June 01, 2020, 09:33:27 AM
Forum Rules: Read This Before Posting


Topic: Why does partial hydrogenation of unsaturated FA's create trans fats?  (Read 6476 times)

0 Members and 1 Guest are viewing this topic.

Offline chiddler

  • Regular Member
  • ***
  • Posts: 36
  • Mole Snacks: +2/-0
If it's easier than reading the title:

Why does partial hydrogenation of unsaturated fatty acids create trans fats?

Thanks very much!

Offline Archer

  • Chemist
  • Sr. Member
  • *
  • Posts: 1001
  • Mole Snacks: +85/-20
  • Gender: Male
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #1 on: August 08, 2013, 01:53:17 AM »
What is a trans fat?

How does this differ from an unsaturated fatty acid?
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4121
  • Mole Snacks: +256/-17
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #2 on: August 08, 2013, 09:39:00 AM »
@OP Do you know the chemistry (reactants and catalyst) of partial hydrogenation?

Offline chiddler

  • Regular Member
  • ***
  • Posts: 36
  • Mole Snacks: +2/-0
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #3 on: August 09, 2013, 10:19:47 AM »
What is a trans fat?

How does this differ from an unsaturated fatty acid?

A fatty acid is a long hydrocarbon chain with a carboxylic acid head. If it is unsaturated, then there are double bonds within this chain. The double bonds are either in cis or trans configuration. That is, the R groups of the double bond either face the same direction (cis) or opposite (trans).

Offline chiddler

  • Regular Member
  • ***
  • Posts: 36
  • Mole Snacks: +2/-0
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #4 on: August 09, 2013, 10:20:37 AM »
@OP Do you know the chemistry (reactants and catalyst) of partial hydrogenation?

Hmm..now that I think about it, I assumed that it is identical to standard catalytic hydrogenation. Meaning Pt catalyst and H2 gas.

If it's "partial", does this make it any different?

Thank you both for the replies. The help is appreciated.

Offline Archer

  • Chemist
  • Sr. Member
  • *
  • Posts: 1001
  • Mole Snacks: +85/-20
  • Gender: Male
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #5 on: August 09, 2013, 11:30:36 AM »
So if you fully hydrogenate under standard conditions you get a saturated fatty acid. The number of moles of H2 can be calculated very accurately.

What is the mechanism of this process?




“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4121
  • Mole Snacks: +256/-17
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #6 on: August 09, 2013, 11:59:13 AM »
@OP Do you know the chemistry (reactants and catalyst) of partial hydrogenation?

Hmm..now that I think about it, I assumed that it is identical to standard catalytic hydrogenation. Meaning Pt catalyst and H2 gas.

If it's "partial", does this make it any different?

Thank you both for the replies. The help is appreciated.
I read that nickel is commonly used as the catalyst and that 2-3 times more catalyst is needed for full than for partial hydrogenation.

Offline chiddler

  • Regular Member
  • ***
  • Posts: 36
  • Mole Snacks: +2/-0
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #7 on: August 10, 2013, 01:32:40 PM »
So if you fully hydrogenate under standard conditions you get a saturated fatty acid. The number of moles of H2 can be calculated very accurately.

What is the mechanism of this process?

I'm not sure I ever learned the mechanism in my ochem class.



BUT I JUST GOOGLED THIS! And it explains it! The double bond region must be in cis form for it to be attached to the catalyst. I'm guessing in partial hydrogenation, there's a chance that the double bond doesn't react with the hydrogen but does react with the catalyst.

Is that right?

Offline Archer

  • Chemist
  • Sr. Member
  • *
  • Posts: 1001
  • Mole Snacks: +85/-20
  • Gender: Male
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #8 on: August 10, 2013, 04:09:23 PM »
What if there is not enough hydrogen to react? Would the substrate remain adhered to the catalyst?
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline chiddler

  • Regular Member
  • ***
  • Posts: 36
  • Mole Snacks: +2/-0
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #9 on: August 11, 2013, 11:24:44 AM »
Looking at this again, the image actually doesn't explain it yet. Because this might allow cis fats but not trans. I mixed them up yesterday.

What if there is not enough hydrogen to react? Would the substrate remain adhered to the catalyst?

Hmm...not enough hydrogen. I don't know. I wouldn't think it remains adhered to the catalyst because I imagine the adhesion being a very transient reaction. Catalysts are not consumed so it only aids the formation of an intermediate which are definitely transient.

What does happen?

Offline Archer

  • Chemist
  • Sr. Member
  • *
  • Posts: 1001
  • Mole Snacks: +85/-20
  • Gender: Male
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #10 on: August 11, 2013, 02:26:12 PM »
The reactions on the catalyst surface are reversible all it is doing is weakening the bonds. Both cis and trans will react with it.

What if instead of the mechanism shown, the second hydrogen was not picked up? What if first hydrogen is lost and the alkene reforms? Can you draw the mechanism for that?
« Last Edit: August 11, 2013, 03:41:02 PM by Archer »
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline chiddler

  • Regular Member
  • ***
  • Posts: 36
  • Mole Snacks: +2/-0
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #11 on: August 12, 2013, 02:35:40 PM »
The reactions on the catalyst surface are reversible all it is doing is weakening the bonds. Both cis and trans will react with it.

What if instead of the mechanism shown, the second hydrogen was not picked up? What if first hydrogen is lost and the alkene reforms? Can you draw the mechanism for that?

Ok I drew it out for myself and it makes sense to me! Here's what I got (textually :():

1. After the hydrogen attaches to the first carbon, it becomes tetrahedral and allows free rotation. The two R groups can now be repelled.

2. The second hydrogen does not react. Double bond reforms and double bond is now trans. Hydrogen probably becomes H2 or reattaches onto the metal catalyst.

I drew it out on paper so I hope you're not bothered by a lack of image.

If i'm right, thanks very much! I understand it well now.

Offline Archer

  • Chemist
  • Sr. Member
  • *
  • Posts: 1001
  • Mole Snacks: +85/-20
  • Gender: Male
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #12 on: August 12, 2013, 02:49:42 PM »
That's a simplified version of events. What actually happens on the catalyst surface is complex and not completely understood (not by me at any rate).

Last question for you as you are nearly there, why would the trans isomer predominate? With your explanation you would get both isomers forming.

I shall give you a clue, draw cis oleic acid and draw trans oleic acid. Why would the trans isomer be favoured in a reversible reaction between the two?
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline chiddler

  • Regular Member
  • ***
  • Posts: 36
  • Mole Snacks: +2/-0
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #13 on: August 12, 2013, 08:28:48 PM »
That's a simplified version of events. What actually happens on the catalyst surface is complex and not completely understood (not by me at any rate).

Last question for you as you are nearly there, why would the trans isomer predominate? With your explanation you would get both isomers forming.

I shall give you a clue, draw cis oleic acid and draw trans oleic acid. Why would the trans isomer be favoured in a reversible reaction between the two?

I can see that cis is curved and trans is a straight chain. Oh it's because it reduces steric repulsion among other molecules when it is in trans, yes? Especially among other lipids.

Offline Archer

  • Chemist
  • Sr. Member
  • *
  • Posts: 1001
  • Mole Snacks: +85/-20
  • Gender: Male
Re: Why does partial hydrogenation of unsaturated FA's create trans fats?
« Reply #14 on: August 13, 2013, 01:59:05 AM »
On a single molecular level which would be more thermodynamically favoured?
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Sponsored Links