converted into chlorine and sodium hydroxide. The reaction, however, takes place at very high temperatures in a very acidic environment with hydrogen chloride vapor and therefore the industrial setting must use highly
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
resistant equipment for the reaction. While the Raschig–Hooker process does recycle the hydrogen chloride it produces, its catalyst experiences carbon deposition and must be frequently regenerated. The harsh chemical environment, use of catalysts, and large energy consumption has made it a target for
green chemistry
Green chemistry, also called sustainable chemistry, is an area of chemistry and chemical engineering focused on the design of products and processes that minimize or eliminate the use and generation of hazardous substances. While environmental che ...
alternatives.
The Raschig–Hooker process suffers from selectivity issues in both steps. In the first step, the reaction is only run to 10% to 15% conversion to prevent the second addition of a chlorine atom to the desired chlorobenzene. Despite this, the overall selectivity of the reaction is 70% to 85%. This second addition can be reversed using the Hooker modification, though it is also costly. The second step shares the low conversion rate and high selectivity of the first step. The small amount conversion per reaction offsets the monetary benefit of recycling the hydrogen chloride due to the large initial cost of the reaction. Therefore, the Raschig–Hooker process needed to be run at high concentrations in large reactors to be industrially economical.
Due to its low productivity, this process is largely unused today. , every plant in the United States that was using the Raschig–Hooker process has been shut down, though it was still used by some plants in countries such as Argentina, India, Italy, and Poland. Rather than using the Raschig–Hooker process, some companies use the Hock or
cumene process
The cumene process (cumene-phenol process, Hock process) is an industrial process for synthesizing phenol and acetone from benzene and propylene. The term stems from cumene (isopropyl benzene), the intermediate material during the process. It was i ...
, which instead synthesizes
acetone
Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour.
Acetone is miscib ...
and phenol from benzene and propylene. This preferred process has dominated the market, especially as acetone is also a highly desired substance.
References
Chemical processes
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