Why must the alkyl halide be sp3 hybridized with respect to the carbon connected to the halogen for the alkylation to occur?
Why, for example, can't we use acetylide with a bromine substituted for hydrogen on one side in conjunction with aluminum tribromide to alkylate benzene?
If anything this seems to be more favorable as an sp carbocation is likely less stable than an sp3 carbocation.
Answer
If anything this seems to be more favorable as an sp carbocation is likely less stable than an sp3 carbocation.
That's the problem, sp2 and sp hybridized carbocations are less stable than sp3. Those carbocations are difficult to form and will likely undergo rearrangement faster than attack of the aromatic ring.
Modern organic synthesis has much better ways of making sp-sp2 and sp2-sp2 carbon-carbon bonds by palladium catalyzed cross-coupling.
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