MCQ Detailed View

In organic chemistry, a nucleophile is a species that donates an electron pair to form a chemical bond. Nucleophiles are usually negatively charged ions or neutral molecules with lone pairs of electrons. They attack positively charged or electron-deficient centers, such as carbocations or polar bonds.

Let’s evaluate the given options:

1. 
   

   C₂H₅O⁻ (Ethoxide ion): Strong nucleophile because it carries a negative charge on oxygen. It readily attacks electrophilic carbon atoms, for example in substitution or elimination reactions.

   
   


2. 
   

   SCN⁻ (Thiocyanate ion): Another good nucleophile, containing a negative charge delocalized between sulfur and nitrogen. Both atoms can act as electron donors, making SCN⁻ an ambident nucleophile.

   
   


3. 
   

   NH₃ (Ammonia): A neutral molecule but still a nucleophile because the nitrogen atom has a lone pair of electrons. Ammonia commonly participates in substitution reactions with alkyl halides to form amines.

   
   


4. 
   

   CH₃⁺ (Methyl cation): This is not a nucleophile. In fact, it is the opposite — an electrophile. CH₃⁺ has only six valence electrons and carries a positive charge, making it electron-deficient. Instead of donating electrons, it seeks electrons from nucleophiles.

   
   

From this analysis, we can conclude that the only species in the list that is not a nucleophile is CH₃⁺.

• 
  

  Nucleophiles donate electrons (C₂H₅O⁻, SCN⁻, NH₃).

  
  


• 
  

  Electrophiles accept electrons (CH₃⁺).

  
  

This distinction is fundamental in organic reaction mechanisms, especially in substitution and addition reactions.