What products would you expect from reaction of the following alkenes with NBS? If more than one

What products would you expect from reaction of the...

Key Concepts

N?Bromosuccinimide (NBS)

NBS is a reagent commonly used to provide a low and steady concentration of bromine in radical reactions, particularly in allylic bromination. It slowly generates bromine radicals in situ, which can abstract an allylic hydrogen from an alkene without causing electrophilic addition across the intact carbon–carbon double bond. This controlled release minimizes the formation of addtion products and ensures high selectivity for substitution at the allylic position.

Allylic Bromination

Allylic bromination refers to the substitution of a hydrogen atom at an allylic position (a carbon adjacent to a double bond) with a bromine atom. In the presence of NBS and light or another initiator, a bromine radical abstracts an allylic hydrogen, forming an allylic radical. The resulting radical then rapidly combines with a bromine atom to afford the allylic bromide. This reaction is selective because the radical is stabilized by resonance with the double bond.

Radical Reaction Mechanism

The mechanism involved in NBS-induced allylic bromination is a free radical chain mechanism. It begins with the formation of a bromine radical (initiation), followed by the abstraction of an allylic hydrogen to generate an allylic radical (propagation), and concludes with the combination of this radical with another bromine atom, resulting in the substituted product. The mechanism also includes termination steps where radicals combine, but these are typically minor under controlled conditions.

Resonance Stabilization

Resonance stabilization is a key concept in understanding allylic radical reactions. Once the allylic hydrogen is abstracted, the resulting radical is stabilized by delocalization of the unpaired electron over the pi system of the adjoining alkene. This delocalization lowers the energy of the radical intermediate and directs the bromination to occur at the allylic positions, often leading to mixtures of regioisomers if more than one allylic site is available.

Regioselectivity in Allylic Bromination

In reactions employing NBS, regioselectivity is observed because the process preferentially targets the allylic positions over the double bond itself. The distribution of products is influenced by the relative stabilization of possible radical intermediates; thus, substituents on the alkene can direct the formation of one isomer over another. Understanding the pattern of substitution requires analysis of the structure of the substrate and the availability of abstractable allylic hydrogens.

Transcript

00:01 Okay, to solve this problem, we're looking at what possible products can be generated from the following nbs reactions.

00:09 Now, nbs reactions are allelic ruminations, and we know that that means we're going to put a bromine at a position allulic to our starting alkyne.

00:19 So in this first part, part a, what we have is a psycho -heptine molecule, and the allelic positions are identified as followed.

00:29 Allelic 1 and allelic 2.

00:33 That means there's two possible spots for our bromings to go.

00:36 And the first easiest thing to do is just draw the products as if you substituted a bromine into each of those allelic positions.

00:43 So that's number two.

00:45 And if you do this, sometimes you'll notice, hey, maybe i've actually drawn the same molecule.

00:52 In this case, we're going to find out that is true.

00:55 That's only six -member during.

00:56 So we can put a br in alilip position 1, or we can put a br in a lilac position 2.

01:24 And what we find out is actually these are the same molecule, because the molecule is symmetric.

01:29 It doesn't matter which side it's on.

01:31 The ch3 is always in the same position relative to the bromine.

01:34 If we were to flip this across this kind of mirror plane, what we're going to see is they're the same.

01:44 So this is product one.

01:49 So this is a possibility from the nbs reactions.

01:52 Now, how do we find out if there's other possibilities from the nbs reactions? and the way we can kind of explore that is just going through the mechanism.

02:00 So if we start again with our cyclo -heptane ring, and we go through our nbs reaction mechanism, we draw in our allelic hydrogen because those are sites that we absolutely know we care about when dealing with allelic bromination reactions.

02:23 So the first thing that happens is that a br radical abstracts one of the allolic hydrogones.

02:28 So let's go ahead and do that.

02:30 The radical breaks the sigma bond of the ch bond here, generating a radical on carbon.

02:49 So we have the nice radical there.

02:51 And this radical, if it went and it grabbed the br2 from the reaction of what we also have hbr generated here.

02:59 Just a reminder that the hbr will react with another equivalent of n -bromosiccinamide.

03:12 And that's going to give br2, which is our brominating reagent.

03:22 And, oops, not another n -bromorosacetamide, but an n -hydrosixenamide.

03:31 There we are.

03:33 So that's just the side reaction that happens to generate our br2, which is going to be our chrominating reagent.

03:41 And what we see here is that, okay, we have this radical.

03:44 And if this radical reacted with our br2, we're actually going to get the exact same thing as product one.

03:57 So if that's product one and we're going to make the same thing, we know that, well, that's not part of the answer.

04:02 We can only make one product.

04:03 That's the same.

04:04 But we do have a radical next to a pie bond, which means resonance could be in play here.

04:10 So if we draw out the resonance structure of this cyclo -hypon, what we can see is that the pie bomb breaks apart, and we generate a new radical option.

04:44 Now, if we compare both of these radicals, we call it radical a and radical b.

04:55 There's really not any difference between them in terms of their stability.

04:59 So they're both equally as likely to exist during this reaction.

05:04 And this is because both of them are secondary radicals.

05:09 They both have ch's.

05:11 There's really no difference in stability there.

05:14 So another secondary radical.

05:16 And that means that this is absolutely a possibility that this radical could react with br2.

05:25 We'll just go ahead and draw that.

05:36 It's best to show the bonds of br2.

05:40 So we can indicate the electron movement and how it's breaking.

05:44 So what we get is the radical here, homolithic cleavage of our weak brbr bond.

05:51 And we're going to get a different product generated.

06:17 So if we just go back here and we compare, in this case, our alkyng is actually much closer to the ch bond than it was before.

06:29 It's just one, two carbons away versus one, two, three, one, two, three.

06:33 And that means that this is a different product.

06:36 It's a different molecule.

06:37 And this is the other option for this reaction...

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