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Can this reaction occur;

(2-Bromopentane) + Br2 --(300°C)--> (2,3-Dibromopentane)

As far as I know, halogenation can only occur in UV or sunlight, not with just a high temperature. So how does this work out?

Sorry about the formatting.

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  • $\begingroup$ Ever heard of radical initiators? $\endgroup$ – Waylander Mar 25 at 15:25
  • $\begingroup$ @HarryHolmes The given reaction though would exchange a hydrogen by a bromine, a substitution. And because radicals are involved, it is a radicial substitution (e.g., en.wikipedia.org/wiki/Radical_substitution). $\endgroup$ – Buttonwood Mar 25 at 15:39
  • $\begingroup$ This reaction is not an addition. It is a substitution, and some HBr is simultaneously produced. So the written equation is wong. $\endgroup$ – Maurice Mar 25 at 17:22
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There multiple means to generate the radicals you would need for a reaction of

$$\ce{\mbox{2-Bromopentane} + Br2 -> \mbox{2,3-Dibromopentane} + HBr}$$

An alternative to radiation is the combination of heat and some of radical initiator which is typically deployed at gentle $70-90\,\pu{^\circ{}C}$ like AIBN or dibenzoyl peroxide. To moderate the radical reaction, though, elementary bromine may be replaced by NBS, which lowers the accessible concentration of elementary bromine in the reaction mixture.

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I think a very interesting way to perform this reaction, absence light or elevated temperature, especially since Br2 is a liquid at room temperature, is via sonolysis of either bromine or an alkyl bromide:

$\ce{Br2 + Energy (sonolysis) -> {*}[Br2] -> .Br + .Br}$

Here is a description of the mechanics from comments in ScienceDirect on sonolysis (sonochemistry) again based on a radical chain mechanism:

Apparently, the mechanisms involved in sonolysis are based on the same radical chain reaction as thermolysis. As the vapor pressure of the solvent decreases, the crushing effect of cavitation (cavitation energy) collapse of a bubbles increases. Although the reaction rate generally rises with higher temperatures, in sonochemistry an increase in temperature does not always contribute to reaction acceleration. Since the vapor pressure of a liquid goes up with a rise in the ambient temperature, cavitation occurs more readily but the cavitation collapse energy decreases. Therefore, depending on the positive and negative effects of the physical properties of the liquid undergoing sonication, such as the vapor pressure and viscosity, there are cases where the effects of ultrasound on a reaction can be somewhat more pronounced as the bulk temperature is lowered. Therefore, one should be mindful that such an apparent anti-Arrhenius effect may happen in sonochemistry [5].

The extent of a solute, reactant, or solvent molecule being incorporated within a cavitation bubble is dependent on the amount in the system, and under equilibrium conditions is controlled by Henry's and Raoult's laws. In general, the higher the vapor pressure of the compound and its solution composition, the higher will be its relative amount within a cavitation bubble. There are of course exceptions to this due to the nonideal effects of mixing, and the specific effects of adsorption of a solute to the cavitation bubble/solution interface. If a reactant cannot enter a cavitation bubble to any significant extent, then its only means of being activated is at the interfacial region of the bubble and/or in the bulk liquid. It has been shown that the activated species of a reaction was produced by ultrasonic cavitation in the study on the participation of various alkyl bromides in the isomerization of 1,2-dichloroethene Eqn (7.6) [6]. The isomerization process was initiated by bromine radicals, produced from the sonolysis of alkyl bromide.

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  • $\begingroup$ Um, that seems a bit above my level. It's just, is sunlight absolutely necessary? $\endgroup$ – Harry Holmes Mar 25 at 16:55
  • $\begingroup$ This will also breaks $\ce{R-Br}$ bond and major product may be a alkene. See that equation (7.6). $\endgroup$ – Mathew Mahindaratne Mar 25 at 17:00
  • $\begingroup$ @HarryHolmes - no sunlight is not necessary if another source of radicals is available $\endgroup$ – Waylander Mar 25 at 17:11
  • $\begingroup$ I would compare this method to photolysis, where you have to know when to stop (for good yield) and some limited temperature adjustment during the treatment may be advantageous (as is the case in photolysis), $\endgroup$ – AJKOER Mar 25 at 23:36

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