major product formed in wurtz reaction in the given molecule[ 1,3-dibromo-2,2-di(1-bromomethyl)propane ]

This is the reactant molecule which undergoes wurtz reaction. All possible products are-

What will be the most favorable in all of these? In my opinion that the last one should not be the one because it forms a three member ring which is quite unstable due to angle strain. Still it is the answer to the question.

why any other molecule is not the major product?

• According to doi.org/10.1021/jo01118a042, it is indeed the last one, spiropentane (approx. 50 % conversion). You can foster the search process knowing your precursor's more common name (pentaerythrityl tetrabromide) and CAS number (3229-00-3). Nov 17 '21 at 9:05
• @ashank Yes, it is a strained species, but there are very many cyclopropanes known so clearly the strain is not a determining factor. Nov 17 '21 at 9:27
• @ashank No prob, this was just a published confirmation for the given answer. It doesn't seem authors provide an in-depth explanation as to why spiropentane is favored, so your question remains. Nov 17 '21 at 10:21
• Spiropentane is preferred, because it does not contain any bromine atom. If enough sodium is available, all bromine atoms will be transformed into $\ce{NaBr}$, so that the final product must be a hydrocarbon with $5$ atoms, or a polymer. Nov 17 '21 at 10:50
• @ashank Exactly so. That is why chemistry remains an experimental science. You cannot predict the outcome of many reactions so you have the run the experiment Nov 17 '21 at 11:22