Name: hbr selectivity in alkene addition thermodynamics
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 32 s
Channel: JoVE

Recall that anti-Markovnikov addition is only observed with HBr. The radical addition with HCl and HI is thermodynamically unfavorable.
But why?
Let&#39;s examine the thermodynamics for each propagation step involved in the radical mechanism.
For a reaction to be spontaneous, the change in Gibbs free energy, &#8710;G, must be negative.
For the first propagation step, the sign of &#8710;G is dictated by the competition between the enthalpy and the entropy terms.
In the case of HBr and HCl, the enthalpy term dominates at low temperatures, and &#8710;G becomes negative, implying a thermodynamically favorable reaction.
However, in the case of HI, both the enthalpy and entropy terms are positive, implying that &#8710;G is positive, regardless of temperature, indicating a thermodynamically unfavorable reaction.
For the second propagation step, the entropy term is close to zero because the number of reactants and product molecules are equal. So, the enthalpy term dictates the sign of &#8710;G.
As evident, &#8710;G is positive for HCl, rendering the reaction thermodynamically unfavorable.
Overall, the two propagation steps are thermodynamically favorable only in the case of HBr.

radical anti-markovnikov addition to alkenes: thermodynamics

Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics

The anti-Markovnikov addition of hydrogen halides to an alkene is thermodynamically feasible only with HBr. The radical addition reaction with other hydrogen halides like HCl and HI is thermodynamically unfavorable.
<img alt="Chemical reaction mechanism, HBr addition, alkene, peroxide diagram, thermodynamic favorability." src="/files/ftp_upload/12904/12904_PT_Figure_1.svg" style="text-align: center; height: 125px;" data-altupdatedat="1754904545000" data-alt="Free radical addition mechanism, organic reaction diagram, HBr, HCl, ROOR, product formation path." />
Thermodynamic factors
The temperature influences the spontaneity of a reaction, which can be evaluated based on the change in the Gibbs free energy, &#916;G. If the change in Gibbs free energy, &#916;G, is negative, the reaction occurs spontaneously. As shown below, &#916;G can be evaluated directly from the values of the two terms, &#916;H and &#8722;T&#916;S.
<img alt="Standard Gibbs free energy equation &#916;G&#176;=&#916;H&#176;&#8722;T&#916;S&#176;, thermodynamics, formula." src="/files/ftp_upload/12904/12904_PT_Figure_2.svg" style="text-align: center; height: 14px;" data-altupdatedat="1754904545000" data-alt="Thermodynamic equation &#916;G&#176; = &#916;H&#176; + (-T&#916;S&#176;), formula for spontaneity analysis." />
The thermodynamic feasibility of anti-Markovnikov addition with HBr can be predicted from each propagation step involved in the radical mechanism.
In the first propagation step of the radical reaction, the &#8710;G sign is determined by the competition between the enthalpy and the entropy terms. At lower temperatures, radical addition with HBr and HCl dominates the enthalpy term. Consequently, &#916;G is negative, indicating that the reaction is thermodynamically favorable. However, at high temperatures, the reaction is endothermic and thermodynamically unfavorable. In contrast, with HI, irrespective of temperature, both the enthalpy and entropy terms are positive. As a result, &#8710;G becomes positive, signifying that the reaction is thermodynamically unfavorable.
In the second propagation step, the entropy term is close to zero as the number of reactants and product molecules are equal, meaning that the &#8710;G sign depends on the enthalpy term. Since the enthalpy term dominates for HBr and HI, the reaction is exothermic and thermodynamically favorable. However, with HCl, &#8710;G is positive, meaning that the reaction is endothermic and thermodynamically unfavorable.
In summary, the two propagation steps in the reaction are thermodynamically favorable only with HBr.

The anti-Markovnikov addition of hydrogen halides to an alkene is thermodynamically feasible only with HBr. The radical addition reaction with other ...

radical-anti-markovnikov-addition-to-alkenes-thermodynamics

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Organic Chemistry

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Radical Chemistry

KEY TERMS AND CONCEPTS

hbr selectivity in alkene addition thermodynamics

Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable.

Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term.

The first propagation step depends on the balance between enthalpy and entropy. At lower temperatures, radical addition with HBr and HCl is driven by the enthalpy term, so ΔG is negative and the step is favorable. At higher temperatures, the reaction becomes endothermic and thermodynamically unfavorable. With HI, both the enthalpy and entropy terms are positive at any temperature, so ΔG stays positive.

The second propagation step is different because the number of reactant and product molecules is equal. That makes the entropy term close to zero, so ΔG depends mainly on enthalpy. For HBr and HI, the enthalpy term makes the step exothermic and favorable. For HCl, ΔG is positive, so the step is endothermic and unfavorable. Taken together, both propagation steps are thermodynamically favorable only with HBr.

HBr Selectivity in Alkene Addition Thermodynamics

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Anti-Markovnikov addition of hydrogen halides to alkenes depends on thermodynamics. In this radical reaction, only HBr makes the overall process feasible. With HCl and HI, the same addition is thermodynamically unfavorable. Temperature affects whether the reaction is spontaneous. Spontaneity is measured with Gibbs free energy, or ΔG. When ΔG is negative, the reaction can occur on its own. ΔG is determined by two terms: ΔH, which is the enthalpy change, and −TΔS, which is the entropy term. The...

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key terms and concepts

See why anti-Markovnikov addition to alkenes is thermodynamically favorable only with HBr, and how ΔG, enthalpy, entropy, and temperature affect each step.

HBr Selectivity in Alkene Addition Thermodynamics | JoVE K12

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HBr Selectivity in Alkene Addition Thermodynamics

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