The order of reactivity of alkyl halides towards halogenation

Options

A. `RCl gt Rbe gt RF gt Rl`
B. `RF gt RCl gt RBr gt Rl`
C. `Rl gt RBr gt RCl gt RF`
D. `RBr gt RCl gt Rl gt RF`

Answer

To determine the order of reactivity of alkyl halides towards halogenation, we need to consider the bond dissociation energies of the carbon-halogen bonds and the stability of the carbocations formed during the reaction. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Mechanism
The halogenation of alkyl halides typically follows an SN1 mechanism, which involves two main steps:
1. Formation of a carbocation by breaking the R-X bond (where R is the alkyl group and X is the halogen).
2. Nucleophilic attack by the halogen on the carbocation.

### Step 2: Analyze the Bond Dissociation Energy
The reactivity of alkyl halides is influenced by the bond dissociation energy (BDE) of the carbon-halogen bond. The lower the BDE, the easier it is for the bond to break, leading to a more reactive alkyl halide.

### Step 3: Compare the Halogens
The halogens in order of decreasing bond strength (and thus increasing BDE) are:
- Fluorine (F)
- Chlorine (Cl)
- Bromine (Br)
- Iodine (I)

As we move down the group in the periodic table, the size of the halogen increases, which leads to a decrease in bond strength due to poorer overlap between the orbitals of carbon and the larger halogen atoms.

### Step 4: Establish the Order of Reactivity
Since the bond dissociation energy decreases from fluorine to iodine, the reactivity of alkyl halides towards halogenation increases in the order:
- R-F < R-Cl < R-Br < R-I

This means that alkyl iodides (R-I) are the most reactive towards halogenation, while alkyl fluorides (R-F) are the least reactive.

### Final Order of Reactivity
The final order of reactivity of alkyl halides towards halogenation is:
**R-I > R-Br > R-Cl > R-F**