The major product formed in the reaction of 1,5-cyclooctadiene with 0.5 equivalent of diborane is A.
1,5-Cyclooctadiene is a diene, which means it has two double bonds. Diborane is a Lewis acid, which means it can accept electrons. When 1,5-cyclooctadiene reacts with diborane, the boron atoms from diborane bond to the two carbon atoms that are part of the double bonds. This results in the formation of a diborane complex, which is a compound that contains boron and carbon atoms.
The diborane complex can then undergo a number of different reactions, depending on the conditions. In the presence of water, the diborane complex will hydrolyze to form 1,5-cyclooctadiene-1,2-diol. This is the major product formed in the reaction of 1,5-cyclooctadiene with 0.5 equivalent of diborane.
The other options are incorrect because they do not represent the major product formed in the reaction of 1,5-cyclooctadiene with 0.5 equivalent of diborane. Option B is the product of the reaction of 1,5-cyclooctadiene with 1 equivalent of diborane. Option C is the product of the reaction of 1,5-cyclooctadiene with 2 equivalents of diborane. Option D is the product of the reaction of 1,5-cyclooctadiene with 3 equivalents of diborane.