Relative Rates: Free-Radical Bromination

BroIn this experiment of the relative rates of free-radical chain bromination, we were expected to be able to determine the relative reactivates of the many types of hydrogen atoms involved toward bromine atoms. Bromination is defined to be a regioselective reaction meaning bromine has preference of making or breaking a bond over all other directions that it may have had available.

In this case, Markovnikov’s rule is revealed to be the case in this situation that states that adding a protic acid represented as HX to an alkene (sp2 hybridized), the hydrogen from that HX would be attracted to a carbon with the least alkyl groups and the halide (X) would become more attracted and will attach to the carbon containing more alkyl groups. Within the intermediate stage of the bromination reaction, the bromine radical will have already formed and the electronegatively charged radical will have a choice of how to protonate in order to create a stable carbocation.

In order to determine these sp3 hybridized carbons stability, it can be determined by knowing that while comparing, if there are less alkyl groups attached to a carbocation, then there would not be enough electrons to slightly mask over the positive charge on that certain carbocation. Due to this, the more alkyl groups that are attached, the more stable the carbocation would be due to the fact that the electron flow in the electron cloud slightly donates to the carbocation making it almost completely stable.

During the experiment, we were expected to organize two groups of tubes with methylene chloride with their respective hydrocarbon (10 drops) as well as the addition of a small amount of bromine. One group of tubes was set in the light and one was set in the dark and when compared, you were already able to see that the ethylbenzene and toluene were the fastest to react in both situations.

Due to this fact we can tell since they were the fastest reacting, they can be associated with the fact that they were more than likely secondary and primary benzylic carbons due to their excellent stability and fast reactions. The only other tube that reacted through observation (by eye) by the end of the experiment was the methylcyclohexane; and we could tell that it had slightly reacted by it’s slight orange color, but was not still completely red like the others(sign of bromine).

The next most stable carbocation form would be the tertiary aliphatic carbon, so we will infer that this is what has occurred in this case. Due to the fact that methylcyclohexane is sp3 hybridized, we can pin point that our assumption that the bromination is tertiary aliphatic because it is stable with there being three alkyl groups present which means that the electrons within these groups are delocalized and are contained within an orbital that extends over the adjacent atoms involved. The last two tubes to react within the group were cyclohexane and t-butylbenzene in that order.

When first added the bromine, these two were the only ones that seemed to remain red longer and remained the same shade of red even after all of the other tubes had reacted. The reaction of cyclohexane however was expected to react slightly more than the t-butylbenzene due to the fact that the positive charge on the carbocation is delocalized “better” when there are more alkyl groups involved. This will match up the two with secondary (cyclohexane) and primary aliphatic (t-butylbenzene) bromination.