[Urbanium]

Hello, I have a question regarding the definition of conjugation.

I had a discussion with one postdoc at my University about farnesol and farnesyl acetate. For some reason I can not see the TLC spot of farnesyl acetate under UV light, while the spot for farnesol is easily detectable. The postdoc I was discussing with told me that I should bear in mind that farnesol is a conjugated system, while farnesyl acetate isn't.

That really puzzled me, since all the double bonds of the carbon chain remain the same as they were before the conversion of farnesol to farnesyl acetate? Why and how does simple conversion of hydroxy group to alkyl ester interrupt the conjugation??

What would then be a simple and understandable criteria for conjugation?

[Dan]

That really puzzled me, since all the double bonds of the carbon chain remain the same as they were before the conversion of farnesol to farnesyl acetate? Why and how does simple conversion of hydroxy group to alkyl ester interrupt the conjugation??

Your reasoning is sound, the posdoc made a mistake.

[AlphaScent]

Dan is right.  Your post-doc is wrong.  Neither one of those compounds are conjugated.  If I were you I would use a 20% solution phosphomolybdic acid in ethanol for TLC stain.  It is a great universal stain.  I do natural products synthesis for my job and use this stain every time; never fails. 

http://www.sigmaaldrich.com/catalog/product/sial/319279?lang=en&region=USftp://

[Urbanium]

Dan is right.  Your post-doc is wrong.  Neither one of those compounds are conjugated.  If I were you I would use a 20% solution phosphomolybdic acid in ethanol for TLC stain.  It is a great universal stain.  I do natural products synthesis for my job and use this stain every time; never fails.

Thanks for the answer.

I actually used that stain. However, the point of my question is in your answer. WHY neither of these compounds are conjugated, given all of these double bonds??

[spirochete]

Dan is right.  Your post-doc is wrong.  Neither one of those compounds are conjugated.  If I were you I would use a 20% solution phosphomolybdic acid in ethanol for TLC stain.  It is a great universal stain.  I do natural products synthesis for my job and use this stain every time; never fails.

Thanks for the answer.

I actually used that stain. However, the point of my question is in your answer. WHY neither of these compounds are conjugated, given all of these double bonds??

Wow. What is conjugation?

[AlphaScent]

A) I am surprised you used UV, heat works every time.

B) I have both of these compounds in my lab and both stain nicely.  Tried it myself.

C) Look at the structure of both and then google conjugation of organic compounds, and you will have your answer.



Thanks for the chuckle spirochete.

[Urbanium]

A) I am surprised you used UV, heat works every time.

B) I have both of these compounds in my lab and both stain nicely.  Tried it myself.

C) Look at the structure of both and then google conjugation of organic compounds, and you will have your answer.



Thanks for the chuckle spirochete.

A), B) after I realized that UV light was useless, I tried to stain the plates with KMnO4, but the same postdoc suggested me to use the phosphomolybdate, and it works great

C) I googled the term, however I still do not understand one thing: why (in the case of chain compounds with more double bonds) only one sp3 carbon inserted between these double bonds is enought to break the conjugation? Shouldn't there be some possibility of at least weaker overlap between HOMO orbitals, in the cases where only one sp3 carbon is between them?

[Corribus]

C) I googled the term, however I still do not understand one thing: why (in the case of chain compounds with more double bonds) only one sp3 carbon inserted between these double bonds is enought to break the conjugation? Shouldn't there be some possibility of at least weaker overlap between HOMO orbitals, in the cases where only one sp3 carbon is between them?

Technically, yes, there is overlap between nonadjacent carbon centers, no matter how far apart (in the LCAO-MO treatment, the AO wavefunction extends to infinity, though it drops of exponentially) but it is incredibly small.  This is (part of) the basis of the Huckel approximation, which assumes that overlap integrals of non-adjacent carbon centers are zero.  The tetrahedral geometry at the sp3 carbon center certainly doesn't make interaction any more favorable, either.  So you've got extermely unfavorable distance and geometry working against you.  In essence, there is no interaction at all.

[spirochete]

C) I googled the term, however I still do not understand one thing: why (in the case of chain compounds with more double bonds) only one sp3 carbon inserted between these double bonds is enought to break the conjugation? Shouldn't there be some possibility of at least weaker overlap between HOMO orbitals, in the cases where only one sp3 carbon is between them?

Wow. You are a much more advanced student then I expected from your original question!