Name: how concentration steers radical additions
Uploaded: 2023-04-30T11:21:06.000Z
Duration: 1 min 20 s
Channel: JoVE

The concentration of starting materials significantly affects the selectivity of radical reactions.
Consider the reaction between an alkyl halide and an alkene. In the presence of tributyltin hydride and AIBN, a radical initiator, the alkyl group is added to the alkene.
Typically, the reaction begins with the generation of a tin radical, which subsequently abstracts halogen from the alkyl halide, producing an alkyl radical.
The alkyl radical now has a choice. It can either abstract hydrogen from the tin hydride producing an alkane, or add to the alkene forming a nitrile-stabilized radical, which further reacts to form the addition product.
This is because the rate constants for the two reactions are about the same.
To drive the reaction towards the addition product, the alkene concentration should always be at least 10 times higher than that of the tin hydride.
This would increase the rate at which alkyl radical adds to the alkene.
In conclusion, the concentration of the starting materials governs the selectivity of radical reactions by favoring one reaction over the other.

radicale reactiviteit: concentratie-effecten

Radical Reactivity: Concentration Effects

Bij een radicaalreactie bepaalt de concentratie van de uitgangsmaterialen de selectiviteit van een radicaal. De reactie tussen een alkylhalogenide en een alkeen begint bijvoorbeeld, in aanwezigheid van tinhydride en AIBN, met de vorming van een tinradicaal. Het gegenereerde radicaal onttrekt vervolgens halogeen aan het alkylhalogenide, waardoor een alkylradicaal ontstaat. Dit alkylradicaal kan reageren met tinhydride waardoor een alkaan ontstaat of kan worden toegevoegd aan een alkeen, waardoor een nitril-gestabiliseerd radicaal ontstaat en uiteindelijk het additieproduct vormt. De vorming van het alkaan en het additieproduct heeft dezelfde mogelijkheid omdat de snelheidsconstante voor de reactie tussen het alkylradicaal en tinhydride vrijwel hetzelfde is als voor de reactie tussen een alkylradicaal en het alkeen. Maar de reactie naar het additieproduct kan worden gestuurd door de alkeenconcentratie minstens tien keer hoger te verhogen dan die van het tinhydride. Dit komt omdat de hogere alkeenconcentratie de reactiesnelheid tussen het alkylradicaal en het alkeen tien keer zal verhogen, waardoor de vorming van het additieproduct ten opzichte van het alkaan wordt bevorderd.

Radicale reactiviteit: concentratie-effecten

In a radical reaction, the concentration of starting materials governs the selectivity of a radical. For example, the reaction between an alkyl halide and ...

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how concentration steers radical additions

Radical reaction selectivity can change when the concentrations of the reactants change. In this chemistry example, an alkyl halide reacts with an alkene in the presence of tin hydride and AIBN, an initiator that helps start the radical process. The reaction begins when a tin radical is formed.

The tin radical removes the halogen from the alkyl halide. This step creates an alkyl radical. That radical then has two possible paths. It can react with tin hydride to form an alkane, or it can add to the alkene to form a nitrile-stabilized radical and then the addition product.

These two paths are about equally likely because the rate constant for the alkyl radical reacting with tin hydride is almost the same as the rate constant for the alkyl radical reacting with the alkene. The reaction can be pushed toward the addition product by making the alkene concentration at least ten times higher than the tin hydride concentration. A higher alkene concentration raises the reaction rate with the alkyl radical and helps favor the addition product over the alkane.

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key terms and concepts

See how reactant concentration affects radical selectivity as an alkyl radical can form either an alkane or an addition product in this reaction.

How Concentration Steers Radical Additions | JoVE K12

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How Concentration Steers Radical Additions

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