For the reaction: The correct statement from the following is:

Step 1: Identify the reaction

This appears to be an iodide exchange reaction (Finkelstein-type):

$\ce{R-CH2-I + I- -> R-CH2-I + I-}$

Wait, this is the same compound on both sides. Let me reconsider.

Actually, this might be showing isotopic exchange or the mechanism of $S_N2$ with iodide.

Mechanism: $S_N2$

$\ce{I- + R-CH2-I -> [I---CH2R---I]^- -> I-CH2R + I-}$

Step 2: Analyze the transition state

$S_N2$ transition state:

• Trigonal bipyramidal geometry
• Partial bonds to both incoming and leaving I
• Negative charge delocalized over two iodine atoms

Localized anion (reactant):

• Free $\ce{I-}$ ion
• Full negative charge on single iodine
• More polar (concentrated charge)

Transition state:

• Charge spread over larger area (two I atoms)
• Less polar than localized anion

Statement (a) is TRUE ✓

Step 3: Evaluate other statements

(b) "The reaction can occur in acetic acid also."

• Acetic acid is polar protic solvent
• Strongly solvates anions (H-bonding)
• $S_N2$ is slower in protic solvents
• Acetone (polar aprotic) is much better
• Statement is misleading/false ✗

(c) "The solvent used in the reaction solvates the ions formed in rate determining step."

• Acetone is polar aprotic
• Does NOT strongly solvate anions (no H-bonding)
• This is why it's good for $S_N2$ (nucleophile is "naked")
• Statement is FALSE ✗

(d) "Br can act as competing nucleophile."

• Not relevant to this specific reaction
• No Br mentioned in the reaction
• Statement is irrelevant/false ✗

Answer: (a)

Key Concepts:

$S_N2$ Transition State:

• Charge delocalization
• Less polar than separated ions
• Stabilized by charge dispersal

Solvent Effects:

| Solvent Type | Examples | Effect on $S_N2$ | |--------------|----------|------------------| | Polar aprotic | Acetone, DMSO, DMF | Fast (doesn't solvate Nu⁻) | | Polar protic | H₂O, ROH, RCOOH | Slow (solvates Nu⁻) |

Finkelstein Reaction:

• Halogen exchange using NaI in acetone
• Driven by precipitation of NaCl/NaBr
• $S_N2$ mechanism