[kirushanth]

In allylic bromination using NBS, why is that low concentration substitutes in allylic position?

[discodermolide]

In allylic bromination using NBS, why is that low concentration substitutes in allylic position?

Have a look here:
http://www.organic-chemistry.org/namedreactions/wohl-ziegler-reaction.shtm

[camptzak]

would it do something different in high concentration of NBS?

it seems like the allylic position is the most readily attacked due to resonace stabilization.
but maybe in higher concentrations Br radical attack the allylic position and then there is still some Br₂ left to attack the double bond?

and then maybe if there really a great molar excess of Br₂ with hv initiation the bromine radicals would start ripping hydrogens off the carbon chain?

[discodermolide]

would it do something different in high concentration of NBS?

it seems like the allylic position is the most readily attacked due to resonace stabilization.
but maybe in higher concentrations Br radical attack the allylic position and then there is still some Br₂ left to attack the double bond?

and then maybe if there really a great molar excess of Br₂ with hv initiation the bromine radicals would start ripping hydrogens off the carbon chain?

A quote from the link "It is very important to keep the concentration of Br2 and HBr low to prevent side reactions derived from simple ionic addition with the alkene."