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Dimethyl dichlorosilane will undergo hydrolysis to form siloxane polymers. But why would its carbon analog dimethyl dichlorocarbon not be able to undergo a similar hydrolysis reaction to form polymers?

My guess is that $\ce{Si-Cl}$ bond being much weaker than $\ce{C-Cl}$ bond would be easier to break and undergo hydrolysis, and that the geminal diol is unstable, hence it is less likely to form.

I'm unsure of my answer though. Could anyone please explain and elaborate?

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  • $\begingroup$ Nah, it's mostly sterics and angles. Polymethyleneoxides $\ce{(CH2O)_{n}}$ is well known, methylmethyleneoxide also known as acetaldehyde forms trimer. Acetone, though, is sterically hindered enough to make it unprofitable to form polymere. As for the hydrolisys process itself, it also mostly sterics: 5-coordinated intermidiate state in hydrolisys is much, much easier to form for silicon than for carbon. $\endgroup$ – permeakra Oct 15 '14 at 15:59
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Actually a $\ce{Si-Cl}$ bond is much stronger than a $\ce{C-Cl}$ bond. It takes about 90 kcal/mole to break a $\ce{Si-Cl}$ bond, but only around 81 kcal/mole to break a $\ce{C-Cl}$ bond. The strengths of the corresponding $\ce{Si-O}$ (110 kcal/mole) and $\ce{C-O}$ (85 kcal/mole) bonds also need to be considered.

In the silicon reaction we need 90 kcal/mole to break the $\ce{Si-Cl}$ bond, but we get 110 kcal/mole back when we form the $\ce{Si-O}$; the reaction is exothermic by roughly 20 kcal/mole - a large amount. In the analogous carbon case, we need 81 kcal/mole to break a $\ce{C-Cl}$ bond, and we only get 85 kcal/mole back when we form the $\ce{C-O}$ bond; the reaction is exothermic by only 4 kcal/mole - not much. There is quite a difference in the overall driving force for the two reactions with the silicon case being energetically much more favorable than the carbon case!

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Your guess is correct. The geminal diol is unstable because carbon atom can't hold more than one $\ce{-OH}$ group. Where as Si atom can hold three $\ce{-OH}$ groups. It is this property of silicon that makes the formation of organosilicon polymers possible.

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  • $\begingroup$ well known example of stable gem-diol en.wikipedia.org/wiki/Chloral_hydrate $\endgroup$ – permeakra Oct 15 '14 at 16:37
  • $\begingroup$ It's not that you couldn't make polyacetal. In fact Dupont will sell it to you (under the name of Delrin) and it's a favourite with machinists because it holds shape extremely well. $\endgroup$ – Abel Friedman Oct 15 '14 at 17:26

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