苯与溴的取代反应(Substitution reaction of benzene with bromine)

苯与溴的取代反应（Substitution reaction of benzene withbromine）Two. Substitution reaction of benzene with bromineOne of the substitution reactions of benzene with bromine[principles]With iron as catalyst (the actual catalysis is FeBr3), benzene can react with bromine and hydrogen atoms in the benzene molecule can be generated - Br atom substitution.[supplies]Iron, flask, tube, conical flask, benzene, bromine[operation](1) device as shown. Check the tightness of the device first. In the small round bottom flask add 5mL of benzene and 2mL bromine, gently make oscillation, benzene and bromide mixture. At this point, benzene and bromine do not react because of the absence of a catalyst.(2) after the mixture is cooled, the prepared reduced iron (about 0.5g) or a plurality of rusted small nails are rapidly placed into the flask, immediately with a single hole rubber stopper with a long glass tube. Under normal circumstances, the reaction can start, the surface will have small bubbles, and then the reaction gradually fierce, half a minute after theliquid boiling state. In a conical flask duct near the mouth of the emergence of a large number of white mist (hydrogen bromide formed in the reaction solution of acid and water). (3) after the end of the reaction, test the hydrogen bromate in the conical flask. The cone of the liquid in the bottle in two tubes each pour a little, add litmus test solution in which a tube (turns red); another tube with a few drops of AgNO3 solution (with pale yellow AgBr precipitation).HBr H++Br- Ag++Br-: AgBr(4) the flask filled with liquid into the beaker of cold water, red brown liquid insoluble in water at the bottom of the beaker, which is generated in the reaction of bromobenzene. Pure bromobenzene is a colorless liquid preparation often due to dissolution of a small amount of bromine and reddish brown. Wash with water or 10%NaOH solution, wash off FeBr3 and reactive bromine, and obtain colorless transparent oily liquid.[remarks](1) the tightness of the device must be good. (2) a pipe in the device that is vertical to the mouth of the bottle and acts as a condensation to prevent the volatilization of bromine and benzene vapors. Therefore, it should be of a certain length, generally not less than 25cm. (3) in the bottle containing bromine, bromine is above a layer of saturated aqueous solution of bromine, use must be inserted into the lower part of Straw bromine, to absorb pure bromine. The benzene used is dried with anhydrous calcium chloride, and the flasks and ducts used shall be dry. Otherwise, the reaction is difficult, or even reactive.(4) generally does not need to heat, if starts to heat, only uses the hot water bath to heat slightly. (5) this reaction should not be too severe; if the reaction is too severe, the flask can be immersed in a beaker of cold water to cool down. The reaction temperature is too high, will increase the product of bromobenzene benzene and bromine volatilization and side reaction yield of two. (6) in order to prevent the reaction to be too violent, the iron filings should not be too fine, and iron powder should not be used instead of iron filings. (7) bromine volatile and highly corrosive, with bromine must be Caution!, do not stick in the hand. Better put on rubber gloves and take them in the fume hood. In case of touching the skin, rinse immediately with water, wipe with alcohol, and then apply glycerin.Two substitution reaction of benzene and bromine[supplies]The flask, iron, glass tube, tube, U tube, conical flask[principles]Same[operation]The 1. device, as shown, adds cold water to the glass tube.2. place a small amount of carbon tetrachloride in the U tube and add some water into the cone.3. in the round bottom flask, add 8~10mL benzene, a small amount of iron filings and 1~2mL bromide, the reaction is beginning.4. after the experiment, a generation of oily bromobenzene in the flask. A small amount of silver nitrate is added to the conical flask to produce a pale yellow precipitate.[remarks]1. this reaction is exothermic, with benzene and bromine is quite volatile, so the device in addition to the use of long catheter, put cold water glass tube is increased on the pipe and make a better effect of reflux, reduce benzene and bromine consumption and pollution of the environment.2. carbon tetrachloride is used to remove bromine vapor.Three of the substitution reaction of bromine with benzene[principles]The hydrogen atoms in benzene molecules can be substituted by halogen atoms to produce corresponding halogenated benzene under the action of catalysts. The reaction is as follows:The reaction mechanism between benzene and halogen substitution is different from alkane substitution reaction. First of all, halogen molecule polarization occurs in the presence of catalyst, cathode polarization can occur in atomic molecular substitution reaction with a hydrogen atom on thebenzene ring; the cathode polarization molecules and hydrogen atoms are replaced with a hydrogen halide molecules. This substitution reaction is called electrophilic substitution reaction.