Amirg3
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Hay guys,
Why the more a fatty acid has double c=c bounds the more it’s soluble in water.
Is solubility related to number of Cis enantiomer?
Looking toward your answers, thanks ❤️
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Sumi Shanks
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The greater the number of carbons or carbon-carbon bonds the lower the solubility

The greater the number of COOH the more the solubility due to H-bonds
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Amirg3
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(Original post by Sumi Shanks)
The greater the number of carbons or carbon-carbon bonds the lower the solubility

The greater the number of COOH the more the solubility due to H-bonds
I think something is misunderstood, I meant why a fatty acid with more c=c in comparison with another one with the same number of carbons but less c=c is more soluble? But thanks 🙏
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ALikesGeetars
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Hi! I may have an idea, it is to do with something called Contact Surface Area. An unsaturated fatty acid has C=C bonds, the more C=C bonds there are in a fatty acid molecule, the more 'kinks' or 'bends' are present in the molecule. A molecule with less C=C bonds will be a straighter chain whereas a molecule with more C=C bonds will be more bent. Contact surface area refers to the area that is exposed allowing for intermolecular forces to form. A straighter chain has a greater contact surface area allowing for more intermolecular forces to form (which may increase solubility), whereas a more bent chain will have a lower contact surface area so less intermolecular forces can form. I hope this helps in someway (I'm not a Biochemistry student, but a Biomed student on a gap year still doing some studying when I get the chance, feel free to message me if you need anything and we can even be friends if you like )
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Amirg3
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(Original post by ALikesGeetars)
Hi! I may have an idea, it is to do with something called Contact Surface Area. An unsaturated fatty acid has C=C bonds, the more C=C bonds there are in a fatty acid molecule, the more 'kinks' or 'bends' are present in the molecule. A molecule with less C=C bonds will be a straighter chain whereas a molecule with more C=C bonds will be more bent. Contact surface area refers to the area that is exposed allowing for intermolecular forces to form. A straighter chain has a greater contact surface area allowing for more intermolecular forces to form (which may increase solubility), whereas a more bent chain will have a lower contact surface area so less intermolecular forces can form. I hope this helps in someway (I'm not a Biochemistry student, but a Biomed student on a gap year still doing some studying when I get the chance, feel free to message me if you need anything and we can even be friends if you like )
Thanks it was quite helpful 👌
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Wuxinyu
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(Original post by Amirg3)
Hay guys,
Why the more a fatty acid has double c=c bounds the more it’s soluble in water.
Is solubility related to number of Cis enantiomer?
Looking toward your answers, tha
let talk about solubility first,
solubility is related to the likehood to form hydrogen bonding and i.e., the ability to break the original intermolecular forces between ionic compounds and the ability to reform a hydrogen bond between the lp of highly electronegative atom and H.
a longer carbon chain is less likely to be dissolved in water and a carbon chain with more side chains is less likely to be dissolved in water, as the polar groups are kind of being covered by those things.
for your question, we can solve it by looking at sth in bio, a fatty acid chain with many double bonds, we refers them as UNSATURATED fatty acids, they are insoluble in water but soluble in oil, for those with no double bonds , they are SATURATED fatty acids, as the number of carbon atom increases, solubility in water decreases as the molecule be less polar when carbon atom accumulates, and in this case the solubility in oil increases.
you mentioned the double bonds, so they are unsaturated fatty acids and I personally think they are not soluble in water or if more accurately, they have tiny solubility in water. and a chain with more double bonds tend to be less soluble in water as they are regarded as more unsaturated compare with those with same c numbers but no double bonds.
Last edited by Wuxinyu; 2 weeks ago
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Wuxinyu
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(Original post by Amirg3)
Hay guys,
Why the more a fatty acid has double c=c bounds the more it’s soluble in water.
Is solubility related to number of Cis enantiomer?
Looking toward your answers, thanks ❤️
I think there is sth wrong with this statement, more double bonds should be less soluble. but I think there is no need to find out sth about the contacting areas,,,,,,,you know the h bond forms is nothing related to carbon,,,,it is H in the carbon chains. and refer to that even more carbon double bonds means less H ,right?are less likely to form H bonding, so that less soluble in water, although this way seems a bit silly. but it makes sense in some way.
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Amirg3
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(Original post by Wuxinyu)
let talk about solubility first,
solubility is related to the likehood to form hydrogen bonding and i.e., the ability to break the original intermolecular forces between ionic compounds and the ability to reform a hydrogen bond between the lp of highly electronegative atom and H.
a longer carbon chain is less likely to be dissolved in water and a carbon chain with more side chains is less likely to be dissolved in water, as the polar groups are kind of being covered by those things.
for your question, we can solve it by looking at sth in bio, a fatty acid chain with many double bonds, we refers them as UNSATURATED fatty acids, they are insoluble in water but soluble in oil, for those with no double bonds , they are SATURATED fatty acids, as the number of carbon atom increases, solubility in water decreases as the molecule be less polar when carbon atom accumulates, and in this case the solubility in oil increases.
you mentioned the double bonds, so they are unsaturated fatty acids and I personally think they are not soluble in water or if more accurately, they have tiny solubility in water. and a chain with more double bonds tend to be less soluble in water as they are regarded as more unsaturated compare with those with same c numbers but no double bonds.

