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Question: Which of the following is most reactive for S$_N$2 reaction?...

Which of the following is most reactive for SN_N2 reaction?

A

2-bromopropane

B

Cyclopropane

C

Ethylene oxide

D

Ethylene sulfide

Answer

Ethylene sulfide

Explanation

Solution

The SN_N2 reaction is a bimolecular nucleophilic substitution reaction. Its rate is influenced by several factors, including steric hindrance at the reaction center, the leaving group ability, and the nature of the substrate.

  1. Steric Hindrance: SN_N2 reactions are favored by less steric hindrance. The reactivity order for alkyl halides is: methyl > primary > secondary > tertiary.
  2. Leaving Group: A good leaving group (e.g., halides like I−^-, Br−^-, Cl−^-) increases reactivity.
  3. Ring Strain: Cyclic compounds with strained rings, such as epoxides and thiiranes, undergo nucleophilic ring-opening reactions which are often considered SN_N2-like. The relief of ring strain provides a significant driving force, making these compounds very reactive.

Analyzing the options:

  • 2-bromopropane: This is a secondary alkyl halide, exhibiting moderate steric hindrance.
  • Cyclopropane: This is a strained ring but typically does not have a good leaving group unless substituted.
  • Ethylene oxide (Oxirane): This is a three-membered cyclic ether with primary carbons, thus minimal steric hindrance. The high ring strain makes it susceptible to nucleophilic attack.
  • Ethylene sulfide (Thiirane): This is a three-membered cyclic sulfide. Similar to ethylene oxide, it has primary carbons and high ring strain. Thiiranes are generally more reactive than epoxides because the C-S bond is weaker than the C-O bond, and sulfur is a larger, more polarizable atom, making it a better leaving atom.

Comparing reactivity: The relief of ring strain in thiiranes and epoxides makes them highly reactive towards nucleophiles, often exceeding the reactivity of secondary alkyl halides like 2-bromopropane. Between ethylene oxide and ethylene sulfide, the thiirane is more reactive due to the weaker C-S bond and better leaving ability of sulfur.

Therefore, ethylene sulfide is the most reactive for SN_N2 reaction among the given options.