Electrophilic Aromatic Substitution help needed. Objectives: Design an experimen

Question

Electrophilic Aromatic Substitution help needed. Objectives: Design an experimental procedure that allows you to determine the relative reactivity between anisole, chlorobenzene, diphenylether, toluene, orthoxylene, metaxylene and paraxylene.

In this lab you are challenged with determining the relative rates for reaction between different substituted benzenes and a common electrophile (Br2).  Bromine was chosen as the electrophile because it is colored.  Acetic acid (solvent), and all of the substituted benzenes we will be using are colorless.  Also all products, HBr and the brominated substituted benzenes are all colorless.  Thus we can know when the reaction is complete if we use bromine as the limiting reagent and watch for the color to disappear.  Before you come to lab you must develop a plan to test relative rates of reactions of anisole, chlorobenzene, diphenyl ether, toluene, mxylene, o xylene and pxylene with bromine.  In the lab you will be provided with 0.20 M solutions of each of the substituted benzenes, and 0.05 M solution of bromine.  You have your standard glassware found in your drawer, and we ask that you use no more than 1 mL of any given solution for any given test (this means you can use 1 mL of Br2 to test anisole and another 1 mL .  Working on this scale would require small reaction vessels, so I would recommend carrying out individual tests in 3inch testtubes (your small ones).  They can easily be held in a water bath by wrapping a coil of copper wire around them and putting a hook in the wire that catches the top of a beaker (see figure). If you need to explore different temperatures, we recommend using a beaker of boiling water as a 100oC bath, a beaker of ice and water for a 0oC bath and a beaker of tap water that has been left out for 20 minutes or so as a room temperature bath which can be monitored with a thermometer.  We also have stopwatches and a clock that you can use to measure time. With these resources available, come up with a plan to determine experimentally the order of reactivity between the above substituted benzenes.  To accurately determine this order you will have to take great care to ensure that all other variables are consistent between different tests.

Explanation / Answer

WE can test the relative rate of the given aromatic compounds b reacting bromine with the following compounds

1) polycyclic aromatic compounds are more reactive than monocylic aromatic compound due to more charge distribution in them

2) chlorine makes ring deactivated for substitution due to -inductive effect

3) two methyl groups in xylene makes it more reactive towards monomethyl substituted ring (toluene)

4) out of methyl and methoxy groups, methoxy group is more activating then methyl group due ot +Mesomeric effect

so reactivity order will be

5) out of three xylenes, meta xylene undergoes bromination faster than para and orthoxylene,due to more stabilization of tranistion state.

diphenyl ether > anisole > meta xylene > ortho xylene > para xylene > toluene > chlorobenzene.

So if will take limiting reagent bromine and rest of the compounds then the order of reactivtiy will be as above.

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