Q.Alkyl fluorides are synthesised by heating an alkyl chloride/bromide in presence of ____________ or ____________. (Two or more than two options may be correct.)

Concept: Swarts Reaction (Halogen Exchange) — This reaction is a halide exchange for preparing alkyl fluorides. Alkyl fluorides are not easily made by direct fluorination (too violent); instead, we use a metathesis reaction where a heavier silver or mercurous fluoride drives the equilibrium.

Reasoning:

1. Alkyl chlorides/bromides react with metallic fluorides in a double displacement: $\mathrm{R-X + M-F \rightarrow R-F + M-X}$.
2. The reaction works best with fluorides of metals that form a very insoluble metal halide (AgCl, Hg₂Cl₂, etc.), pulling the equilibrium forward. …

The question is about a classic method in organic chemistry: converting an alkyl chloride or bromide into an alkyl fluoride. Direct fluorination with $\mathrm{F_2}$ gas is too violent and unselective — it would tear the molecule apart. So chemists use a gentler approach: a halogen exchange reaction, where a heavier halogen (Cl or Br) is swapped for fluorine using a metal fluoride.

This is the Swarts reaction. The key insight is that not all metal fluorides work equally well. The metal fluoride must be able to donate a fluoride ion (i.e., be sufficiently ionic or have a suitable metal-fluorine bond) and the byproduct (the metal chloride or bromide) must be stable enough to drive the reaction forward. Silver fluoride ($\mathrm{AgF}$) and mercurous fluoride ($\mathrm{Hg_2F_2}$) are classic reagents. Antimony trifluoride ($\mathrm{SbF_3}$) is also common, often with a catalyst. Cobalt(II) fluoride ($\mathrm{CoF_2}$) is another known reagent for this exchange.

Let’s examine each option:

1. Option (i): $\mathrm{CaF_2}$

   Calcium fluoride is extremely stable and insoluble — it’s the mineral fluorite. Its lattice energy is so high that it does not readily release fluoride ions under normal reaction conditions. Heating an alkyl chloride with $\mathrm{CaF_2}$ gives essentially no reaction. So this is not a correct choice.

2. Option (ii): $\mathrm{CoF_2}$

   Cobalt(II) fluoride is used in the Swarts reaction. It can exchange its fluorine for chlorine or bromine, producing alkyl fluorides. For example:

This is a valid reagent. So (ii) is correct.

3. Option (iii): $\mathrm{Hg_2F_2}$ Mercurous fluoride is a classic Swarts reagent. It reacts smoothly with alkyl halides:

The byproduct mercurous chloride is insoluble and drives the equilibrium forward. This is one of the most well-known reagents for this purpose. So (iii) is correct.

4. Option (iv): $\mathrm{NaF}$ …

Concept: Swarts Reaction (Halogen Exchange)

This is a classic nucleophilic substitution where a heavier halogen (Cl or Br) is replaced by fluorine using a metal fluoride. The reaction works best with silver fluoride ($\mathrm{AgF}$) or mercurous fluoride ($\mathrm{Hg_2F_2}$) and cobalt(II) fluoride ($\mathrm{CoF_2}$).

Method: Swarts Reaction — Steps

1. Identify the substrate: An alkyl chloride or alkyl bromide.
2. Choose the fluorinating agent: The metal fluoride must have a high lattice energy and be able to exchange halogens. The most common reagents are:
   • $\mathrm{AgF}$ (silver fluoride) — most effective
   • $\mathrm{Hg_2F_2}$ (mercurous fluoride)
   • $\mathrm{CoF_2}$ (cobalt(II) fluoride)
3. Heat the mixture: The reaction is carried out by heating the alkyl halide with the metal fluoride.
4. Product: Alkyl fluoride is formed, along with the metal chloride/bromide as byproduct.

Applying to the options …

Common Mistakes & How to Avoid Them

Mistake 1: Choosing $\mathrm{NaF}$ (Option iv) as a correct reagent

Why students make this mistake:

Students often assume that since $\mathrm{NaF}$ is a fluoride salt, it should work like $\mathrm{NaCl}$ or $\mathrm{NaBr}$ in halogen exchange reactions. They think "any fluoride salt will do."

Why it's wrong:

The reaction is a Finkelstein-type halogen exchange, but fluorine is much less nucleophilic than chlorine or bromine. $\mathrm{NaF}$ is insoluble in common organic solvents (like acetone) and its fluoride ion is too tightly held in the crystal lattice to participate effectively. The reaction requires metal fluorides that are either soluble or have a high affinity for the heavier halogen.

How to avoid:

Remember: For fluorination, you need heavy metal fluorides (like $\mathrm{AgF}$, $\mathrm{Hg_2F_2}$, $\mathrm{CoF_2}$) or $\mathrm{CaF_2}$ in specific conditions. $\mathrm{NaF}$ and $\mathrm{KF}$ are not used for this conversion.

Mistake 2: Selecting only one option when the question says "two or more"

Why students make this mistake:

Students see "alkyl fluorides are synthesised by heating..." and recall only one common reagent (often $\mathrm{Hg_2F_2}$ from the Swarts reaction) and stop there.

Why it's wrong:

The question explicitly allows multiple correct answers. The Swarts reaction uses $\mathrm{Hg_2F_2}$ or $\mathrm{CoF_2}$, and $\mathrm{CaF_2}$ is also used in some industrial fluorination methods.

How to avoid:

Read the instruction carefully: "Two or more than two options may be correct." Always check if multiple reagents fit the condition. For this concept, three reagents are valid.

Mistake 3: Confusing $\mathrm{Hg_2F_2}$ with $\mathrm{HgF_2}$

Why students make this mistake:

The subscript "2" in $\mathrm{Hg_2F_2}$ is often overlooked. Students think it's just mercuric fluoride ($\mathrm{HgF_2}$).

Why it's wrong:

$\mathrm{Hg_2F_2}$ (mercurous fluoride) is the correct reagent for the Swarts reaction. $\mathrm{HgF_2}$ (mercuric fluoride) is not used for this purpose. The reaction mechanism involves the dimeric nature of $\mathrm{Hg_2^{2+}}$ which facilitates halogen exchange.

How to avoid:

Pay attention to oxidation states and subscripts. Write the formula carefully: $\mathrm{Hg_2F_2}$ (not $\mathrm{HgF_2}$). Memorise the Swarts reagent as mercurous fluoride.

Mistake 4: Assuming CaF2 must be valid because it is a fluoride source used industrially somewhere

Why students make this mistake: …