Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C201/00—Preparation of esters of nitric or nitrous acid or of compounds containing nitro or nitroso groups bound to a carbon skeleton
  • C07C201/06—Preparation of nitro compounds
  • C07C201/08—Preparation of nitro compounds by substitution of hydrogen atoms by nitro groups

Definitions

• the present invention relates to nitration of aromatic compounds.
• Nitration of aromatic compounds is of considerable commercial importance, as nitrated aromatic products find utility as dyes, explosives, pharmaceuticals, perfumes, plastics and solvents.
• Nitration processes are known that use a mixture of nitric acid and sulfuric acid (hence, these methods are referred to as "mixed acid methods"), wherein sulfuric acid acts as a catalyst.
• mixed acid methods Unfortunately, these processes produce large quantities of waste dilute sulfuric acid, and the disposal or recovery of this acid waste presents a serious environmental problem. Recovery involves an energy-intensive and expensive process of reconcentrating sulfuric acid , that has been diluted by the water produced in the nitration reaction.
• the mixed acid method also produces nitrocresol and cyanide by-products, which require expensive waste-water treatment to remove. Further, the mixed acid method is not selective and produces a mixture of isomers and polynitrated compounds that are difficult to separate from each other.
• nitrating reagents such as acyl and alkyl nitrates
• acyl and alkyl nitrates used in these methods are explosive (as is nitric acid itself in the presence of solid acidic catalysts) and are therefore hazardous to use.
• Nitration of aromatics with alkyl nitrates requires protic or Lewis acid activation.
• Copper (II) nitrate supported on montmorillonite clay quantitatively nitrates toluene in the presence of acetic anhydride, but this reaction achieves para- regioselectivity only under conditions of high dilution and long reaction times (i.e. over 120 hours).
• Para-selectivity may be achieved using H ⁇ zeolite or HY zeolite as a solid inorganic catalyst and a combination of liquid nitrogen dioxide and gaseous oxygen as the nitrating agent.
• Zeolites have also been used in the vapour phase nitration of aromatic compounds using nitrogen dioxide and other methods.
• the present invention provides a process for the nitration of an aromatic compound, the process comprising contacting an aromatic compound with a nitrating agent in the presence of a phosphonium salt ionic liquid.
• the current invention provides a novel process for the nitration of aromatic compound that may be used to obtain nitrated aromatic compounds in high yield and with high selectivity.
• This process provides several advantages over existing methods.
• the current process can avoid the use of sulphuric acid and thereby avoid many of the hazardous waste products that are associated with conventional mixed acid nitration methods.
• the current process also avoids the use of chlorinated organic solvents (e.g. methylene chloride), which are environmentally hazardous, in favour of phosphonium salts, which have zero vapour pressure and are therefore more easily contained.
• the current process does not require explosive nitrating agents (for example the acyl or alkyl nitrating agents) .
• the current nitration process does not require solid acid catalysts, nor Lewis acids. The current process results in a mononitrated product, without concommitant dinitrated compounds .
• the preferred nitrating agent is nitric acid, especially fuming nitric acid.
• Nitric acid is a preferred ntirating agent because it is relatively inexpensive and readily available.
• other nitrating agents may be used for the nitration of an aromatic compound in a phosphonium salt.
• Suitable nitrating agents include nitrate salts, and mention is made of NaN0 3 , and KN0 3/ in combination with H 2 S0 4 .
• H 2 S0 4 can react with NaN0 3 or KN0 3 to produce HN0 3 and Na 2 S0 4 or K 2 S0 4 , respectively, and the production of waste H 2 S0 4 can be avoided.
• the nitrating agent, say nitric acid, and the aromatic compound to be nitrated may be used in approximately stoichiometric amounts to produce mononitrated products in high yield, with little or no production of polynitrated aromatic compounds .
• the molar ratio of nitrating agent to aromatic compound can be in the range 1:1.3 to 1.3:1. Preferably a modest excess of nitrating agent, say 1.2:1, is used.
• the efficient use of nitric acid which is a relatively inexpensive nitrating agent, contributes to the overall economy of the current nitration process.
• the phosphonium salt solvent may be recovered for reuse. Phosphonium salts may be stable under treatment with fuming nitric acid and moderate heating, for example 80 °C, for extended periods of time (at least 3 days) .
• nitration of an aromatic compound with nitric acid proceeds according to the following scheme, using optionally substituted benzene as example, to produce a nitrated aromatic compound and water:
• the process of nitration may be carried out over a wide range of temperatures, for example from -75 C C up to the upper limit at which ionic liquids decompose, at about 300°C.
• the reaction is carried out at a temperature where the reaction mixture
• reaction (which comprises an aromatic compound, a nitrating agent and a phosphonium salt) is a liquid.
• the reaction may be carried out at temperatures between 0°C and 120°C, more preferably between room temperature and 100 °C.
• the pressure can range between 1 bar and 100 bar, but the reaction is conveniently carried out at atmospheric pressure.
