Structural isomers are compounds of the same chemical formula that possess different structures and properties based on how their constituent atoms are ordered. For example, there are two structural isomers with the same chemical formula C4H10, namely normal butane CH3CH2CH2CH3 and methylpropane (CH3)2CHCH2CH3. It is interesting to note normal butane boils at -0.5 degrees Celsius, whereas methylpropane boils at +28 degrees Celsius. As the number of atoms grows, the number of isomers increases. There are three structural isomers with chemical formula C5H12, five with formula C6H14 and nine with formula C7H16.
Structural isomers of carbon are not restricted only to carbon and hydrogen, though those are the best known instances of structural isomerism. In the household medicine cabinet one may find C3H8O, or isopropyl alcohol, sometimes identified as "rubbing alcohol." Its structural formula is CH3CH(OH)CH3. Additionally, there is n-propyl alcohol, CH3CH2CH2(OH) and even methylethyl ether, CH3OCH2CH3, though neither of these two compounds is likely to be found in the home. There are structural isomers of carbon compounds containing other atoms, as well.
What makes such an abundance of isomeric forms possible is the ability of the atoms of a few elements — most notably carbon — to join to one another. This is due to the nature of the bonds between the atoms. Adjacent carbon atoms are joined by covalent bonds, bonds in which participating atoms share electrons equally, rather than transfer them from one atom to the other. To illustrate, in ordinary table salt, NaCl, the participating sodium atom gives over its one available electron to the chlorine atom, and the two atoms are attracted electrostatically. Nothing like this exists between the joined carbon atoms in ethane, C2H6.
Silicon and boron possess the same ability to bond to each other without electron transfer. The structural isomers of silicon and boron are well illustrated in the silanes — compounds of silicon and hydrogen — and the boranes — compounds of boron and hydrogen. Compounds of carbon and hydrogen begin with the molecule methane, CH4. Analogous to this, the compounds of silicon and hydrogen begin with silane, SiH4. Interestingly, the compounds of boron and hydrogen start out differently with borane, BH3 — a compound known only in the gaseous state that rapidly dimerizes to form B2H6.
The ability to form structural isomers greatly increases the number of possible compounds with a nearly endless range of properties. In the case of carbon, structural isomers make possible the compounds of life. For silicon and boron, the large variety of compounds affords the scientific and manufacturing worlds a plethora of reagents. One application of silane derivatives is in coatings that allow biologically harmless materials to be attached to titanium implant structures. As for boranes, they may be used in specialized organic syntheses, in exotic fuel cells, and even for rocketry fuel.