Process for preparing 2,3-dibromo-2-alkylalkanes
United States Patent 1191 McNulty et al.  
 PROCESS FOR PREPARING 2,3-DlBROMO-2-ALKYLALKANES lnventors: John G. McNulty, Glenshaw, Pa.;  
 Glen A. Russell, Ames, Iowa; William L. Walsh, Glenshaw. Pa.  
 Gulf Research &amp; Development Company, Pittsburgh, Pa.  
 Filed: Dec. 1, 1972 Appl. No.: 311,368  
 Assignee:  
 US. Cl 260/658 R Int. Cl. C07c 17/10 Field of Search 260/658 R References Cited UNITED STATES PATENTS 7/l9l4 Groul 260/658 R 1451 Apr. 8, 1975 OTHER PUBLICATIONS Russell and Brown, JACS, 77, p. 4030 (1955) QDlAS.  
 Hughes, lngold and Scott, J. Chem. Soc. of London, (1937), PP- 1274-1276 QDlC6.  
 Primary Examiner-Delbert E. Gantz Assistant Examiner-Joseph A. Boska [57] ABSTRACT 8 Claims, No Drawings PROCESS FOR PREPARING 2,3-DIBROMO-2-ALKYLALKANES This invention relates to a process for preparing 2,3-dibromo-2-alkylalkanes while inhibiting the formation of 2,3,3-tribromo-Z-alkylalkanes.  
  When a 2-bromo-2-alkyalkane is reacted with bromine to form a 2,3-dibromo-IZ-alkylalkane undesirable amounts of 2,3,3-tribromo-2-alkylalkanes are also formed. For example, Russell and Brown (J.A.C.S., Aug. 5, 1955, page 4030) reacted equimolar quantities (0.25 mo1) of bromine and t-amylbromide (2-bromo-2- methylbutane) in 120 grams of carbon tetrachloride to give 34 grams of 2,3-dibromo-2-methylbutane (59 per cent yield) and 8.5 grams of 2,3,3-tribromo-2- methylbutane (11 per cent yield). The presence of the carbon tetrachloride effectively removes formed hydrogen bromide from further reaction. Carbon tetrachloride has been used because of the belief that the removal of hydrogen bromide promotes the bromination reaction. Shriner and Fusan (Identification of Organic Compounds, pages 93 and 94, John Wiley and Sons, Third Edition, 1948) state: Carbon tetrachloride is a good solvent for bromine and for many organic compounds but does not dissolve hydrogen bromide.&#34; Reducing the amount of 2,3,3-tribromo-2-alkylalkanes is important. For example, 2-bromo-2-methylbutane can be reacted with bromine to form 2,3-dibromo-2- methylbutane which, in turn, can be dehydrobrominated to form isoprene. To the extent 2,3,3-tribromo-Z-methylbutane is also formed during the course of the reaction, obviously reduced amounts of 2,3-dibromo-2-methylbutane are obtained for use in the production of isoprene.  
  We have found that a 2-bromo-2-alkylalkane can be reacted with bromine alkylalkane while inhibiting the formation of 2,3,3-tribromo-2-alkylalkane by the relatively simple expedient of carrying out the reaction in the presence of HBr while maintaining the reaction system substantially anhydrous. That HBr would have the desired effect herein of inhibiting formation of 2,3,3-tribromo-2- alkylalkane is surprising in view of the fact that in the reaction ofa 2-bromo-2-alkylalkane with bromine HBr is formed as a side product.  
