Substitution via Benzyne and Related Intermediates

Where there are strongly donating or withdrawing substituents on the aromatic ring, it is sometimes possible to predict the outcome by considering the reaction as having some ElcB character (so that the most acidic proton is removed) and then considering the relative 

Pyridynes can also be produced from halopyridines. In 3-chloropyridine, the hydrogen at the 4-position is significantly more acidic than that at the 2-position (H-4 exchanges with DjO in base 53 times more rapidly than H-2), so that the main pyridyne produced is that with the 

Explain the following observation, drawing a complete mechanism  

The two products from this process should tell us that the mechanism of this process involves a benzyne intermediate. Only one benzyne can possibly be produced in the first elimination step  

Two charged resonance forms can be written for the benzyne intermediate. Given the out-come. It IS clear which one is more important. We can rationalize this by remembering that a 

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