the topic of this lecture is allylic bromination allylic bromination is a free radical halogenation of alkanes containing double bonds for the most part free radical halogenation reactions are not particularly useful to synthesize products because we get a mixture of products for example if we perform the free radical chlorination of pentane we'll get a mixture of products take a moment to pause the video now and see if you can draw all of the possible mono halogenated products of pentane that we would get in this reaction and label them major and minor there are three possible products that result from the mono halogenation of pentane we get one chloro pentane 2-chloro pentane and three chloro pentane one clora pentane which results from a primary radical would be the minor product to chloro pentane and three clora pentane which results from secondary radicals would be the major products because we get a mixture of products this is not a particularly useful synthetic reaction and for the most part free radical halogenation is not useful except in special cases such as for example if we try to mono halogen 8 cyclohexane we will get only a single product because of symmetry there's only one type of hydrogen on this molecule on so we only get a single product the other type of special case involves reactions where there is a single very stable radical that can be formed for example if we turn to brominate 2 methyl propane because bromination is so much more selective than chlorination we will only get the very stable tertiary radical which will give us a single product another example of a molecule that has a very stable radical are allylic compounds an allylic position is a position right next to a carbon-carbon double bond a radical on the carbon next to a double bond is very stable because it is resonance stabilized take a moment to draw these two resonance contributors in their Lewis structures so you can tell what's going on Lewis structures for these resonance contributors look like this because of the stability of the allylic radical combined with the fact that bromination is so selective we can choose some reactions of a little combination that will give us a single product for example let's look at the radical bromination of cyclohexene radical bromination of cyclohexane gives us a single product and very good yield the reason that we get a single product is because of the stability of the allylic radical combined with the steel activity of the bromination reaction combined with the symmetry in this molecule there are two allylic positions the pronation at either of those two positions gives us the same product now let's look at the mechanism the first step of radical halogenation is abstraction of a hydrogen atom this can occur at the alone position or at the position next to the allylic visitin abstraction of a hydrogen from the allylic position does this be a little radical this radical is resonance stabilized take a moment to draw the other resonance contributor for this allylic radical the other resonance contributor looks like this because the symmetry in the molecule and the next step of propagation which is legit enough bromine to the radical will give us the same product no matter which resonance contributor reacts in both cases we get addition of bromine to the allylic carbon of the molecule now conserve what occurs if we abstract a hydrogen from the position next to a theological position this results in a secondary radical that is not resonance stabilized because the secondary radical is so much less stable than the resonance stabilized radical it does not form and we don't get any of the product that would result from this radical because of the selectivity in a little bromination this is a very useful reaction organic synthesis however side reactions can occur if our concentration of bromine is two sine reactions are bad because they produce additional products that we then have to remove using purification we'll talk about some of these side reactions later in the semester however what I want to focus on now is how we avoid these side reactions in a lot of bromination we do this using a molecule called n-bromosuccinimide n-bromosuccinimide allows us to maintain a constant low concentration of bromine which allows these allylic bromination reactions to work without reducing side reactions and ballistics an amide or NBS produces bromine in the presence of HBR we do not need to know the mechanism for this reaction n-bromosuccinimide is used in elimination reactions to replace bromine for example the reaction of propane to give bromo propane in this case we've replaced bromine BR 2 with NBS note that we still need to use light or heat to activate the radical reaction