[asa029]

Hello everyone. I'm working on a new method for halogenation which I suspect to be a radical-mechanism. Are there compounds that can be added to the reaction-mixture to detect the pressence of said radicals?

[Honclbrif]

There's all sorts of radical quenchers available which should shut down your reaction if its going through such an intermediate. One you might consider is DPPH. Its a persistent radical with a dark purple color. It gets used frequently in radical scavenging assays because it produces a yellow colored product when quenched.

http://en.wikipedia.org/wiki/DPPH

There's also ESR/EPR.

http://en.wikipedia.org/wiki/Electron_paramagnetic_resonance

[mpv]

You could try adding nitic oxide. It is said to react with radicals and give nitrozo derivatives witch are colored(in case it is a tertiari radical, otherwise it will give oximes).

[asa029]

There's all sorts of radical quenchers available which should shut down your reaction if its going through such an intermediate. One you might consider is DPPH. Its a persistent radical with a dark purple color. It gets used frequently in radical scavenging assays because it produces a yellow colored product when quenched.

http://en.wikipedia.org/wiki/DPPH

There's also ESR/EPR.

http://en.wikipedia.org/wiki/Electron_paramagnetic_resonance

I've tried to use DPPH in quantative amounts. The reaction proceeded as normal and gave the product (which might indicate that there is no radical mechanism). I'm unsure about what conditions the DPPH can withstand (ie, if DPPH has been quenched by some other reagent). Do you know if it can withstand sulphuric acid?

I've tried to find litterature studies where DPPH has been used to detect radicals (normally its used to meassure anti-oxidant properties) but have found none. Any tips?

[dipesh747]

This may be a strange answer but why don't you run the reaction in an Electron paramagnetic resonance spectrometer. If there is a radical, a spectrum you could detect the structure of the intermediate.

[asa029]

This may be a strange answer but why don't you run the reaction in an Electron paramagnetic resonance spectrometer. If there is a radical, a spectrum you could detect the structure of the intermediate.

The answer is that we dont have the necessary equipment. If we did though, I would definitely do EPR:D

[Halogenator]

Hello everyone. I'm working on a new method for halogenation which I suspect to be a radical-mechanism. Are there compounds that can be added to the reaction-mixture to detect the pressence of said radicals?

I recently shut down a radical side process (olefin E/Z isomerization) in my electrophilic bromination system by adding 0.5 mol% 3,5-ditbutyl-4-hydroxyanisole. It is like BHT but better. alpha Tocopherol should be even more effective but I couldn't find any around the lab. Everyone suggests TEMPO, another stable radical but it can catalyze other processes like oxidation of alcohols. TEMPO is also only really effective against C-centered radicals except in strong acid. I assume that you have already tried running the reaction in the dark? That doesn't always work, but it is easy. If think you know where the radical is going to occur you might be able to build a radical clock into your substrate.

Also: EPR only shows that you have radicals, not that they are responsible for the desired reaction. Radical side reactions happen a lot in halogenation chemistry (at least to me)

[asa029]

I recently shut down a radical side process (olefin E/Z isomerization) in my electrophilic bromination system by adding 0.5 mol% 3,5-ditbutyl-4-hydroxyanisole. It is like BHT but better. alpha Tocopherol should be even more effective but I couldn't find any around the lab. Everyone suggests TEMPO, another stable radical but it can catalyze other processes like oxidation of alcohols. TEMPO is also only really effective against C-centered radicals except in strong acid. I assume that you have already tried running the reaction in the dark? That doesn't always work, but it is easy. If think you know where the radical is going to occur you might be able to build a radical clock into your substrate.

Also: EPR only shows that you have radicals, not that they are responsible for the desired reaction. Radical side reactions happen a lot in halogenation chemistry (at least to me)

Thanks for the tips. I have not thought about the use of antioxidants to potentially shut down the reaction. I thought about TEMPO (our group has done some work on it), but decided not to use it. The dark reaction was one of the first things I did:D