Boron anomalous behavior

In sharp contrast to the behavior of 9-BBN, hydroboration of an internal allene with disiamylborane and dicyclohexylborane reveals preferential attack of the boron at the central carbon of the allenic chain, forming vinylboranes. For example, 2,4-dimethyl-2,3-pentadiene is converted [7] essentially, quantitatively, to the vinylborane with dicyclohexyl- and disiamylboranes, but exclusively to the allylborane with 9-BBN (Chart 5.24). In sharp contrast to acyclic allylborane, these allylborane products of 9-BBN do not react with acetone however, it undergoes usual alkaline hydrogen peroxide oxidation to afford the corresponding allylic alcohol. No protonolysis is observed on oxidation. This anomalous behavior of the allylborane is due to the steric bulk surrounding the boron atom, which prevents the coordination of oxygen of the acetone or water, necessary for allylboration or protonolysis [2, 8]. 

This review deals with studies performed at the Oak Ridge National Laboratory concerning the chemical fractionation of boron isotopes between BF3 and its molecular addition compounds. This research resulted in the development of a new separation process which was superior to methods previously employed. The work also led to a theoretical explanation of the exchange reaction which accounted for the anomalous concentration of boron-10 in the molecular addition compound, and the observed variations of the isotopic equilibrium constants as a function of different donors. The model predicted maximum isotopic equilibrium constants for the exchange reaction which were consistent with the experimental data. It also predicted the behavior of the other boron halides. 

Boron. Boron is structurally the most bizarre element in the periodic table. Simple bonding rules that are applicable to the other elements have to be bent considerably in order to accommodate the behavior of boron. Boron should be a metal, but it is a semiconductor with unique structures and anomalous physical properties. Combined with metals it participates in metallic bonding but boron atoms are simultaneously covalently bonded to other boron atoms in the higher metal borides. Similar to carbon it does not form mononuclear ions, its halides are molecules, and its other compounds with nonmetals are solids. 

Looking down Group 3A(13), we see a wide range of chemical behavior. Boron, the anomalous member from Period 2, is the first metalloid we ve encountered so far and the only one in the group. It is much less reactive at room temperature than the other members and forms covalent bonds exclusively. Although aluminum acts like a metal physically, its halides exist in the gas phase as covalent dimers— molecules formed by joining two identical smaller molecules (Figure 14.3)—and its oxide is amphoteric rather than basic. Most of the other 3A compounds are ionic, but they have more covalent character than similar 2A compounds because the 3A cations can polarize nearby electron clouds more effectively. 

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