[discodermolide]

The benzyllic position, i.e. Ph-CH₂- is usually very reactive as the radical, or the cation, and even the anion can be stabilised by resonance interaction with the pi system of the aromatic ring.
Typically in the mass spec. of compounds containing a benzyllic substituent you often see a mass of 91 which is the benzyl radical cation (in the EI mass spec). This is really quite stable. In fact it can form the tropylium cation. See
http://en.wikipedia.org/wiki/Tropylium_tetrafluoroborate

Now that is digging in the deep recesses of my addled brain cell.

[Rutherford]

Thanks for the extra info.
I just wanted to return to the question I posted: "Is it really true that not even a single molecule is made with a primary hydrogen replaced by bromine?"

[curiouscat]

"Is it really true that not even a single molecule is made with a primary hydrogen replaced by bromine?"

Not true.

[discodermolide]

What happens with toluene + bromine + light?

[Rutherford]

The primary hydrogen will be replaced as the C₆H₅CH₂· is stabilized now.

"Is it really true that not even a single molecule is made with a primary hydrogen replaced by bromine?"

Not true.

Then the question in my first post is wrong. Very rigorously, two derivates are made in reality.

[discodermolide]

What happens with hexane + bromine + light?

[Rutherford]

No resonance now, but I remember that I read somewhere that primary, secondary and tertiary hydrogens in alkanes have different reactivities (I think that the tertiary are the most reactive). Not sure what affects this.

[discodermolide]

No resonance now, but I remember that I read somewhere that primary, secondary and tertiary hydrogens in alkanes have different reactivities (I think that the tertiary are the most reactive). Not sure what affects this.

This is the stability of the radical that is formed. This stability parallels that of carbocations.
So a primary radical is not very stable it is likely to rearrange through the abstraction of a H radical to a secondary radical, this can also abstract another H radical or you can get alkyl shifts to generate tertiary radicals.
So to answer my question, hexane + bromine + light will give a mixture of brominated products.

[Rutherford]

The only thing I forgot to wrote in my post was the answer  .

[discodermolide]

Happy new year

[Rutherford]

Happy new year  !

[curiouscat]

The primary hydrogen will be replaced as the C₆H₅CH₂· is stabilized now.

"Is it really true that not even a single molecule is made with a primary hydrogen replaced by bromine?"

Not true.

Then the question in my first post is wrong. Very rigorously, two derivates are made in reality.

Very rigorously, all possible derivatives are made in almost all reactions you can think of. If you frame the question the way you did: "even a single molecule".

When you have ~10²³ molecules flying around even the most probablistically unlikely event is very sure to sometimes happen (at Room Temp. ). Even a Markovniov addition has almost surely at least one molecule of the anti-Markovnikov product.

I'd love to see cases where a product is strictly forbidden at a molecular level; that'd have to be some electronic or quantum mechanical restriction. But based on energetics or activation energies alone I think molecular level absolute selectivity is very difficult.

I don't think this makes the question wrong. You are only mis-interpreting it.

[Rutherford]

Okay, when it is said that only one product is obtained, it is actually meant that there is a major product and a very little abundant product. I think that everything is clear here now. Thanks again.