bromination of arenes - ch220c

bond dissociation energy

amt of energy required to effect a free radical reaction

initiation for chlorine atoms

initiator In-In

homolysis of 1,1’ - azobis[cyclohexanenitrile] (ABCN) — rate of rxn sufficiently fast at

80 - 100 degrees celsius

propagation steps of radical rxn include

abstraction of hydrogen atom from hydrocarbon by chlorine atom to produce new free radical, R*, which attacks sulfuryl chloride to yield alkyl chloride and radical SO2Cl; radical fragments in SO2 and another Cl*

termination reactions 

chain reaction interrupted by these side reactionsi

initiation

radicals formed in low conc from neutral molecules, resulting in net increase in conc of free radicals w/in the system

propagation

radicals produced in 1st step react w/ molecules to yield new molecules and new radicals; no net change in conc of radicals

termination

various radicals combine to give molecules, resulting in net decrease in radical conc and decrease in rate of rxn

transition state energies controlled by

statistical factor AND energy factor

statistical factor

determined by number of hydrogen atoms whose replacement will give specific constitutional isomer

energy factor

related to strength of type of C-H bond being brokenen

energy factor aka

relative reactivity

aliphatic

hydrogens bound to sp3 hybridized carbons

aliphatic hydrogens further categorized as 

primary, secondary, tertiary depending upon number of other carbon atoms attached to reference carbon atomc

compounds tested in experiment

toluene, ethyl benzene, isopropyl benzene, tertbutyl benzene

bromination of arenes procedure

1. Obtain 8 disposable test tubes.

2. Label 4 tubes with the number that corresponds to the compound that will be added.

3. Add 2 mL of DCM to each of the tubes.

4. Add 0.5 mL of each substrate to its corresponding numbered tube.

5. Add 0.5 mL of the bromine solution to the 4 remaining tubes.

6. Place all 8 tubes into an ice-water bath for 5 minutes.

7. Add one of the bromine tubes to one of the 4 substrate tubes and record how long it takes to react.  Record the time and then repeat step 7 for the remaining substrates.

8. Once the order has been determined for each set of tubes, all of the tubes can be poured into an Erlenmeyer flask.

9. Quench any residual bromine by added a few (3-4) drops of cyclohexene to the flask or enough such that the solution is colorless.

10. Dispose of the solution in the halogenated waste container.

if Br2 in DCM spilled

pour some of sodium thiosulfate around spill, red color disappears and can be treated as halogenated organic waste

cyclohexene

only need one drop for each test tube/rxn that is still red

glassware that contained bromine

never wash with acetone

order of reactivity should be:

isopropyl benzene, ethylbenzene, toluene, tert-butylbenzene (1 & 2 might flip)