ethylene (ch2=ch2) is the answer. basically, the Mg reacts with the halide, making it
I ch2 ch2(-)(+)MgI
then, the negative on the carbon creates a double
bond with the other carbon causing the elimination of the I.
ch2=ch2 + I(-)(+)MgI
at least I think that's how the reaction goes. I'm not sure if the final answer has I2 and Mg,ether again, but I do know that ethylene is the major product.
I'm not 100% sure this is how the mechanism is, but i'm pretty sure that this is the gist of it. hope it helps!😉