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My textbook tells me that when carbon bonds with oxygen, the electrons are too close to the oxygen, and therefore they cannot just string together and form large molecules like glucose. It goes on to say that because, during photosynthesis, carbon becomes bonded to less electronegative atoms like hydrogen, it becomes more capable of forming larger compounds. Why is that?

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    $\begingroup$ Glucose is a large organomolecule? Since when? (But of course, being large is relative) $\endgroup$ – M.A.R. Apr 24 '15 at 16:03
  • $\begingroup$ Might be referring to glucose polymers. $\endgroup$ – John Snow Apr 24 '15 at 16:17
  • $\begingroup$ @MARamezani Much larger than carbon dioxide anyway. $\endgroup$ – lightweaver Apr 24 '15 at 16:27
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If we take a look at glucose we can see there are oxygen atoms in it, just with extra presence of the hydrogen.

The assumption :

carbon bonds with oxygen, the electrons are too close to the oxygen, and therefore they cannot just string together and form large molecules like glucose.

is wrong.

Take a look at Polymeric carbon oxides

Carbon suboxide spontaneously polymerizes at room temperature into a carbon-oxygen polymer, with 3:2 carbon:oxygen atomic ratio. The polymer is believed to be a linear chain of fused six-membered lactone rings, with a continuous carbon backbone of alternating single and double bonds. Physical measurements indicate that the mean number of units per molecule is about 5–6, depending on the formation temperature.

Carbon monoxide compressed to 5 GPa in a diamond anvil cell yields a somewhat similar reddish polymer with a slightly higher oxygen content, which is metastable at room conditions. It is believed that $CO$ disproportionates in the cell to a mixture of $\ce{CO2}$ and $\ce{C3O2}$ ; the latter forms a polymer similar to the one described above (but with a more irregular structure), that traps some of the $\ce{CO2}$ in its matrix.

Another carbon-oxygen polymer, with C:O ratio 5:1 or higher, is the classical graphite oxide and its single-sheet version graphene oxide.

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