Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: adianadiadi on 06/04/2014 08:19:59
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Why chloral hydrate is stable? Is it due to hydrogen bonding. If so, are there any such hydrates of aldehydes, which are stable like this?
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Chloral hydrate is stable not so much because the geminal diol is stabilized, but because the aldehyde form is destabilized as compared to, say, acetaldehyde (which also hydrates reversibly in acidic aqueous solution). The sp2 hybridized carbon in the aldehyde is very electron deficient, so it reacts with electron-rich nucleophiles (like water). Acetaldehyde, with a methyl group, slightly reduces the electron deficiency by "donating" some electron density. Trichloroacetaldehyde, however, cannot donate nearly as much electron density because the electronegative chlorine atoms are pulling electron density away from the already electron-deficient carbon, making it even more reactive.
Formaldehyde, like trichloroacetaldehyde, exists primarily as the diol form, and even oligomerizes and polymerizes with itself in the absence of water (1,3,5 trioxane and "paraformaldehyde"). Here the issue is not electronegativity (the hydrogen substituents are fairly electron-rich, and energetically favorable to share). Unfortunately, there is a symmetry mismatch in the orbitals involved, so it is impossible for the hydrogen substituents to ease the sp2 carbon's deficiency. Therefore, formaldehyde reacts with water, alcohols, proteins, even itself, to relieve the strain.
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Another note on the stability of chloral hydrate:
Under basic conditions it is not so stable, and will spontaneously decompose into formic acid (which is rapidly deprotonated under the reaction conditions) and chloroform (which may or may not be deprotoned, depending on pH)
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Thank you. I would like to ask you a follow up question. Why the carbon with two -OH groups is unstable?
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I think the main driving force is entropic. Fragmentation is generally favorable entropically. However, the reason this type of molecule fragments *more quickly* than other molecules has to do with the two oxygen atoms on the same carbon. These two play tug-of-war with the electrons. When they are balanced perfectly the molceule stays together, but as soon as an imbalance forms, one of the oxygen atoms steals the electron density and runs away. When there is a large concentration of oxygen with high electron density (water and alcoholic solvents) the reverse reaction also takes place, where the C=O is reacts to form -O–C–O-. This also happens *more quickly* than with other types of double bonds (like C=C and C=N), but it still needs a fair amount of heat or a catalyst (even a simple protic acid will work wonderfully, even in low concentrations)