The Dow process is the
electrolytic method of
bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
extraction from
brine
Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for ...
, and was
Herbert Henry Dow's second revolutionary process for generating bromine commercially.
This process was patented in 1891. In the original invention, bromide-containing brines are treated with
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
and
bleaching powder to oxidize bromide to bromine, which remains dissolved in the water. The aqueous solution is dripped onto burlap, and air is blown through causing bromine to volatilize. Bromine is trapped with iron turnings to give a solution of
ferric bromide. Treatment with more iron metal converted the ferric bromide to
ferrous bromide via
comproportionation
Comproportionation or synproportionation is a chemical reaction where two reactants containing the same element but with different oxidation numbers, form a compound having an intermediate oxidation number. It is the opposite of disproportionation. ...
. Where desired, free bromine may be obtained by thermal decomposition of ferrous bromide.
Before Dow got into the bromine business, brine was evaporated by heating with wood scraps and then crystallized
sodium chloride
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
was removed. An oxidizing agent was added, and bromine was formed in the solution. Then bromine was distilled. This was a very complicated and costly process.
Preparation of phenol
Dow's Process may also refer to the hydrolysis of
chlorobenzene in the preparation of
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 r ...
. Benzene can be easily converted to chlorobenzene by
nucleophilic aromatic substitution via a
benzyne intermediate. It is treated with aqueous sodium hydroxide at 350 °C and 300 bar or molten sodium hydroxide at 350 °C to convert it to
sodium phenoxide
Sodium phenoxide (sodium phenolate) is an organic compound with the formula NaOC6H5. It is a white crystalline solid. Its anion, phenoxide, also known as phenolate, is the conjugate base of phenol. It is used as a precursor to many other organic ...
, which yields phenol upon acidification.
When 1-
14C">sup>14C1-chlorobenzene was subjected to aqueous NaOH at 395 °C, ''ipso'' substitution product 1-
14C">sup>14Cphenol was formed in 54% yield, while ''cine'' substitution product 2-
14C">sup>14Cphenol was formed in 43% yield, indicating that an elimination-addition (benzyne) mechanism is predominant, with perhaps a small amount of product from addition-elimination (S
NAr).
References
Chemical processes
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