Patented Nov. 3, 1953 2,658,086 UNITED STATES PATENT OFFICE 2,658,086 BROMINATION OF POLYHALOMETHANES Robert E. Ruh and Ralph A. Davis, Midland, Mich, assignors to The Dow Chemical Com pany, Midland, Mich., a corporation of Dela Ware No Drawing. Application July 2, 1952, Serial No. 296,927 9 Claims. (C. 260-653) 2 This invention relates to the bromination of carried out employing an equimolecular propor polyhalomethanes, particularly difluoromethane tion of halogen, i.e., total bromine and chlorine, and trifluoromethane. theoretically required to effect the desired de In the vapor phase halogenation of fluorometh gree of bromination. To completely monobro anes, it has been observed that chlorine reacts minate a polyhalomethane, it is theoretically much more rapidly than bromine. For instance, di necessary to employ at least an equimolecular fluoromethane Will react with chlorine at a prac proportion of halogen consisting of a mixture of tical rate at temperatures of 100 to 400° C. On bromine and chlorine and containing at least 0.5 the other hand, when bromine is to be reacted mole of bromine per mole of polyhalomethane. with difluoromethane, temperatures of 400° to O Although reaction will proceed with less than 600° C. are required for satisfactory results. equimolecular proportions of halogen to poly It has now been found, quite surprisingly, that halomethane, it is usually desirable to employ When a mixture of chlorine and bromine is con greater than an equimolecular proportion of tacted at elevated temperatures with a polyhalo halogen. The halogen mixture should prefer methane, particularly a fluoromethane, the pre 5 ably contain an equimolecular or greater than ponderant reaction is bromination rather than equimolecular proportion of bromine to chlorine, chlorination. Further, bromination in the preS With up to a 10:1 or higher ratio being operable. ence of chlorine occurs more readily, and at a Depending on other variables such as tempera lower temperature, than when bromine alone is ture, contact time, and degree of bromination de used as the brominating agent. At the preferred 20 Sired, a mole ratio of halogen (total bromine and conditions for the reaction, most of the bromine chlorine) to polyhalomethane of greater than undergoing reaction enters the polyhalomethane 15:1 is generally preferred although any ratio molecule and by-product hydrogen halide is from 1:1 to 10:1 or higher may be employed largely hydrogen chloride rather than hydrogen When the molecular proportion of bromine to bronide. 25 chlorine is greater than 0.7:1. A large excess of The principal object of the invention is to pro total halogen, though unnecessary, may be used. vide an improved method for producing bromo This is generally not desirable in plant Operation, fuoromethanes, particularly bromodifluorometh however, due to the problems connected with the ane, dibromodifluoromethane, and bromotrifluo recovery of unreacted halogen. romethane. More specific objects and advan The bromination of a polyhalomethane by the tages are apparent from the specification, which 30 method of the invention is carried out in the illustrates and discloses, but is not intended to vapor phase at a temperature in the range of limit the invention. about 200 to 500° C. Within this range, other According to the invention, a polyhalomethane factors remaining constant, the optimum tem containing at least one hydrogen atom and at perature for bromination varies with the poly least one fluorine atom and no iodine in the 35 halomethane to be brominated. For example, molecule is brominated. When heated at a reace difluoromethane can be brominated in accord tion temperature in the range of 200° C. to 500 With the invention at about 200° C. and bromina C. With a mixture of bromine and chlorine. The tion will proceed at a practical rate within the invention is particularly applicable to a fluoro temperature range of about 300° to 4000 C. Tri nethane of the group consisting of difluorometh 40 fluoromethane on the other hand, requires a tem ane and trifluoromethane. Other polyhalo perature of from 400° C. to 500° C. for bromina nethanes which may be brominated by the proc tion. It is significant, however, that difluoro ess of the invention are bromodifluoromethane, methane and trifluoromethane may be bromi chlorodifluoromethane, chlorofluoromethane, di nated at a practical rate at a Considerably lower chlorofluoromethane, etc. 45 temperature with a mixture of bromine and chlo It is not understood by what mechanism chlo rine, i.e. according to the process of the inven rine and bromine react with a polyhalomethane tion, than has heretofore been possible with bro to brominate the latter. However, it has been mine alone. discovered that halogenation is preponderantly Temperature to a large extent governs the con bromination rather than chlorination when bro 50 tact time preferably employed in the practice of mine and chlorine, in a molecular proportion the invention. In general, a Sufficiently long greater than 0.7:1, are reacted. With a polyhalo time of contact is employed to effect the desired methane in the temperature range of about 200 annount of reaction between the reactants at a to 500° C. More desirably, an equimolecular or given temperature. Contact times of from 1 to greater than equimolecular ratio of bromine to 55 30 seconds are usually employed in carrying out chlorine is preferred. the invention. Longer contact times up to 60 The bromination of polyhalomethanes may be seconds may be employed, e. g. in the brominal 2,658,086 3 4. tion of difluoromethane