MCQ Detailed View

The reactivity of alkyl halides in nucleophilic substitution reactions mainly depends on the strength of the carbon–halogen bond and the ability of the halogen atom to act as a leaving group.

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  Carbon–fluorine bond (C–F): This bond is the strongest among all carbon–halogen bonds due to the high electronegativity and small atomic size of fluorine. As a result, R–F compounds are very unreactive in substitution reactions because breaking the C–F bond requires high energy.

  
  


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  Carbon–chlorine bond (C–Cl): The bond strength decreases compared to C–F. R–Cl compounds are more reactive than fluorides but still less reactive than bromides and iodides.

  
  


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  Carbon–bromine bond (C–Br): Weaker than C–Cl, making bromides more reactive in substitution reactions. Bromine is also a better leaving group than chlorine.

  
  


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  Carbon–iodine bond (C–I): This bond is the weakest among alkyl halides. Iodine is a large atom and can easily stabilize the negative charge when it leaves as I⁻. Because of its low bond strength and excellent leaving group ability, R–I compounds are the most reactive alkyl halides.

  
  

Reactivity order:
R–I > R–Br > R–Cl >> R–F

Therefore, for a particular alkyl group, the reactivity is highest when the halogen is iodine.

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  R–F: Unreactive due to very strong bond.

  
  


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  R–Cl: Moderate reactivity.

  
  


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  R–Br: High reactivity.

  
  


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  R–I: Highest reactivity.

  
  

This trend is important in both SN1 and SN2 substitution mechanisms, as well as elimination reactions.