Explain why iodine (I 2 ) does not react with ethane, even though I 2 is more easily cleaved homolytically than the other halogens.

All right. Hi, everyone. So for this question, it says, or it's asking us why does iodine not react with methane under ordinary conditions, even though I two is cleaved easily to form radicals. So the thing about these kinds of questions, right is we have to understand or ask ourselves whether or not the reaction in question is energetically feasible or realistic. And in order to consider that we have to go ahead and discuss the bond dissociation energies that are relevant for this reaction, right? Because recall that the bond association energy is the energy required to break a certain chemical bond. Now, when it comes to the halogen nation of methane, we're talking about two bonds in question, right? There is a carbon hydrogen bond that's being broken and a new bond between hydrogen and iodine is forming now here, right, the bond association energy of the bond between carbon and hydrogen and methane is actually quite high. It's about 105 kg calories per mole. And by contrast, right, the energy of formation of the bond between hydrogen and iodine is relatively low. It's about 71.3 kg calories per mole. So just to clarify. I'm going to write on the side here that 105 kilocalories per mole is bond association energy, whereas 71.3 kilocalories per mole is energy of formation because here, methane is being cleaved and a new hydrogen iodine bond is in theory forming. So the problem here is that there is quite an energy difference between are two chemical bonds here specifically, if we subtract 71.3 from 100 and five, that leaves us with a difference of about 33.7 kg calories per mole. Now the 33.7 kg calories per mole of difference, right? It is the energy that the reaction requires to be inputted in order for it to take place. So what that means is that this reaction is quite endothermic. So what I'm trying to say here, right is that the iotation of methane is not energetically feasible because it is highly endothermic and a lot of energy is required to be put into it to force the reaction, so to speak. So here, right, our answer for the reason why iodine does not react with methane is due to the low bond energy of H I, it is due to the low bond energy of H I because this results this difference in energy results in an incredibly endothermic reaction that is simply not energetically feasible or favorable. So that's pretty much it. So thank you very much for watching and I hope you found this helpful.

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1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

11. Radical Reactions

Free Radical Halogenation

Organic Chemistry

11. Radical Reactions

Free Radical Halogenation

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4:38 minutes

Problem 28

Textbook Question

Explain why iodine (I₂) does not react with ethane, even though I₂ is more easily cleaved homolytically than the other halogens.

Verified step by step guidance

Iodine (I2) is more easily cleaved homolytically than other halogens because the I-I bond is weaker due to the larger atomic size of iodine, which results in poorer orbital overlap and a longer bond length.

For a reaction to occur between iodine (I2) and ethane (C2H6), the homolytic cleavage of the I-I bond must produce iodine radicals (I•), which can then abstract a hydrogen atom from ethane to form ethyl radicals (C2H5•).

However, the C-H bond in ethane is relatively strong, with a bond dissociation energy of approximately 410 kJ/mol. This means that a significant amount of energy is required to break the C-H bond and form the ethyl radical.

The iodine radicals (I•) generated from the homolytic cleavage of I2 are not sufficiently reactive to overcome the high bond dissociation energy of the C-H bond in ethane. As a result, the reaction does not proceed.

In contrast, halogens like chlorine (Cl2) and bromine (Br2) are more reactive because their radicals (Cl• and Br•) are more energetic and can more readily abstract hydrogen atoms from ethane, leading to a reaction.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Homolytic Cleavage

Homolytic cleavage refers to the breaking of a covalent bond in such a way that each atom involved in the bond retains one of the shared electrons, resulting in the formation of two radicals. In the case of iodine (I2), this process occurs more readily than in other halogens due to its weaker bond strength. However, the presence of radicals alone does not guarantee a reaction with ethane, as other factors must also be considered.

Reactivity of Alkanes

Alkanes, such as ethane, are generally unreactive due to their stable C-C and C-H bonds, which do not easily participate in reactions without the presence of strong electrophiles or radical initiators. The lack of functional groups in alkanes means they do not readily undergo reactions like halogenation unless conditions are favorable, such as the presence of heat or light to initiate radical formation.

Radical Stability and Selectivity

The stability of radicals plays a crucial role in determining the outcome of reactions involving radical species. Iodine radicals, while formed easily, are less stable compared to other halogen radicals, such as those from chlorine or bromine. This instability leads to a lower likelihood of reaction with ethane, as the formation of less stable radicals is energetically unfavorable, resulting in a lack of reactivity.

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