To determine the reactivity order of halides in dehydrohalogenation, we need to consider the factors that influence the reaction, particularly the bond length and bond dissociation enthalpy of the halogen-carbon bonds.
### Step-by-Step Solution:
1. **Understanding Dehydrohalogenation**:
- Dehydrohalogenation is a reaction where a hydrogen halide (HX) is removed from an alkyl halide (R-X), resulting in the formation of an alkene (R=R).
- The reactivity of the halides in this reaction depends on the strength of the carbon-halogen bond.
2. **Analyzing Bond Length and Strength**:
- The halogens (F, Cl, Br, I) belong to the same group in the periodic table, and as we move down the group, the size of the atoms increases.
- The bond length increases from R-F to R-I, which means the bond strength decreases. The weaker the bond, the easier it is to break, leading to higher reactivity in dehydrohalogenation.
3. **Bond Dissociation Enthalpy**:
- The bond dissociation enthalpy is the energy required to break a bond. For halides, the order of bond dissociation enthalpy is:
- R-F > R-Cl > R-Br > R-I
- This indicates that the R-F bond is the strongest and the hardest to break, while the R-I bond is the weakest and the easiest to break.
4. **Reactivity Order**:
- Since the reactivity in dehydrohalogenation is inversely related to bond strength, we can conclude:
- R-I will be the most reactive (weakest bond), followed by R-Br, then R-Cl, and finally R-F (strongest bond).
- Therefore, the reactivity order of halides in dehydrohalogenation is:
- R-I > R-Br > R-Cl > R-F
5. **Conclusion**:
- Based on the analysis, the correct answer is option B: R-I > R-Br > R-Cl > R-F.