“The longer the fatty acyl chain and
the fewer the double bonds, the lower is the solubility in water...” that’s a statement directly from Lehninger Biochemistry principles. When I read it the question popped up in my mind. But you said unsaturated is not soluble in water in your comment, I think it seems wrong because the book doesn’t say that.
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Wuxinyu
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(Original post by Amirg3)
“The longer the fatty acyl chain and
the fewer the double bonds, the lower is the solubility in water...” that’s a statement directly from Lehninger Biochemistry principles. When I read it the question popped up in my mind. But you said unsaturated is not soluble in water in your comment, I think it seems wrong because the book doesn’t say that.
that statement is exactly what I said , and I had given some explanations in my comments.
sorry I did not state clearly about the unsaturated fatty acid ' solubility, It do dissolve in water in some extent,but just the ability is very poor. you know all fatty acids tend to dissolve more in oil. as fatty acid are non polar and water is polar so they may hardly be mixed. but if you have to compare the solubility of saturated fatty acids with unsaturated ones, that is what you truly asked in your question. then saturated fatty acids are more soluble, although in general, they all have poor solubility as they are mostly non polar
by the way, where did you find this statement? is that in your textbook? I am a student who study IAL AS and I have met these sorts of things in our bio textbook.
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Amirg3
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(Original post by Wuxinyu)
that statement is exactly what I said , and I had given some explanations in my comments.
sorry I did not state clearly about the unsaturated fatty acid ' solubility, It do dissolve in water in some extent,but just the ability is very poor. you know all fatty acids tend to dissolve more in oil. as fatty acid are non polar and water is polar so they may hardly be mixed. but if you have to compare the solubility of saturated fatty acids with unsaturated ones, that is what you truly asked in your question. then saturated fatty acids are more soluble, although in general, they all have poor solubility as they are mostly non polar
by the way, where did you find this statement? is that in your textbook? I am a student who study IAL AS and I have met these sorts of things in our bio textbook.
Aha that’s right, thanks 🙏
But one thing still is left bro, I asked about the reason of direct relation between having double bond and solubility. Why having more double bonds means being more soluble even though that’s unsaturated and not quite soluble.
I’m a medicine student 1st yr. we’ve got a biochemistry book which is written based on three references however a major part of it comes from Lehninger.
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Wuxinyu
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(Original post by Amirg3)
Aha that’s right, thanks 🙏
But one thing still is left bro, I asked about the reason of direct relation between having double bond and solubility. Why having more double bonds means being more soluble even though that’s unsaturated and not quite soluble.
I’m a medicine student 1st yr. we’ve got a biochemistry book which is written based on three references however a major part of it
OOPS! I think I misunderstood sth, sorry, first , saturated or unsaturated acids are not distinguished as their solubility in water! they are all INSOLUBLE and they are differs as they have different amount of hydrogen or hydrogen to carbon ratio, and saturated and unsaturated is nothing about the solubility. they are about the stability and bp.
I personally think when fatty acyl dissolves in water, this undergos hydrolysis. when their are more double bonds the OH- nucleophile is more likely to attack the carbon atom in the fatty acyl, as the electrons might move by delocalisation. so charge density decreases and the compound formed after hydrolysis is more stable. so from more products formed we can deduct that this reaction is more likely to occur and the acyl is more soluble consequently.

I have no idea if my deduction is right or not...but hopefully it will help u, sorry if my previous commments confused u as some points are incorrect.
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Amirg3
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Thanks anyway it was useful 🙏
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Sumi Shanks
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(Original post by Amirg3)
I think something is misunderstood, I meant why a fatty acid with more c=c in comparison with another one with the same number of carbons but less c=c is more soluble? But thanks 🙏
Mostly to do with no H bonds being formed with C so they are non polar and insoluble
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