• the time of reaction may vary with temperature, but is usually about 12 to 24 hours.
• the nitrated aromatic products may be purified from the reaction mixture by any of several methods.
• the reaction product may be purified by the method of steam distillation, the method comprising:
• the nitrated aromatic may be convenient to isolate by vacuum distillation, provided that the product has a boiling point that is below the temperature at which either the nitrated aromatic or the phosphonium salt decomposes.
• an organic solvent for example petroleum ether or cyclohexane
• the phosphonium salt may be recovered for reuse
• the phosphonium salt can be re-used many times without loss of activity or selectivity.
• the aromatic compounds for use in the inventive process may be any known hydrocarbon compound containing one or more aromatic ring systems .
• aromatic ring systems include: phenyl, naphthalenyl , anthracenyl, phenanthrenyl , pyrenyl and coronenyl .
• the aromatic compounds to be nitrated may contain substituents, provided that the substituents do not interfere with the nitration process. When there is more than one substituent present, the substituents may be the same or different.
• substituents include: alkyl, alkenyl and alkynyl, especially C; L -C 6 alkyl, C 2 -C 6 alkenyl or C 2 -C 6 alkynyl, any of which may optionally be substituted with one or more substituents selected from, for example, halogen or hydroxy; halo e.g. fluoro, chloro, bromo or iodo; alkoxy, especially C ⁇ -C e alkoxy optionally substituted by halogen e.g.
• R 4 and R 5 are each independently hydrogen or Ci-Cg alkyl optionally substituted with one or more halogen atoms and R 6 is a halogen atom or a group R 4 .
• aromatic compounds that comprise a phenyl ring, substituted or unsubstituted. Mention is also made of aromatic compounds comprising a diphenyl ether, the phenyl rings of which are independently optionally substituted by one or more groups selected from: halo; hydroxy; COOR 4 , COR 6 , CONR 4 R 5 or CONHS0 2 R 4 , wherein R 4 and R s are each independently hydrogen or C ⁇ -C 6 alkyl optionally substituted with one or more halogen atoms and R 6 is a halogen atom or a group R 4 .
• the ionic liquid used in the current invention may be a phosphonium salt according to the general formula:
• each of R 1 , R 2 , R 3 , and R 4 is independently a hydrocarbyl group or a hydrogen, provided that not more than one of the R 1 to R 4 groups is a hydrogen;
• X " is an anion, provided X " is not a hydroxyl group; for example, suitable anions include halides, phosphinates, alkylphosphinates, alkylthiophosphinates, sulphonates, tosylates, aluminates, borates, arsenates, metallates; cuprates, sulfates, triflate, bistriflamide, and carboxylates , for example trifluoroacetate.
• the phosphonium salt will be a tetrahydrocarbylphosphonium salt, wherein each of R 1 , R 2 , R 3 , and R 4 is independently an alkyl group of 1 to 30 carbon atoms, a cycloalkyl group of 3 to 7 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, an aryl group of 6 to 18 carbon atoms, or an aralkyl group. It is possible for two of R 1 , R 2 , R 3 , and R 4 together to form an alkylene chain.
• the phosphonium salt should be liquid at the desired temperature for carrying out the nitration reaction, but it is not necessary for the phosphonium salt to be liquid at room temperature in all cases.
• Phosphonium salts that melt at low temperatures may be suitable for nitration reactions carried out at slightly elevated temperatures (i.e. in the range of 50°C to 100°C) . Since alkyl- groups with 4 carbon atoms or less can increase the melting point for the ionic liquid, more preferred are phosphonium salts according to formula (I) wherein each of R 1 , R 2 , R 3 , and R 4 is independently an alkyl group of 4 to 20 carbon atoms.
• R 1 , R 2 , R 3 , and R 4 may be n-butyl, isobutyl, n-pentyl, cyclopentyl, isopentyl, n-hexyl, cyclohexyl, (2,4 ,4'-trimethyl)pentyl, cyclooctyl, tetradecyl, etc.
• the degree of asymmetry and the degree of branching of the hydrocarbyl groups are important determinants of the melting point of the phosphonium salt: the melting point tends to decrease as the degree of asymmetry and branching is increased. Therefore, preferred compounds are those in which R 1 , R 2 , R 3 , and R 4 are not identical and/or are branched.
• Phosphonium salts include compounds according to formula (I) wherein each of R 1 , R 2 , R 3 , and R 4 is independently an aryl or aralkyl group.
• Aryl-containing salts may be less preferred in view of the possibility that the aryl and/or aralkyl groups may become nitrated under the reaction conditions used.
• an aryl-containing phosphonium salt that has become nitrated may also be a suitable solvent for nitration of aromatic compounds. Examples of aryl and aralkyl groups include phenyl, phenethyl, toluyl , xylyl, and naphthyl .
• R 1 , R 2 , R 3 , and R 4 may bear substituents, or to include heteroatoms, provided that the substituents or heteroatoms are inert (e.g. do not undergo nitration or oxidation) under the reaction conditions used, do not adversely affect the desired reaction, and do not adversely affect the desired properties of the ionic liquid.