  The Z-bromo-Z-alkylalkanes that can be used herein can be defined by the following structural formula:  
 I-I-C-C-C-R I U I BrH H R H to form 2,3-dibromo-2- H Br Br H R H and n-c-c-c-a I I I Br Br H R2 Br wherein R, and R are as defined above. In a preferred embodiment herein Z-bromo-Z-methylbutane is reacted with bromine to form 2,3-dibromo-2- methylbutane while inhibiting the formation of 2,3,3- tribromo-2methylbutane. Specific examples of Z-bromo-Z-alkylalkanes that can be used as charge herein include 2-bromo-2-methylbutane, 2-bromo-2- ethylbutane, 2-bromo-2-propylbutane, 2-bromo-2- butylbutane, 2-bromo-2-amylbutane, 2-bromo-2- isopropyl-butane, 2bromo-2-t-butylbutane, Z-bromo- 2-isobutylbutane, 2-bromo-2-isoamylbutane, 2-bromo-2-t-amylbutane, 2-bromo-2-methylhexane, 2-bromo-2-ethylhexane, 2-bromo-2-propylheptane, 2-bromo-2-butyl-heptane, 2-bromo-2- isopropylheptane, Z-brOmo-Z-amylheptane, 2-bromo- 2-isobutylhexane, etc.  
  The reactants can be employed over a wide range. Thus, the molar ratio of 2-bromo-2-alkylalkane to bromine can be from about 100:1 to about 1:1, but, in general, can be from about 50:1 to about 20: 1 The amount of HBr relative to the 2-bromo-2-alkylalkane necessary to inhibit formation of undesired 2,3,3-tribtromo-2- alkylalkane, on a molar basis, can be from about 100:] to about 1:1, preferably from about 50:1 to about 20: l.  
  The reaction herein is simply effected by bringing together the 2-bromo-2-alkylalkane, bromine and HBr while maintaining the reaction system substantially anhydrous, preferably by using substantially anhydrous components. The components of the reaction system are miscible in each other. The reaction conditions are not critical. Thus, the temperature can be maintained in a range of about 31 30 to about 100 C., preferably from about 0 to about 50 C. Any pressure sufficient to maintain HBR in the reaction system at reaction temperature will suffice, for example, from about atmospheric to about 1000 pounds per square inch gauge, preferably from about 100 to about 500 pounds per square inch gauge. A reaction period of about 5 minutes to about 2 hours, preferably in the range of about 10 to about 60 minutes, will suffice.  
  The desired 2,3-dibromo-2-alkylalkane can be recovered from the reaction system in any suitable manner. A convenient procedure involves bringing the reaction system to atmospheric pressure and temperature and then subjecting the same to distillation conditions. The components will come off in the following order and can be recovered as desired: HBr, bromine, the monobromoalkane, the desired dibromoalkane and, finally,  
 whatever tribromoalkanes that may also have been produced.  
  The process herein can be further illustrated by the following.  
 EXAMPLE I To a 300-milli1iter pressure glass reactor equipped with a magnetic stirrer and a thermowell, which had been cooled below C., there was added 46.7 grams of tertiary amyl bromide and 15.9 grams of bromine. The reactor was connected by a rubber O-ring seal to a manifold having a pressure gauge and a connection to an anhydrous hydrogen bromide cylinder. HBr was added to the reactor until the pressure therein rose to 140 pounds per square inch gauge and the temperature rose to 45 C. At the end of 45 minutes the pressure was released and a test sample taken. A gas liquid chromatographic analysis of the reaction mixture showed 76.4 mol per cent 2-bromo-2-methylbutane. 22.6 mol per cent 2.3-dibromo-2-methylbutane and 1.0 mol per cent 2,3,3-tribromo-2-methy1butane. The reaction product was again cooled to below 0 and 20.7 grams of bromine was added thereto. After reassembly, anhydrous HBr was again added to the reaction system until the pressure reached 140 pounds per square inch gauge and the reaction mixture 45 C. Altogether 15.6 grams of HBr was added to the system during the course of this run. After 1 hour the pressure was released and the product was analyzed by gas liquid chromatography and found to contain 37.4 mol per cent 2-bromo-2- methylbutane, 60.0 mol per cent, 2,3-dibromo-2- methylbutane and 2.6 mol per cent 2,3,3-tribromo-2- methylbutane. Thus, efficiency to desired 2,3-dibromo- 2-methy1butane was 95.8 mol per cent.  
