Sciencemadness Discussion Board - Organic Compound witch Si or PB instead of C

Organic Compound witch Si or PB instead of C

Blind Angel - 8-3-2004 at 10:22

I always wondered if it was possible to create organic compound with Si or Pb instead of C, like instead of Benzene it would be Hexasiline. I think that Pb compound wouldn't exist but maybe it would be possible to make simple one using Si. I once asked my Biology teacher about that and he told me about Silicious Algae but this didn't give me what i want. What do you think about that?

vulture - 8-3-2004 at 13:07

Silanes are analoguous to hydrocarbons, but they're very unstable because the bond distance between the Si atoms is too big to form a good overlap.

thunderfvck - 8-3-2004 at 13:53

My organic chemistry book did a little thing on this subject.

I remember one thing.

Instead of exhaling CO2, we'd exhale SiO2. Ha!

The Extensive Chemistry of Carbon

BromicAcid - 8-3-2004 at 14:20

From Descriptive Inorganic Chemistry, 3rd edition 2002, Geoff Rayner-Canham; Tina Overton

Quite a bit to quote but I felt most would consider it interesting.

Quote:

Carbon has two properties that enable it to form such an extensive range of compounds: catenation (the ability to form chains of atoms) and multiple bonding (that is the ability to form double and triple bonds). Extensive use of multiple bonding is found in compounds of carbon, nitrogen, and oxygen. Carbon shows the greatest propensity for catenation of all elements. [Note: Sulfur shows the second most] For catenation, three conditions are necessary:

A bonding capacity (valence) greater then or equal to 2.
An ability of the element to bond with itself; the self-bond must be about as strong as its bonds with other elements.
A kenetic inertness of the catenated compound toward other molecules and ions.

We can see why catenation is frequently found in carbon compounds but only rarely in silicon compounds by comparing bond energy data for those two elements. Notice that the energies of the carbon-carbon and carbon-oxygen bonds are very similar. However, the silicon-oxygen bond is much stronger than that between two silicon atoms. Thus, in the presence of oxygen, silicon will form -Si-O-Si-O- chains rather then -Si-Si- linkages. We will see later that the silicon-oxygen chains dominate the chemistry of silicon. There is much less of an energy "incentive" to break carbon-carbon bonds in favor of the formation of carbon-oxygen bonds.

It is sobering to realize that two "quirks" of the chemical world make life possible: the hydrogen bond and the catenation of carbon. Without these two phenomena, life of any form (that we can imagine) could not exist.

C-C BE = 346 KJ/Mol
C-O BE = 358 KJ/Mol
Si-Si BE = 222 KJ/Mol
Si-O BE = 452 KJ/Mol

Of course this all comes down to what Vulture said about overlap, that is the main contributing factor to the bond energy. The best you're going to come to similar bonding in my opinion is the combo elements, such as the boron-nitrogen chemistry, the alternating boron-nitrogen atoms by a terrible oversimplification basically averages out to a carbon atom. Same as how with semi-conductors we use gallium arside semi conductors or cupuric bromide to obtain different resistivites akin to germanium.

vulture - 9-3-2004 at 08:44

The best you're going to come to similar bonding in my opinion is the combo elements, such as the boron-nitrogen chemistry, the alternating boron-nitrogen atoms by a terrible oversimplification basically averages out to a carbon atom. Same as how with semi-conductors we use gallium arside semi conductors or cupuric bromide to obtain different resistivites akin to germanium.

What you're referring to are isoelectric compounds and surprisingly the theory of isoelectricity can explain quite a few phenomena, despite it being so "rough".

Re: Compounds of Si analogous to organic compounds

JohnWW - 3-8-2004 at 01:05

I always thought that the reason why compounds with Si-Si bonds, and especially Si=Si double bonds, are much more difficult to obtain than the corresponding organic compounds is, not because of the Si-Si bond length, but instead because the Si atom has empty 3d orbitals, unlike C, through which oxidants and other reagents can readily attack through expansion of the number of electrons in valence shells beyond 8. This is why SiH4 and H3Si-SiH3 and the higher silanes (and germanes) spiontaneously ignite in air. There are compounds with P-P, S-S and S-P bonds, but in such cases the compounds, although still highly inflammable, have functional groups providing some steric protection from oxidants.

This availability of empty 3d orbitals for bonding also enables Si to form the orthosilicate anion, SiO4----, polymeric silicates with bridging O atoms, and SiO2 with a giant-molecule structure, which CO2 cannot. For the same reason, P and S can form compounds in which they have 5 or 6 covalent bonds like PF5 and SF6, while N and O cannot.

In addition, Si=Si double bonds would be less stable than C=C double bonds, including in any benzene homolog, because of the larger size and lesser stability of overlapping 3sp2 hybid orbitals compared to the 2sp2 hybrid orbitals in C=C bonds. I do not think that any compounds with Si=Si bonds have yet been isolated, but compounds with P=P double bonds have recently been.

John W.

Nick F - 3-8-2004 at 03:15

Yeah, Si etc have vunerable d orbitals. Also, Si generally doesn't like making pi bonds to heteroatoms, because it's p electrons are relatively high in energy. So you can easily get ketones, because O and C p electrons are similar in energy, but a silone is not favourable, because Si's p electrons are so much higher in energy than O's p electrons. Hence why CO2 is a gas, but SiO2 is a covalent solid, because there are sigma bonds linking everything together.