A siloxane is a functional group in organosilicon chemistry with the Si−O−Si linkage. The parent siloxanes include the oligomeric and polymeric hydrides with the formulae H(OSiH2)nOH and (OSiH2)n. Siloxanes also include branched compounds, the defining feature of which is that each pair of silicon centres is separated by one oxygen (O2-) atom. The siloxane functional group forms the backbone of silicones, the premier example of which is polydimethylsiloxane (PDMS). The functional group R3SiO− (where the three Rs may be different) is called siloxy. Siloxanes are manmade and have many commercial and industrial applications because of the compounds’ hydrophobicity, low thermal conductivity, and high flexibility. Structure Siloxanes generally adopt structures expected for linked tetrahedral ("sp3-like") centers. The Si−O bond length is 1.64 Å (vs Si–C distance of 1.92 Å) and the Si-O-Si angle is rather open at 142.5°. By contrast, the C−O distance in a typical dialkyl ether is much shorter at 1.414(2) Å with a more acute C−O−C angle of 111°. It can be appreciated that the siloxanes would have low barriers for rotation about the Si−O bonds as a consequence of low steric hindrance. This geometric consideration is the basis of the useful properties of some siloxane-containing materials, such as their low glass transition temperatures. Synthesis of siloxanes The main route to siloxane functional group is by hydrolysis of silicon chlorides: 2 R3Si−Cl + H2O → R3Si−O−SiR3 + 2 HCl The reaction proceeds via the initial formation of silanols (R3Si−OH): R3Si−Cl + H2O → R3Si−OH + HCl The siloxane bond can then form via a silanol + silanol pathway or a silanol + chlorosilane pathway: 2 R3Si−OH → R3Si−O−SiR3 + H2O R3Si−OH + R3Si−Cl → R3Si−O−SiR3 + HCl Hydrolysis of a silyldichloride can afford linear or cyclic products. Linear products are terminated with silanol groups: n R2Si(OH)2 → H(R2SiO)nOH + (n − 1) H2O Cyclic products have no silanol termini: n R2Si(OH)2 → (R2SiO)n + n H2O The linear products, polydimethylsiloxane (PDMS), are of great commercial value. Their production requires the production of dimethylsilicon dichloride. Starting from trisilanols, cages are possible, such as the species with the formula (RSi)nO3n/2 with cubic (n = 8) and hexagonal prismatic (n = 12) structures. The cubic cages are cubane-type clusters, with silicon centers at the corners of a cube oxygen centres spanning each of the twelve edges. Reactions Oxidation of organosilicon compounds, including siloxanes, gives silicon dioxide. This conversion is illustrated by