The Question is from neet-chemistry haloalkanes-haloarenes , and it was created by the author A.RAMA NAGESWARARAO

Question

Which of the following reactions is most likely to proceed via a $S_N2$ mechanism?

A.RAMA NAGESWARARAO · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: $S_N2$ reaction mechanism
 **Related Topics**: Nucleophilic substitution, reaction kinetics, steric hindrance
 **Prerequisites**: Understanding of $S_N1$ and $S_N2$ mechanisms, knowledge of alkyl halide reactivity.
## SOLUTION PATHWAY
**Given Information**:
Four different reactions involving alkyl halides and nucleophiles.
**Method & Steps**:
1. Identify the characteristics of an $S_N2$ reaction. $S_N2$ reactions are favored by:
    * **Strong nucleophiles:**  e.g., $CN^-$, $RO^-$
    * **Primary or methyl alkyl halides:** Less steric hindrance allows for backside attack.
    * **Aprotic solvents:** Solvents that don't have O-H or N-H bonds, which can hydrogen bond and hinder the reaction.  DMF (dimethylformamide) is an example.
2. Analyze each reaction:
    * **A:** Reaction of tert-butyl bromide with aqueous sodium hydroxide.  tert-butyl bromide is a tertiary alkyl halide, highly sterically hindered, making $S_N2$ very unlikely.  It favors $S_N1$.
    * **B:** Reaction of 2-bromopropane with sodium methoxide in methanol. 2-bromopropane is a secondary alkyl halide.  $S_N2$ is possible but competes with $S_N1$. Methanol is a protic solvent, disfavoring $S_N2$.
    * **C:** Reaction of methyl iodide with sodium cyanide in DMF. Methyl iodide is a primary alkyl halide, ideal for $S_N2$. Sodium cyanide is a strong nucleophile. DMF is an aprotic solvent. This combination strongly favors $S_N2$.
    * **D:** Reaction of 2-chloro-2-methylpropane with ethanol. 2-chloro-2-methylpropane (tert-butyl chloride) is a tertiary alkyl halide, highly hindered, making $S_N2$ impossible. It favors $S_N1$.
3. Conclusion: Reaction C is the most likely to proceed via an $S_N2$ mechanism because it meets all the favorable conditions.
  Result: C
  Verification:  The reaction conditions and substrate in option C align perfectly with the requirements for a successful $S_N2$ reaction.
 ## OPTION ANALYSIS
A :x: Tertiary alkyl halides are highly hindered and do not undergo $S_N2$ reactions effectively.
B :x: Secondary alkyl halides can undergo $S_N2$, but the protic solvent and the possibility of $S_N1$ competition make this less likely than option C.
C ✓: Primary alkyl halides are ideal substrates for $S_N2$ reactions, and the strong nucleophile and aprotic solvent further enhance the likelihood of this mechanism.
D :x: Similar to A, tertiary alkyl halides are poor substrates for $S_N2$ reactions due to steric hindrance.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing $S_N1$ and $S_N2$ mechanisms: Pay close attention to substrate structure, nucleophile strength, and solvent type.
  2.Ignoring steric hindrance: Steric hindrance around the carbon atom bearing the leaving group significantly affects the reaction mechanism.
 **Quick Tips**:
  1.Remember the key factors favoring $S_N2$: strong nucleophile, primary/methyl halide, aprotic solvent.
  2.Use a mnemonic device to remember the characteristics of $S_N1$ and $S_N2$ reactions (e.g., $S_N1$ = Sterically hindered, $S_N2$ = Sterically unhindered).

A.RAMA NAGESWARARAO · Answer

Reaction of tert-butyl bromide with aqueous sodium hydroxide

A.RAMA NAGESWARARAO · Answer

Reaction of 2-bromopropane with sodium methoxide in methanol

A.RAMA NAGESWARARAO · Answer

The correct Option is "Reaction of methyl iodide with sodium cyanide in DMF"

A.RAMA NAGESWARARAO · Answer

Reaction of 2-chloro-2-methylpropane with ethanol

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