Isomerization of Fatty Acid Tails

[CTAATGT]

How does the isomerization of an unsaturated tail of a triacylglycerol during saponification result in an extra fatty acid salt?

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[Bruinsfinalfour]

How does the isomerization of an unsaturated tail of a triacylglycerol during saponification result in an extra fatty acid salt?

Each glycerol is attached to 3 fatty acid tails in a triacylglycerol (Lets say 2 are fully saturated and one is unsaturated). A percentage (lets say 50%) of the unsaturated fatty acid tail can isomerize at the double bond position (trans to cis or vice versa). This leaves you with the 2 saturated fatty acid salts and .5 trans unsaturated fatty acid salt and .5 cis unsaturated fatty acid salt. You now have 4 different types of fatty acid salts

 

[CTAATGT]

Each glycerol is attached to 3 fatty acid tails in a triacylglycerol (Lets say 2 are fully saturated and one is unsaturated). A percentage (lets say 50%) of the unsaturated fatty acid tail can isomerize at the double bond position (trans to cis or vice versa). This leaves you with the 2 saturated fatty acid salts and .5 trans unsaturated fatty acid salt and .5 cis unsaturated fatty acid salt. You now have 4 different types of fatty acid salts

But to be a fatty acid salt, doesn't an OH need to perform an addition-elimination at a carbonyl, producing an R-COOH fatty acid? This R-COOH fatty acid then deprotonates and then becomes a true fatty acid salt, R-COO-Na+, right? I can't see how that can happen at the location of alkene isomerization because it doesn't have a carbonyl functional group.

 

[Bruinsfinalfour]

But to be a fatty acid salt, doesn't an OH need to perform an addition-elimination at a carbonyl, producing an R-COOH fatty acid? This R-COOH fatty acid then deprotonates and then becomes a true fatty acid salt, R-COO-Na+, right? I can't see how that can happen at the location of alkene isomerization because it doesn't have a carbonyl functional group.

The COO-Na+ section of all 4 fatty acids salts are the same. Its the R section that differs in all 4 fatty acids which leads to 4 different fatty acid salts
Think of it like this: you have R'COO-Na+ , R''COO-Na+ , R'''COO-Na+ , R''''COO-Na+ (where R' and R'' are isomers of each other)

 

[boaz]

How does the isomerization of an unsaturated tail of a triacylglycerol during saponification result in an extra fatty acid salt?

If you recall, one of the first reactions taught in organic chemistry is acid-catalyzed hydration of alkenes. In an unsaturated fatty acid there's a double bond somewhere in the chain and it can be either cis or trans. In acid the double bodn will get hydrated. But since this reaction is readily reversible it will form a double bond again-either cis or trans irrelevant of what it was before.

 

[CTAATGT]

The COO-Na+ section of all 4 fatty acids salts are the same. Its the R section that differs in all 4 fatty acids which leads to 4 different fatty acid salts
Think of it like this: you have R'COO-Na+ , R''COO-Na+ , R'''COO-Na+ , R''''COO-Na+ (where R' and R'' are isomers of each other)

Oh! I know where my confusion stemmed from. I've always thought that ONE single triacylglycerol molecule individually gives rise to FOUR different fatty acid salts. A triacylglycerol only has three tails. Because of this, I keep thinking that a carboxylic acid has to somehow insert itself in the middle of an unsaturated tail where the alkene is. Only then will one single triacylglycerol gives rise to four different tails. But now I realize that in a mixture with many triacylglycerol molecules, a few them would have unsaturated tails. Thus, in a mixture of many triacylglycerols, there can be up to four different tails. I hope my reasoning is correct.

 

[EFDoc]

I just made the same mistake on one of the AAMC practice tests! Makes perfect sense. Got to think macroscopically.

 

[wavvvvvves123]

omg, i'm doing the AAMC Chemistry Question Pack right now and this exact question came up! I was confused for the exact same reasons as OP; it was awesome to read this explanation haha