Structural isomers are compounds with the same chemical formula but different structures and properties. Carbon and hydrogen compounds have many isomers, with the number increasing as the number of atoms increases. Silicon and boron also have isomers. The ability to form isomers allows for a wide range of compounds with different properties, including those necessary for life and for scientific and manufacturing applications.
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 regular butane CH3CH2CH2CH3 and methylpropane (CH3)2CHCH2CH3. Interestingly, regular butane boils at -0.5 degrees Celsius, while methylpropane boils at +28 degrees Celsius. As the number of atoms increases, the number of isomers increases. There are three structural isomers with the chemical formula C5H12, five with the formula C6H14, and nine with the formula C7H16.
Structural isomers of carbon are not limited to just carbon and hydrogen, although these are the best known cases of structural isomerism. In the home medicine cabinet you can find C3H8O, or isopropyl alcohol, sometimes referred to as “denatured alcohol.” Its structural formula is CH3CH(OH)CH3. Additionally, there is n-propyl alcohol, CH3CH2CH2(OH), and even methylethyl ether, CH3OCH2CH3, although neither of these compounds is likely to be found in the home. There are also structural isomers of carbon compounds containing other atoms.
What makes such an abundance of isomeric forms possible is the ability of the atoms of some elements, especially carbon, to join together. This is due to the nature of the bonds between atoms. Adjacent carbon atoms are joined by covalent bonds, bonds in which the participating atoms share electrons equally, rather than transferring them from one atom to another. To illustrate, in ordinary table salt, NaCl, the participating sodium atom gives its one available electron to the chlorine atom and the two atoms are electrostatically attracted. Nothing like this exists between the bonded carbon atoms in ethane, C2H6.
Silicon and boron possess the same ability to bond with each other without electron transfer. The structural isomers of silicon and boron are well illustrated in silanes – compounds of silicon and hydrogen – and boranes – compounds of boron and hydrogen. Carbon and hydrogen compounds start with the methane molecule, CH4. Similarly to this, compounds of silicon and hydrogen begin with a silane, SiH4. Interestingly, compounds of boron and hydrogen start 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 an almost infinite range of properties. In the case of carbon, structural isomers make compounds of life possible. For silicon and boron, the great variety of compounds offers the scientific and manufacturing world 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 the boranes, they can be used in specialized organic syntheses, in exotic fuel cells and even for rocket fuel.
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