Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: lepidopter on 01/06/2005 21:30:02

Title: Why Ylide species are colored ?
Post by: lepidopter on 01/06/2005 21:30:02

I wanted to know where the color comes from in YLIDE species, I have found no clue on the net. Is this due to interaction between negative charge and the sigma star ??

So, if somebody has an answer or a site talking about this fact, please let me know

Thanks a lot.

Title: Re: Why Ylide species are colored ?
Post by: anthony on 03/06/2005 08:06:11

Wikipedia says:
ylide

A ylide is a neutral molecule with positive and negative charges on adjacent atoms.

The most common ylides are used in the Wittig reaction for double bond synthesis from carbonyl groups (C=O). The positive charge is carried by a phosphorus atom with three phenyl substituents and one bond to a carbon bearing a negative charge and two substituents, commonly alkyl groups.

Ylides can be 'stabilised' and 'non-stabilised', non stabilised react readily with both aldehydes and ketones whereas stabilised will only react with aldehydes.

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Colours are created by electronic transitions of the correct energy in which an electron is promoted to a higher energy level. This is gernally from the HOMO (Highest Occupied Molecular Orbital) to the LUMO (Lowest Unoccupied Molecular Orbital). In an ylide these are generally where the negative and positive charges are respectively located. Though positioning a charge on a molecule is can be dangerous. Your instinct in this case has lead you to more-or-less the correct place. But not all ylides are coloured, or all coloured molecules ylides.

Title: Re: Why Ylide species are colored ?
Post by: lepidopter on 03/06/2005 19:54:25

Ok thanks a lot, so for example in my case (Non-Stabilized ylides), the colour should be due to electronic transition between the HOMO (of the C negatively charged) and the LUMO (of the P+ adjacent) I think