Name: allylic bromination with nbs
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 27 s
Channel: JoVE

Halogenowanie alkenu przez allil wymaga niskiego stężenia halogenu i wysokiej temperatury.
Aby przeprowadzić bromowanie allilu w temperaturze pokojowej, jako odczynnik stosuje się N-bromosuccinimid lub NBS. Utrzymuje niskie stężenie bromu i działa jako źródło rodników bromowych.
Na przykład propen reaguje z NBS w obecności światła lub nadtlenku, tworząc 3-bromopropen.
Mechanizm bromowania allilu obejmuje etapy inicjacji, propagacji i zakończenia.
Na etapie inicjacji NBS ulega indukowanemu światłem homolitycznemu rozszczepieniu wiązania N-Br, tworząc rodnik bromowy.
W pierwszym etapie rozmnażania rodnik bromu reaguje z profenem i abstrahuje wodór allilowy, tworząc rodnik allilowy i HBr.
Wygenerowany HBr reaguje z NBS, tworząc cząsteczkę bromu niezbędną do dalszej propagacji reakcji.
W drugim etapie rozmnażania cząsteczka bromu reaguje z rodnikiem allilowym, dając bromek allilu i atom bromu.
Wreszcie, na etapie zakończenia, różne rodniki łączą się ze sobą, aby dać produkty nierodnikowe, zatrzymując dalszą reakcję.

podstawienie rodnikowe: bromowanie allilowe

Radical Substitution: Allylic Bromination

W syntezie organicznej powstawanie produktów można zmieniać poprzez zmianę warunków reakcji. Na przykład produkt addycji dibromu powstaje gdy propen traktuje się bromem w temperaturze pokojowej. Natomiast propen ulega podstawieniu allilowemu w niepolarnych rozpuszczalnikach w wysokich temperaturach tworząc 3-bromopropen. Aby uniknąć reakcji addycji, podczas całej reakcji należy utrzymywać możliwie najniższe stężenie bromu. Można to osiągnąć stosując N-bromosukcynimid (NBS) jako odczynnik zamiast bromu cząsteczkowego.

Propen reaguje z NBS w obecności światła lub nadtlenku poprzez podstawienie rodnikowe, tworząc bromek allilu lub 3-bromopropen. Podobnie jak w przypadku reakcji rodnikowych, mechanizm bromowania allilowego obejmuje trzy etapy: inicjację, propagację i terminację. Na etapie inicjacji NBS ulega homolitycznemu rozszczepieniu słabych wiązań N – Br w obecności światła lub nadtlenku, tworząc rodnik bromu. Podczas pierwszego etapu propagacji wytworzony rodnik bromu oddziela wodór allilowy, dając rodnik allilowy stabilizowany rezonansowo i HBr. Powstały HBr natychmiast reaguje z NBS w reakcji jonowej, w wyniku której powstaje Br_2, który bierze udział w drugim etapie propagacji. Ostatecznie, na etapie zakończenia, różne rodniki łączą się, co skutkuje powstaniem produktów nierodnikowych, co prowadzi do zakończenia reakcji. Przez całą reakcję stężenia HBr i Br_2 utrzymuje się na minimalnym poziomie. W tych warunkach, to znaczy w niepolarnym rozpuszczalniku o bardzo niskim stężeniu bromu, jonowy dodatek Br_2 nie stanowi skutecznej konkurencji dla bromowania rodnikowego.

Rodnikowe bromowanie allilowo podstawionych alkenów tworzy mieszaninę produktów. Dzieje się tak ze względu na stabilizację rezonansową utworzonego pośredniego rodnika allilowego, który może odciągać halogen z dowolnego miejsca (Rysunek 1).

<img alt="Figure1" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

Podstawienie rodnikowe: bromowanie allilowe

W syntezie organicznej powstawanie produktów może być zmienione poprzez zmianę warunków reakcji. Na przykład produkt z dodatkiem dibromo jest ...

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JoVE Core

Organic Chemistry

Chemia organiczna

Unit 1

Jednostka 1

Radical Chemistry

Chemia rodnikowa

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

allylic bromination with nbs

Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form.

The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene instead undergoes allylic substitution to form 3-bromopropene. When propene reacts with NBS in the presence of light or peroxide, it forms allyl bromide, which is also called 3-bromopropene.

The mechanism follows three familiar radical steps: initiation, propagation, and termination. During initiation, NBS undergoes homolytic cleavage of its weak N–Br bond when exposed to light or peroxide. This forms a bromine radical. In the first propagation step, that radical removes an allylic hydrogen and leaves behind a resonance-stabilized allylic radical plus HBr.

HBr does not build up. It reacts right away with NBS in an ionic step that produces Br2. The Br2 then takes part in the second propagation step. In the termination step, radicals combine to form nonradical products, which stops the chain reaction.

Keeping both HBr and Br2 at very low levels is important throughout the reaction. Under these conditions, the ionic addition of Br2 does not compete well with radical bromination. For allylic substituted alkenes, the reaction can give a mixture of products because the resonance-stabilized allylic radical can abstract halogen from either site.

Allylic Bromination with NBS

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Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form. The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene...

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key terms and concepts

See how NBS helps propene undergo allylic bromination instead of addition. Learn the radical steps, low bromine conditions, and product mix.

Allylic Bromination with NBS | JoVE K12

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Allylic Bromination with NBS

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