Boron is one of the few elements that seems to offer a plausible alternative to carbon as a basis for life elsewhere in the universe. Like carbon and silicon, boron has a strong tendency to form covalent molecular compounds. Being a group III element, however, it has one less valence electron than the number of valence orbitals, which makes its chemistry noticeably different from that of carbon.
There are no direct analogs to hydrocarbons in boron chemistry because, although boron forms a lot of different structural varieties of hydride, in these the boron atoms are linked indirectly through hydrogen bridges. Boron forms bonds with nitrogen that are somewhat like the carbon-carbon bond – two electrons from the nitrogen being donated in addition to the covalent electron sharing. Boron-nitrogen compounds largely match the chemical and physical properties of alkanes (such as methane and ethane) and aromatic hydrocarbons (such as benzene) but with higher melting and boiling points. Borazole especially is both chemically and physically similar to benzene. However, the fact that borazole and its derivatives are more reactive than their benzene counterparts would make any boron-based biochemistry more feasible within the lower temperatures at which ammonia is a liquid solvent since the reactions would then be more controllable. Interestingly, boron has an affinity ammonia as a solvent, which would suit a low-temperature biological scheme.
One of the biggest drawbacks to boron as a basis for life it is scarcity. On Earth, its abundance in the continental crust is only about 10 parts per million, so that any biology would seem to depend on their being present some mechanism for bringing about greater local concentrations of the element.
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