[supplies]Benzene, aluminum, iron, bromine, silver nitrate, sodium hydroxide, litmus paper tray, balance, dropper, iron, glass, rubber hose, pipe sink, distillation flask, rubber plug, cylinder.[operation]The experimental setup is shown in fig..(1) adding 4mL pure benzene and 1g reduced iron powder in a dry steaming flask, and immediately sealing the stopper with a glass tube. The glass tube should be immersed in the liquid and the tube should be slightly thicker for bromine operation.(two) to the sink (or beaker) into distilled water and a small amount of purple litmus test solution. Adjust the height of the funnel so that its mouth is just close to the surface of the water. In order to observe the color change of inverted white mist generation and hydrobromic acid in the indicator of funnel. The black and white lining panels can be enhanced visually.(three) from the crude distillation bottle glass dropper single hole plug pipe is added 1mL benzene layer at the bottom of the flask with bromine into iron reaction, generating three ferricbromide in three, catalytic ferric bromide, bromide and benzene can be a substitution reaction, generate bromobenzene.(four) after stopping the reaction, to the beaker with a few drops of silver nitrate solution, silver bromide formation was observed precipitation, further proof of the generated gas is hydrogen bromide.(five) the liquid into a flask containing dilute alkali solution in the beaker, stir with a glass rod, observed the color of the liquid gradually disappeared, after standing, because was heavier than water, and insoluble in water, sink to the bottom of the beaker. Pure bromobenzene is colorless, prepared by dissolving a small amount of bromine and bromine is light yellow.[remarks]1., the key to success or failureThe catalyst can be either Al or Fe, and in fact the catalytic activity is AlBr3 or FeBr3. Therefore, the surface clean, oil free metal iron must be used. As long as the catalyst is chosen properly, the reaction will proceed very smoothly.2. this experiment requires the flask and catheter to be kept dry.The benzene should also be in a few days before the experiment of anhydrous calcium chloride drying before use filtering, existence of a small amount of water, will make the reactionstarted slowly, even no reaction; there will be water to generate hydrogen bromide is not easy to discharge, generation cannot observe hydrogen bromide.After the 3. reaction stops, the liquid is poured into the dilute alkali solution to absorb the remaining bromine. The reaction is as follows:2NaOH+Br2=NaBr+NaBrO+H2OIn order to observe the outcome of bromobenzene.4. bromine has strong corrosive and irritating, and must pay attention to safety when using, do not drop on the table or the ground, as far as possible to reduce the diffusion of bromine vapor. This experiment added to avoid excessive bromine.5. the requirements of materials of hydrogen bromide derived tube and vertical flask and long, can also play a role in the condenser tube, toshihisa unreacted benzene, bromine vapor reflux. However, many experiments have proved that the short catheter has no adverse effect on the experimental result, and the operation is convenient because of the small amount of the experiment.6. in order to improve the success rate of experiment, this experiment has improved the equipment and operation in the teaching material. Add a thick glass tube (or a thin glass tube is fired into a bell), into the bottom of the flask, the bromine directly to the lower layer of benzene. This prevents bromine vapor from evaporating into the flask, escaping with thereaction gas, dissolving it in the water of the beaker, making the solution yellow, causing the experiment to fail. For the visual effect of outstanding experiment, adding litmus reagent in distilled water absorption of hydrogen bromide. After the experiment, the residual bromine absorption with dilute alkali solution, to observe the students of bromobenzene.7. when demonstrating the substitution reaction of benzene and bromine, it is necessary to highlight the analysis and detection of the product and compare it with the substitution reaction of methane and chlorine so as to consolidate the concept of substitution reaction. Through the analysis of two substitution reaction conditions, the substitution reaction of halogen benzene and inspire students to know than methane easily, so that the students understand the chemical bonds of alkanes and benzene in the molecular structure of different single, to emphasize that the characteristics of benzene.Four substitution reaction of benzene