To answer the question regarding why hydrogen chloride (HCl) and hydrogen iodide (HI) do not give anti-Markovnikov's addition to alkenes in the presence of peroxide, we can break down the explanation into several steps:
### Step-by-Step Solution:
1. **Understanding Anti-Markovnikov's Addition**:
- Anti-Markovnikov's addition occurs when the addition of HX (where X is a halogen) to an alkene results in the halogen attaching to the less substituted carbon atom. This is typically observed in reactions involving peroxides.
**Hint**: Recall the Markovnikov's rule, which states that in the addition of HX to alkenes, the hydrogen atom will attach to the carbon with more hydrogen atoms (more substituted carbon).
2. **Role of Peroxides**:
- In the presence of peroxides, the addition of HBr to alkenes follows a free radical mechanism. This mechanism allows for the anti-Markovnikov addition to occur.
**Hint**: Consider how peroxides influence the reaction mechanism, leading to the formation of radicals.
3. **Free Radical Mechanism**:
- The free radical mechanism involves several propagation steps. For this mechanism to be favorable, all propagation steps must be exothermic (i.e., they release energy).
**Hint**: Think about the energy changes involved in each step of the free radical mechanism.
4. **Exothermic vs. Endothermic Reactions**:
- In the case of HBr, all the propagation steps are exothermic, allowing the anti-Markovnikov addition to occur. However, for HCl and HI, at least one of the propagation steps is endothermic (i.e., it requires energy).
**Hint**: Identify the nature (exothermic or endothermic) of the propagation steps for HCl and HI.
5. **Specific Cases of HCl and HI**:
- For HCl, the propagation step involving the formation of RCH2CH2Cl from RCH=CH2 and HCl is endothermic (ΔH is positive).
- For HI, the first propagation step also shows a positive ΔH, indicating it is endothermic.
**Hint**: Analyze the specific reactions for HCl and HI to determine their energy changes.
6. **Conclusion**:
- Since at least one propagation step for both HCl and HI is endothermic, the free radical mechanism cannot proceed effectively, and thus, they do not exhibit anti-Markovnikov addition in the presence of peroxides.
**Hint**: Summarize the findings to conclude why HCl and HI do not follow the anti-Markovnikov addition pattern.
### Final Answer:
In the presence of peroxide, hydrogen chloride and hydrogen iodide do not give anti-Markovnikov's addition to alkenes because one of the propagation steps in their reactions is endothermic, preventing the free radical mechanism from operating effectively.
