Question To Those With Good Knowledge Of Biochemistry Re PUFA

[Elie]

When i studied chemistry, the double bond was presented as a more stable bond than a single bond. Based on this PUFA should be more stable than SFA. However, it is explained that the double bonds in PUFA (an extra electon?) is more reactive than the single bonds in SFA which happily bind carbons with hydrogens. How do you reconcile these two different explanations / understandings?

 

[Such_Saturation]

SFA are double bonds. PUFA is two double bonds, which are really bent and unstable

 

[Elie]

SFA's bonds are single if I am not mistaken Single bond - Wikipedia

 

[Westside PUFAs]

"Just about everything that goes wrong involves FFA increase. If they are totally saturated fatty acids, such as from coconut oil and butter, those are less harmful, but they still tend to shift the mitochondrial cellular metabolism away from using glucose and fructose and turning on various stress related things; By lowering the carbon dioxide production I think is the main mechanism."-RP

 

[Elie]

Thanks. I understand that.
My question is meant to reconcile the commonly held belief in chemistry that double bonds (as the ones that occur in PUFA) are stronger and more stable than single bonds (as in SFA) with what we are taught by Peat, that PUFA (despite the presence of the supposed strong double bonds) is more reactive than the stable SFA.
If a double bond is supposedly stronger why PUFA more reactive than SFA?

 

[Such_Saturation]

Thanks. I understand that.
My question is meant to reconcile the commonly held belief in chemistry that double bonds (as the ones that occur in PUFA) are stronger and more stable than single bonds (as in SFA) with what we are taught by Peat, that PUFA (despite the presence of the supposed strong double bonds) is more reactive than the stable SFA.
If a double bond is supposedly stronger why PUFA more reactive than SFA?

Yes my bad I was thinking of two electrons as opposed to ionic bond "one electron". It seems that the double bond itself has more strength, but it makes the single bonds next to it more vulnerable in some way, and the hydrogen can be removed more easily which starts the peroxidation.

 

[Elie]

Oh? Its the hudtogen next to it that is implicated in peroxidation? Not the electrons from the double bond?
Mmm....

 

[Travis]

When i studied chemistry, the double bond was presented as a more stable bond than a single bond. Based on this PUFA should be more stable than SFA. However, it is explained that the double bonds in PUFA (an extra electon?) is more reactive than the single bonds in SFA which happily bind carbons with hydrogens. How do you reconcile these two different explanations / understandings?

The double bonds have a higher energy, and can be seen as more stable in that way. If you look at a table of bond energies, and you will see that a C–C single bond has X amount of joules per mole; you will also notice that the C=C double bond has roughly 1.77 that energy—the C≡C triple bond having less than 3× the single.

The carbon–carbon quadruple bond exists only in the imagination of whimsical schoolchildren.

But these extra bonds are not σ‐bonds; the extra bonds are π‐bonds. This means that they are more reactive and have a more pervasive distribution. On paper, in chemical line depictions, the double bond looks like two single bonds; this is not so; this is unrealistic. To get an idea of what the π‐bond looks like, you have to see images of more elaborate depictions.

The electrons of the π‐bond will add to molecular oxygen, while the electrons of the σ‐bond will not. The σ‐bond is actually stronger, remember, as your table of bond energies shows it has more energy: the double bond (~614·kJ·mole⁻¹) is the energy of the σ‐bond (~347·kJ·mole⁻¹) plus that of the π‐bond (~267·kJ·mole⁻¹).

So reactivity and bond energy are not the same thing. A higher total bond energy does not mean lower reactivity. If this was so, then than cyanide (C≡N) would be safer than methylamine (C–N).

Any confusion, and apparent paradox, likely stems from thinking that O₂ adds to the double bond—it doesn't! Molecular oxygen adds to ¹⁄₂ the double bond (as depicted on paper): the π‐bond aspect. Pi‐bond electrons represent much more mobile electrons which exist in resonance disruptable by UV light, sending one of the electrons into an anti‐bonding orbital. Electrons constituting the σ‐bond are fundamentally different, although they are still electrons. A detailed explanation why this is so can perhaps be found in a book on quantum chemistry; Linus Pauling has authored one such book:

Pauling, Linus, and E. Bright Wilson. Introduction to quantum mechanics with applications to chemistry. Courier Corporation, 2012.​

Linus Pauling also has a book on general chemistry. This books tells you the chemical identity of the green pigment found American paper currency (trivia: it's chromium III oxide).

Pauling, Linus. General chemistry. Courier Corporation, 1988.​

 

[Elie]

Brilliant. Thank you.

 

[Miso]

Double bonds are stronger at ~146kcal/mol vs Single bond at ~83kcal/mol; when you react a PUFA double bond, you are breaking the weakest bond of the two (pi bond), which takes less energy (146-83 = ~63kcal/mol), versus breaking the single remaining (sigma) bond (~83kcal/mol).

 

[Elie]

makes sense