Which of the following does not undergo hydrolysis?

According to electronegativities of halogen BF₃ should acts strong lewis acid but not ,why?

In Boron trihalides around the boron 6 electrons are present and hence Boron trihalides are acting as electron deficient compounds.Therefore Boron trihalides are acting as Lewis acids [electron pair acceptors].

The Lewis acidic strength order from BF₃ to BI₃ increases.

                                                This can be explained by using back "π'' bonding phenomina. In BF₃  the Fluorine filled 2p-orbital electrons are donated to the empty 2p-orbital of boron.Their sizes are same therefore good overlapping takes place between 2p-orbital of boron and 2p-orbital of fluorine.Hence the back π bond strength in BF₃ is very high.Hence the electron deficiency around the boron in BF₃ is less and hence the electron pair accepting tendency from electron pair donors is low. Therefore BF₃ is acting as weak lewis acid among all trihalides.

                                             Where as in case of BI₃ in the back π bond formation filled 5p-orbital of iodine involves in ovrlapping with the empty 2p-orbital of boron. Here difference sizes of overlapping orbitals is very high and hence extent of overlapping is very poor. Therefore back π-bond formation is almost not observed in case of  BI₃. So in BI₃ around "B" electron deficiency is very high ,therefore the B in BF₃ easily accepts the electron pair and hence BI₃ is acting as strong Lewis acid.

 

As the size of p-orbital of halogen increases, pπ-pπ  overlapping decreases, Back π-bond strength decreases, electron deficiency around B increases, therefore,electron pair accepting tendency increases, Lewis acidic strength increases. As the size of p-orbital of halogen increases, pπ-pπ  overlapping decreases, Back π-bond strength decreases, therefore  B—X bond double bond character decreases, means B—X bond single bond character increases, and hence B—X bond cleavage with water increases. i.e.,rate of hydrolysis increases. Therefore BI3 easily undergo hydrolysis ,where as BF3 cannot undergo hydrolysis , hence BF3 is using as lewis acid in friedel crafts reactions.