 EXAMPLE 11 To a stirred monel autoclave which had been cooled to below 0, there was added 1 10.2 grams of 2-bromo- Z-methylbutane and 69.0 grams of bromine. The autoclave was sealed, stirrer started and HBr added while the reactor was warmed to 35 C. Altogether 17.6 grams of HBr was added to the reaction system. After 1 hour at 35 C. and 345 pounds per square inch gauge, the pressure was released and the product analyzed by gas liquid chromatography. The results obtainied are shown below as Run No. 1 in Table 1. Additional runs similarly made are also tabulated in Table l.  
  From the above it can be seen that when a 2-bromo- 2-alky1a1kane is brominated in a substantially anhydrous system in the presence of HBr desired 2,3-dibromo-2-alkyla1kane is obtained with but small amounts of undesired 2,3,3-tribromo-2-methylbutane.  
  Obviously, many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.  
 We claim:  
  1. In a process wherein a 2-bromo-2-alkylalkane is reacted with bromine to obtain a 2,3-dibromo-2- alkylalkane and a 2.3,3-tribromo-2-alkylalkane, said reactants being miscible in each other, the improvement which comprises inhibiting formation of said 2,3,3-tribromo-2-alkylalkane by carrying out said process by stirring the 2-bromo-2-alkyl-alkane, bromine and HBr under substantially anhydrous conditions and under sufficient pressure to maintain the HBr in the reaction system, the reaction temperature being in the range of about 30 to about 100C, the reaction time is from about 5 minutes to about 2 hours, the molar ratio of said 2-bromo-2-alkyalkane to bromine is in the range of about 100:1 to about 1:1 and the molar ratio of HBr to said 2-bromo-2-alkylalkane is in the range of about 100:1 to about 1:1.  
  2. The process of claim 1 wherein 2-bromo-2- alkylalkane is 2-bromo-2-methylbutane.  
  3. The process of claim 1 wherein the reaction temperature is in the range of about 0 to about 4. The process of claim 1 wherein the reaction pressure is in the range of about atmospheric to about 1000 pounds per square inch gauge. 1  
  5. The process of claim 1 wherein the reaction pressure is in the range of about 100 to about 500 pounds per square inch gauge.  
  6. The process of claim 1 wherein reaction time is from about 10 to about minutes.  
  7. The process of claim 1 wherein the molar ratio of said 2-bromo-2-alkylalkane to bromine is in the range of about 50:1 to about 20:1.  
  8. The process of claim 1 wherein the molar ratio of HBr to said 2-bromo-2-alkylalkane is in the range of about 50:1 to about 20:1.  
 TABLE I Product Composition Charge HBr in M01 Per Cent Efi&#39;iciency to 2 Brom0- Pressure 2.3- 2.3.3-Tri- 2,3-Dibromo- Z-Methyl Pounds 2-Bromo- Dibromo- Bromo-2- Z-Methyl- Run Butane, Bromine, HBr, Tem Time. Per Square 2-Methyl- Z-Methyl- Methyl- Butane. Mol No. Grams Grams Grams C. Hours Inch Gauge Butane Butane Butane Per Cent UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,876,715  
 DATED :April 8, 1975 INVENTOR(S) John G. McNulty, Glen A. Russell, and  
 v William L. Walsh It 15 certtfled that error appears in the above-tdentltled patent and that said Letters Patent are hereby corrected as shown below:  
  COL. 1, line 7, after a 2,3-dibromo-l2-alkylalkane&#34;, should read 2,3-dibromo-2-alkylalkane-.  
  COL. 2, line 36, after undesired &#34;2,3,3-tribtromo-2- alkylalkane&#34;, should read 2,3, 3-tribromo-2-alkylalkane COL. 2, line 46, after about &#34;31 30 should read -30 C.  
  COL. 2, line 63, after whatever &#34;tribromoalkanes&#34;, should read tribromoalkane COL. 4, line 29, after wherein &#34;2-bromo-2&#34;, should read said 2-bromo-2- COL. 4, line 32, after about &#34;50&#34; should read 50 C.  
 Signed and Sealed this twenty-ninth Day of July 1975 [SEAL] A lies 1:  
 RUTH C. MASON C. MARSHALL DANN Arresting Officer (&#39;umnzissiunor uj&#39;latenls and Trademarks&#34;