and bromine[principles]Same[supplies]Iron, flask, funnel, tube, U tube, bromine, benzene, iron, silver nitrate solution and 5% sodium hydroxide solution[operation]The 1. device is shown in Figure2. in the flask, first add 1mL bromide, and then drop by adding bromine, in the vicinity of the nail has "boiling" phenomenon (reaction is not fast, slightly heating). The gas passes through the U tube, the colorless benzene orange, yellow generation in silver nitrate solution in silver bromide precipitation.3. reaction 2~3min. After the experiment, 30mL5% solution of sodium hydroxide was added into the hopper to oscillate the flask. Then the flask add ice water, after standing down to the upper layer of the liquid, sink to the bottom of the flask liquid is crude bromobenzene.[remarks]1., this experiment should prevent pollution of the environment. The following measures can be adopted: (1) the dosage of bromine should not be too much. (2) the speed of adding bromine is small.(3) absorb the remaining gas with sodium hydroxide solution.(4) treating the reaction product in an original flask.2., the catalyst nails or iron sheet, before the experiment, after pickling, washing and drying, should be immediately put into benzene, cut, do not stay in the air for too long.ThreeThere is no water in the reaction system, otherwise the reaction rate will be affected.Five substitution reaction of benzene and bromine[principles]Same[supplies]With test tubes, flasks, iron, funnel, tube, conical flask, benzene, bromine, iron, glass fiber, sodium hydroxide solution and distilled water[operation]1. to support lower elongation, tube section, device as shown in figure. Put a small amount of glass fiber in the attenuated end, blocking iron does not fall. Fill the tube with iron filings, about 3~4cm high.The mixture of benzene and 2. bromine, at a rate of 3S a drop in iron into. When the iron is wetted by the mixture, the mixture is dripped at a drop of 4s.3. after a while, a colorless oily Bromophenyl drops into the beaker containing sodium hydroxide solution. There is fog in the near conical bottle mouth catheter.4. after the end of the experiment, dropping silver nitrate solution into the conical flask, a pale yellow precipitate of silver bromide.[remarks]1. in this experiment, as the reaction proceeds, the reactants and the products can be separated without polluting the environment.2. benzene and bromine mixed with 5:3 is appropriate.The velocity of 3. drops and mixture is the key of this experiment. The solution is too fast and the effluent is reddish brown; if it is too slow, the experiment takes a long time. In addition, the amount of iron filings should be a little more.The long catheter in the 4. unit acts as condensation and flow.5. this experiment requires about 6min.The substitution reaction of benzene with bromine; this six[principles]Same[supplies]With test tubes (25 * 200mm), conical flask, catheter, iron, a separatory funnel, sucking ball, suction flask, iron powder, asbestos, 1mol L-1 sodium hydroxide solution, benzene, bromo mixture, silver nitrate solutionTake 25 * 200mm with a test tube and change it into a reaction tube. In the lower tube, iron powder and asbestos (iron powder and asbestos column about 2cm) are installed. Two conical bottles, left filled with 1mol/L sodium hydroxide solution, the right side of the distilled water. In the middle, a test tube is loaded with benzene to dissolve bromine mixed with hydrogen bromide. The device is shown as shown.[operation]1., a mixture of bromine and benzene is added to the reaction tube by a single drop. Within a few seconds, a violent reaction can occur. The resulting hydrogen bromide gas is slightly yellow because of bromine vapor. After washing benzene, hydrogen bromide into the conical flask, the formation of fog in the mouth of conduit.2. to the left side of the suction flask drops of silver nitrate solution, producing yellow silver bromide precipitation.After 3.1~2min, a colorless (soluble in a small amount of bromine and slightly yellow) liquid is produced at the lower end of the reaction tube. The suction ball is used to decompress the side tube of the tapered bottle so that the liquid drops into the bottle and sinks to the bottom. This oily liquid is bromobenzene.[remarks]1. bromine and benzene are mixed into liquid according to the volume of 1:2. The benzene dissolved in the washing gas can berecovered and used after being recovered.2. can be used to test the formation of hydrogen bromide with concentrated ammonia water. In the cone of the right, instead of distilled water with a few drops of concentrated ammonia water, a large amount of concentrated white smoke, which is ammonium bromide, can be seen. Visibility is high.3. using the commercially available suction pipe instead of the reaction tube to do experiments, and the effect is also very good.。