[7791-21-1] · Cl2O · Dichlorine Monoxide · (MW 86.91)
(powerful chlorinating reagent; induces
ene-type and aromatic ring/side chain chlorination)
Alternate Names: chlorine oxide; hypochlorite.
Physical Data: mp -116 °C; fp -120.6 °C; bp 2.0 °C; pale orange-yellow gas.
Solubility: sol carbon tetrachloride, water (contact with water forms hypochlorous acid), ether, benzene, CH2Cl2, ethyl acetate, dioxane.
Form Supplied in: usually prepared as needed, as a solution in CCl4.2
Preparative Methods: gaseous dichlorine monoxide is prepared by passing Chlorine through a column containing dry yellow Mercury(II) Oxide.2,4 A carbon tetrachloride solution of dichlorine monoxide is obtained as follows:2,5 into a solution of chlorine in carbon tetrachloride is added 3.36 g of dry yellow mercury(II) oxide per gram of chlorine (1.1 mol/mol of chlorine) at 25 °C. After being stirred for 45 min at this temperature, the solids are removed by filtration through a coarse sintered-glass funnel to give a solution of dichlorine monoxide in ca. 75% yield.
Handling, Storage, and Precautions: explosions of the gas may result from a spark, by heating, or by contact with rubber, cork, or other forms of organic matter. A carbon tetrachloride solution is easier to obtain and safer to handle. It can be stored for a long time at -78 °C, but decomposes at a moderate rate at rt. Strong oxidant. Use in a fume hood.
Gas-phase chlorinations of alkanes such as methane and propane with dichlorine monoxide have been reported.6 More conveniently, a carbon tetrachloride solution of the reagent can be used for chlorination of various saturated compounds, such as 1-chlorobutane, 1-chloropropane, and butyronitrile.7 Each mole of dichlorine monoxide produces nearly 2 moles of chlorinated products and 1 mole of water. In the dark, a very long induction period is required, but this is significantly shortened by exposure to light or addition of Azobisisobutyronitrile (AIBN). A radical chain reaction mechanism has been proposed (eqs 1-7).
The chlorination of alkenes with dichlorine monoxide usually gives a mixture of allylic chlorides, b-chloroalkanols, a-chloroalkanones, and other products.1 For instance, the reaction of Cl2O with a large excess of cyclohexene at -20 °C in the dark proceeds instantaneously to give a mixture of the addition and substitution products (eq 8).8
Ene-type chlorination of alkenes with dichlorine monoxide in various organic solvents, such as methylene chloride, diethyl ether, benzene, dioxane, and ethyl acetate, proceeds smoothly at ambient conditions, affording the corresponding allylic chlorides in good to excellent yields. Each reaction is completed by use of 0.5 mol equiv of dichlorine monoxide, indicating that both chlorine atoms are utilized in the chlorination. Double ene-type chlorination is also performed by use of 1 mol equiv of the reagent (eq 9).
The ene-type chlorination proceeds in a highly regio- and chemoselective manner; indeed, ene-type chlorination of the terminal double bond of azetidinone derivatives is achieved by treatment with dichlorine monoxide at rt for 10 min, affording the desired chlorinated products in 85-93% yields (eqs 10 and 11).
Dichlorine monoxide is a powerful and selective reagent for either side chain or ring chlorination of inactive aromatic substrates; it gives excellent yields under mild conditions where conventional reagents fail or require harsh conditions. For instance, side chain chlorination of inactive alkyl aromatic compounds, such as p-nitrotoluene, can be performed by treatment with dichlorine monoxide in carbon tetrachloride at 25 °C for 96 h, affording the corresponding tri- or dichlorinated compounds in high purity (see Table 1). At 75 °C, the reaction affords equivalent yields in about 3 h.
Exclusive ring chlorination of inactive aromatic compounds e.g. p-nitrotoluene, proceeds smoothly at 0-50 °C if a strong protic acid, e.g. H2SO4, is added to the Cl2O reaction medium. Dichlorine monoxide is among the most reactive and selective agents known for chlorination of inactive aromatic ring systems; the degree of chlorination is controlled by the quantity of dichlorine monoxide employed, yielding mono- and perchlorinated products, respectively (Table 2). The mechanistic aspects of the reaction have been studied based on isotope effects, linear free-energy relationships, and related kinetics.
Chlorinations of phenols, aryl ethers,1 sulfamides,11 lignin,12 hydrazines,13 cyanic acid,14 and polymers15 with dichlorine monoxide have been investigated. The reactions are conducted in carbon tetrachloride, yielding mono- and/or polychlorinated products.
Dichlorine monoxide has been found to be an effective reagent for the bleaching of wood pulp and textiles. This method promises not only to increase process efficiency, but also reduce stream pollution significantly.
Sigeru Torii & Hideo Tanaka
Okayama University, Japan