The correct order of reactivity of the given chlorides with acetate in acetic acid is:

Step 1: Identify the reaction type

Acetate ion ($\ce{CH3COO-}$) in acetic acid → $S_N2$ substitution

Acetic acid is polar protic solvent, but acetate is a decent nucleophile.

Step 2: Factors affecting $S_N2$ reactivity

$S_N2$ rate depends on:

1. Steric hindrance at reaction center (most important)
2. Leaving group ability
3. Nucleophile strength
4. Solvent effects

For cyclohexyl chlorides with different substituents:

• All have same leaving group (Cl)
• Different steric environments

Step 3: Analyze steric effects

From the structures shown:

• (a) Cyclohexyl-Cl with one substituent
• (b) Cyclohexyl-Cl (plain)
• (c) Cyclohexyl-Cl with $\ce{CH2Cl}$ group
• (d) Cyclohexyl-Cl with multiple substituents

Reactivity order (decreasing):

Less hindered > More hindered

Plain cyclohexyl chloride (b) should be most reactive (least hindered).

Substituents increase steric hindrance → decrease reactivity.

Step 4: Common mistakes to avoid

Students often think:

• Electron-withdrawing groups always increase reactivity (true for $S_N1$, not $S_N2$)
• Conformational effects don't matter (they do in cyclohexane)

Solving strategy:

For $S_N2$ reactivity:

1. Check steric hindrance around C-X
2. Less crowded = faster
3. Axial vs equatorial Cl matters
4. Substituents at adjacent positions matter most

Answer: (b)

The order shows plain cyclohexyl chloride as most reactive, with reactivity decreasing as substituents are added.

Key Points:

• $S_N2$: Steric hindrance is the dominant factor
• Cyclohexane: Chair conformation affects accessibility
• Axial Cl: More accessible for backside attack than equatorial
• Adjacent substituents: Significantly hinder $S_N2$