• Acceptable substituents include alkoxy and acetyl, and acceptable heteroatoms include oxygen.
• Preferred anions form liquid salts at temperatures below about 100°C and preferably below about 50°C when combined with a cation described above .
• Suitable types of anions include: anions based on nitrogen, phosphorus, boron, silicon, selenium, tellurium, aluminum, copper, arsenic, antimony, bismuth, or halogens; oxoanions of metals; halides; phosphinates, mono- and dialkylphosphinates, alkylthiophosphinates, sulphonates, tosylates, aluminates, borates, arsenates, cuprates, sulfates, nitrates, and organic anions, for example trifluoroacetate, bistriflamide and triflate.
• alkyl groups each independently has any of the values given to R 1 , R 2 , R 3 , and R 4 of the phosphonium cation (as defined above) .
• sulfur-containing anions such as triflates, bistriflamides or sulfates, may be preferred.
• preferred anions include: chloride; bromide; perchlorate; fluoride; sulfate; sulfonate; fluorosulfonate; trifluoromethylsulfonate; triflate; bistriflamide; dicyclohexylphosphinate; diisobutylphosphinate; bis (2,4, 4'-trimethylpentyl)phosphinate; diisobutyldithiophosphinate; tetrafluoroborate; tetrachloroborate; hexafluorophosphate; hexafluoroantimonate and hexafluoroarsenate .
• phosphonium salts according to formula (I) that are hydrophobic or water immiscible may be preferred.
• some applications may involve washing the reaction mixture with water, in which .case it may be advantageous to use a phosphonium salt that is immiscible with water and forms a two-phase system when mixed with water.
• water immiscible is intended to describe compounds that form a two phase system when mixed with water but does not exclude ionic liquids that will dissolve water, provided that the two-phase system forms. Therefore, phosphonium salts that have a larger total number of carbons, equal to or greater than 20 and in particular greater than 25 or 26, are preferred because they are more hydrophobic .
• the given phosphonium salt ionic liquid consists of two components, which are a positively charged phosphonium cation and a negatively charged anion.
• any salt which can be a fluid at or near the reaction temperature or exist in a liquid state during any stage of the reaction can be used as the ionic liquid.
• Moisture sensitive anions may react with the water that is produced by the nitration reaction, and it is therefore preferred that X " is an anion that is not moisture sensitive.
• Moisture sensitive anions include: transition metal halide complexes such as tetrachloroaluminate, tetrachloroferrate, or trichlorocuprate .
• trioctyl (tetradecyl) phosphonium chloride trioctyl (tetradecyl) phosphonium chloride
• phosphonium salts of formula (I) are novel.
• phosphonium hydrocarbylphosphinates and phosphonium hydrocarbylthiophosphinates are the subject of Canadian Patent Application Serial 2,343,456, filed on March 30, 2001.
• the novel salts can be made from compounds of formula (I) in which the anion is a good leaving group, for example a halogen or acetate or tosylate, in an ion exchange reaction with a salt of the desired anion.
• the salt can be, for example, an ammonium or an alkali metal salt.
• the following aromatic compounds were nitrated essentially as described in Example 2: benzene, toluene, o-xylene, m-xylene, p-xylene and naphthalene.
• the aromatic compound to be nitrated was dissolved in 2.0 g of trihexyl (tetradecyl) phosphonium triflate, and fuming nitric acid was added.
• the equivalent ratio of aromatic compound to nitric acid was 1:1.2. Then, the contents of the flask were heated to 80°C and the reaction was allowed to proceed for 6 hours .
• EXAMPLE 5 Nitration of various aromatic compounds in trihexyl (tetradecyl) phosphonium bistriflamide at room temperature
• Trihexyl (tetradecyl) phosphonium triflate was synthesised by the reaction of trihexyl (tetradecyl) phosphonium chloride (l.Oeq) and sodium trifluoromethanesulfonate (1.05eq) in acetone over a period of 2 to 9 hours. The total reaction mixture was concentrated and the residue was dissolved in chloroform or ether. The organic layer was washed with deionised water till the organic layer did not show the presence of chloride ions by silver nitrate test, then dried over anhydrous MgS0 4 , filtered and concentrated on a rotary evaporator. The ionic liquid thus obtained was subjected to high vacuum.
• EXAMPLE 8 EXAMPLE 8 :
• Trihexyl (tetradecyl) phosphonium bistriflamide was synthesised by the reaction of trihexyl (tetradecyl) phosphonium chloride (l.Oeq) and lithiumbistriflamide (1.05eq) in acetone over a period of 2 to 9 hours.
• the total reaction mixture was concentrated and the residue was dissolved in chloroform or ether.
• the organic layer was washed with deionised water till the organic layer did not show the presence of chloride ions by silver nitrate test, then dried over anhydrous MgS0 4 , filtered and concentrated on a rotary evaporator.
• the ionic liquid thus obtained was subjected to high vacuum.

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