Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: iScience on August 12, 2013, 11:28:57 AM
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This comes down to the question "what is responsible for the chemical difference between sulfur and oxygen?" and i don't know the answer to this. I can see why they would be similar, but i do not see why they are not exactly the same. The only difference i can think of between the two are the sizes. But this doesn't have much effect ....does it?
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Size has a lot of effect. Sulfur and oxygen bond every differently. The valence electrons of sulfur are diffuse, those of oxygen not so much. The affects everything from the ease of oxidation and reduction to the fact that sulfur tends to form sigma bonds only (except in cases of expanded octets) and oxygen having more versatility (single and double bonds prevalent).
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The increased size/decreased electronegativity of sulfur also make thiols better nuceleophiles than alcohols. It again comes down to its diffuse nature - it can "give away" its electrons to form sigma bonds much more easily than oxygen.
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one of the most important features of "-SH vs. -OH" in my opinion is, that thiols can't form any hydrogen bridges worthy to mention (as you can learn from, for example, the boiling points comparison H2S/H2O and CH3SH / CH3OH , for example)
regards
Ingo
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There is a large difference in pKa values between the two, also.
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I suppose that the salient difference between the oxygen and sulfur atom in organic chemistry is the nature of the carbon oxygen vs. carbon sulfur bond (both single and double). Both of them are covalents, but the first is a polar covalent bond (see the difference in electronegativities). The latter is more interesting: although there is no big electronegativity difference, the weaker orbital overlap causes that e.g. 1, the thioamide carbon atom is more electrophilic than the amide and 2, ester carbon atom is less electrophilic than the thioester carbon atom. As others mentioned earlier the electron structure is diffuse around the sulfur which makes a thioalkoxide a ‘soft’ base whilst an alkoxide is a hard base. So it can be attacked easier by soft alkylating agents such as alkyl-iodides.