Question

Arylbromides are not good candidates for

• A. Wurtz-Fittig reaction
• B. Fittig reaction
• C. Friedel-Crafts reaction
• D. Grignard reaction
• E. Gabriel-phthalimide synthesis

Hint:

Aryl halides and vinyl halides are generally unreactive in $S_N2$ and $S_N1$ reactions under standard conditions due to resonance and bond strength.

Answer 1

Answer: (E)

Concept: The Gabriel Phthalimide Synthesis is a method for producing primary amines. It involves an $S_N2$ reaction between potassium phthalimide and an alkyl halide.

Step 1: {The mechanism of Gabriel synthesis.} The nucleophile (phthalimide anion) must attack the carbon bearing the halogen from the back side to displace the halide ion.

Step 2: {The reactivity of Aryl bromides.} In aryl halides (like bromobenzene), the $C-X$ bond has partial double bond character due to resonance. Furthermore, the $sp^2$ hybridized carbon is more electronegative, making the bond stronger and shorter than in alkyl halides.

Step 3: {Explain the failure of the reaction.} Aryl bromides do not undergo nucleophilic substitution ($S_N2$) reactions because the benzene ring prevents back-side attack. Consequently, aryl halides cannot be used to prepare aromatic primary amines (like aniline) using the Gabriel synthesis.