at approximately 200° Eacample 2 c. Conversely, a contact time of only a frac Trifluoromethane was brominated in the same tion of a second may be used when higher tent -equipment. and according to the gelaeral pro peratures are employed. Usually, however, COn cedure of Example 1. Through the reaction tact times in the range of 1 to 30 Seconds are tube heated to about 450° C. were passed 0.78 Suitable. imole of trifluoromethane, 4.8 moles of bromine, Bromination according to the invention pro-'. and 0.94 mole of chlorine during a period of 140 ceeds in the absence of solid catalysts and may minutes. Slightly greater than half of the be carried out either batchwise or continuously: trifluoromethane was converted to halogen It is preferred that the process be conducted 0 ated products. Based on the trifluoromethane continuously, because the reaction is more easily charged, 36.6 mole per cent bromotrifluorome carried out, and because better yields result. thane and 16.7 mole per cent chlorotrifluoro When a continuous process is employed, any methane were separated from the product by convenient reaction vessel, e. g. a cylindrical ce fractional distillation in a Podbielniak column. action tube, which is inert under the conditions What is claimed is: of the reaction may be used. In order to in. 1. A method of brominating polyhalomethanes crease the effective heat transfer surface, the which comprises contacting in a heated Zone at reactor is generally packed with a material inert a reaction temperature in the range of 200' to to the reactants and to the products, such as 500° C. for a time sufficiently long to effect re refractory pebbles, silica beads, etc. Usually 20 'action, a polyhalomethane containing at least atmospheric or slightly greater than atmospheric one hydrogen, at least one fluorine, and no iodine pressures are employed although bromination With a mixture of bromine and chlorine in a mole may be conducted at either subatmospheric or ratio of bromine to chlorine greater than 07:1. superatmospheric pressures. 2. A method according to claim 1 wherein the The following examples illustrate the process 25 polyhalomethane is a fluoromethane of the of the invention, but are not to be construed as group consisting of difluoromethane and tri limiting the scope thereof. fluoromethane. Eacample 1 3. A method according to claim 2 wherein the contact time is from 1 to 30 seconds. Difluoromethane was brominated at three dif 30 4. A method of brominating difluoromethane ferent temperatures in a vertical reactor con which comprises passing through a Zone heated sisting of a silica tube one inch in inside di to a temperature between about 300 and 400 ameter by 30 inches in length packed to a height C. at a rate corresponding to a contact time of of 24 inches with an inert heat transfer medium from 1 to 30 Seconds, a vapor phase mixture consisting of beads of silica gel. A 24 inch muffle 3 5 of difluoromethane and at least an equimolecular furnace surrounding the upright reaction tube proportion of halogen consisting of a mixture of was employed to maintain the desired temper bromine and chlorine in a mole ratio greater ature as measured by a thermocouple placed on than 0.7:1. the outside of the tube near the middle of heated 5. A method according to claim 4 wherein the zone. Difluoromethane gas was measured and 40 mole ratio of halogen to difluoromethane is from passed through a glass vaporizer containing 1:1 to 5:1. liquid bromine, the temperature of Which WaS 6. A method according to claim 5 wherein the regulated to give the desired ratio of CH2F2 to reaction is conducted in a packed tube. Br. Metered chlorine gas, together with the 7. A method of brominating trifluoromethane CHF-Bra mixture, was introduced into the top 5 which comprises passing through a zone heated of the reaction tube. After passing through the to a temperature between about 400 and 500 reactor, the effluent gases were Scrubbed in Water C. at a rate corresponding to a contact time of and aqueous sodium hydroxide, and were then from 1 to 30 seconds, a vapor phase mixture dried with calcium sufate and condensed in a of trifluoromethane and greater than 1.5 molec receiver cooled in a Dry Ice-methylene chloride ular proportions of halogen consisting of a mix mixture. The organic product was later Sep ture of bromine and chlorine in a mole ratio arated into its component parts by fractional greater than 1:1. distillation in a Podbielniak column. The data, 8. A method according to claim 7 wherein the for the three runs are contained in the follow reaction is conducted in a packed tube. ing table. 5 5 9. In a bromination method wherein a poly PROMINATION OF CIF halomethane containing at least one hydrogen, at least one fluorine, and no iodine is passed CEF. -- Moles.------0.65 0.71 0.7. together with bromine through a zone heated Bra------do------0.52 0.78 2.25 Cl2------0.5 0.70 0.92 to a temperature in the range of 200° to 500 CHF/Br/Cl:------do------11.81.77 1/1.11.99 1.f3.16/1. 28 60 Reaction temper- C------250. 300 350 C. for a time sufficiently long to effect reaction, ture. the improvement which comprises carrying out Contact time---- See- - - - 23 2 11. the bromination in the presence of chlorine in Duration of run-i Min-- 90 . , 120 120 Product re a molecular proportion of at least 0.1 that of covery the bromine. CBrF------Moliepercent. 14.5 19.1 36, d 32.0 41.3 39, 65 4.2 3.8 1. 13.9 10.1 4. ROBERT P. R.U.H. 8.2 .7 7. RAPH. A. DAWS. Total organic re------do------2.8. 86.0 89, covery. No references cited. BralClin product Moles------3.61 6.0 18.4