UV re. bromine water and hexane (1 Viewer)

[Paroissien]

What is the chemistry behind this substitution reaction?

 

[helper]

Initiation: The UV breaks the Bromine down into two Bromine free radicals.

Propogation: The free radical starts a chain reactions to form new free radicals. This step occurs by the free radical removing a hydrogen and forming HBr. The alkyl free radical then breaks another Bromine molelcule to form the alkyl halide and a new free radical.

Termination: Occurs when two free radicals join together ending the chain reaction.

 

[Paroissien]

Thanks, but what I don't understand is how the UV breaks the bromine down

 

[helper]

Is called homolytic cleavage and goes way past HSC level chem.

Do a search on the internet.

Here is one explaination

As revealed by several experimental examples, radicals and ion radicals may, in contrast with closed-shell molecules, undergo exothermic homolytic cleavages (.A..B --> A: +.B) with substantial activation barriers. A two-state semiclassical model is proposed for explaining the existence of the barrier and estimating its magnitude. It is based on the intersection of the potential energy surfaces characterizing the dissociation of a bonding state, .A..B -->.A. +.B, on one hand, and the approach to bonding distance of a repulsive state, A: +.B --> A therefore B, on the other. After inclusion of the bond cleavage and formation as Morse curves in the normal-mode analysis, a simple activation driving force relationship is obtained, the two main ingredients of the intrinsic barrier being the triplet excitation energy of the A moiety and the pi*--> sigma* excitation energy in .A-B. The model is then tested by quantum chemical calculations, first on a simplified system to evaluate the calculation techniques and then on a real system. A comparison of the model predictions with experiment is finally performed using the rate data recently gathered for the cleavage of 4-cyanophenyl alkyl ether anion radicals, which cover a respectable range of driving forces, showing satisfactory agreement between theoretical predictions and experimental data.

 

[eth]

How it works doesn't really matter, what we basically need to understand is that the Br water decolourises with the alkene, but the alkane requires energy input in order to decolourise it.

 

[Paroissien]

Yeah I know but I like to understand the processes behind things like that. Helps in memorisation

 

[helper]

Yeah I know but I like to understand the processes behind things like that. Helps in memorisation

In general Iwould agree with you. In this case it doesn't really help due to the complexity.

 

[Paroissien]

Hahah. Yeah I realised that after the posted the explanation, I wasn't expecting it to be that complex. Thanks anyway