When suspended in tetrachloride (CCl 4), NBS reacts very rapidly with the HBr formed during the reaction mechanism to provide bromine (Br 2) which is required for the reaction to continue. Under the correct conditions, NBS provides a constant but very low concentration of Br 2 in the reaction mixture.

We have used free radical bromination of alkanes to illustrate characteristics of free radical reactions and intermediate carbon free radicals. We can apply this knowledge to alkane halogenation reactions with molecular halogens (X2) other than Br2.

Chad introduces Free Radical Halogenation including the basic similarities and differences between using Br2, Cl2, and NBS.

2013年3月15日 · When alkenes (also known as olefins) are treated with bromine (Br 2) or chlorine (Cl 2) in an inert solvent [Note 1] such as carbon tetrachloride (CCl 4) or dichloromethane (CH 2 Cl 2), they are converted into dihalides (specifically, ‘vicinal’ dihalides since the C-halogen bonds are on adjacent carbons).

When suspended in tetrachloride (CCl 4), NBS reacts with trace amounts of HBr to produce a low enough concentration of bromine to facilitate the allylic bromination reaction. Once the pre-initiation step involving NBS produces small quantities of Br 2, the bromine molecules are homolytically cleaved by light to produce bromine radicals.

Free Radical Bromination of Alkanes Explained: The bromination mechanism is the same as for any other free radical halogenation and consists of three stages: initiation, propagation, and termination. Initiation: Creation of radicals – When bromine (Br 2) treated with light (hν) it comes to homolytic cleavage of the Br-Br bond.

Description: Treatment of alkenes with bromine (Br2) gives vicinal dibromides (1,2-dibromides). Notes: The bromines add to opposite faces of the double bond (“anti addition”). Sometimes the solvent is mentioned in this reaction – a common solvent is carbon tetrachloride (CCl4).

overall substitution - replacement of H by halogen. initiated by light (or heat) usually multiple products (not very useful synthetically) e.g., CH4 --> CH3Cl --> CH2Cl2 --> CHCl3 --> CCl4. mechanism: free radical chain reaction. initiation (generation of radicals) propagation (reaction of radicals) termination (loss of radicals)

When treated with Br 2 or Cl 2, radical substitution of allylic C-H generates the allyl halide and HX. The stability of the allylic radical (due to resonance) favours substitution at the allylic position. N-bromosuccinimide (NBS) can be used as an alternative source of Br 2.

2017年1月22日 · Bromine (Br_2) is more selective and will give the most stable (i.e. highly substituted) radical, whereas chlorine (Cl_2) is less selective. The selectivity has much to do with thermodynamics. There are four general steps recognized in a free radical halogenation reaction: initiation, propagation (two steps), and termination.