unsaturated vs saturated fats

[Hemichordate]

Which one has the higher melting point? Boiling point? Also, is the unsaturated one more stable?

---

Members don't see this ad.

 

[OhioDoc]

Which one has the higher melting point? Boiling point? Also, is the unsaturated one more stable?

Given two fatty acids of the same chain length differing only in their presence of a double bond, I would expect the following -

1. The trans isomer of the unsaturated fatty acid would have a similar bp and mp to the saturated fatty acid

2. The cis isomer of the unsaturated fatty acid would have a lower mp (can't pack quite as closely) but higher bp (larger dipole moment) than the trans unsaturated fatty acid and the saturated fatty acid

3. The saturated fatty acid would probably be more stable. In general, alkanes are very non-reactive whereas alkenes will often act as nucleophiles (electrophillic addition rxns)

Be aware that these are harsh generalization and are based upon fundamental organic chemistry principles.

 

[ThirdEye]

An easy way to remember this is to relate them to every day life.

Vegetable oils are unsaturated and liquid at room temperature.
Animal fats have a lot of saturated fatty acids in them and are solid at room temperature.

 

[matth87]

Saturated Fatty Acids have a higher heat of combustion too!

If you ever feel like combusting some fatty acids.

 

[wanderer]

Saturated Fatty Acids have a higher heat of combustion too!

If you ever feel like combusting some fatty acids.

Maybe I'm mixing up concepts but don't unsaturated compounds of the same length generally have higher heats of combustion?
In hydrogenating a double bond you're exchanging a C-c pi bond and a H-H sigma bond for 2 C-H sigma bonds, so hydrogenation is exothermic.
Also a fully saturated hydrocarbon can be more oxidized than an unsaturated hydrocarbon (which is already somewhat oxidized).

 

[matth87]

Maybe I'm mixing up concepts but don't unsaturated compounds of the same length generally have higher heats of combustion?
In hydrogenating a double bond you're exchanging a C-c pi bond and a H-H sigma bond for 2 C-H sigma bonds, so hydrogenation is exothermic.
Also a fully saturated hydrocarbon can be more oxidized than an unsaturated hydrocarbon (which is already somewhat oxidized).

I didn't think to much into, I just read the answer from EK #4 Organic Lecture Question #76.

 

[RogueUnicorn]

I didn't think to much into, I just read the answer from EK #4 Organic Lecture Question #76.

then it's probably wrong.

 

[matth87]

then it's probably wrong.

lol, the way I thought about it was that fully saturated FA are less "fluid" and more rigid unlike unsaturated fatty acids. I used that to reason they would have a higher heat of combustion but if you can find some information on if this is correct, then that would be awesome.

the EK explanation is very vague, just say that FA carry more energy then proteins or carbs.

 

[RogueUnicorn]

they do carry more energy than proteins or carbs. the "fluidity" has nothing to do with heats of combustion, which is just straight up enthalpy. let's take for a hypothetical example two 18C FAs, one monounsat (X) and the other fully sat (Y_. you can convert the X to saturated by hydrogenating it. this is an exothermic process. after this point, X and Y are indistinguishable and have the same heat of combustion. by hess's law, the heat of hydrogenation is added to the heat of combustion of the now saturated X to give the total, which is necessarily larger than that of Y, since, as we discussed, hydrogenation is exothermic.

 

[matth87]

General Formula for Combustion

CxHy + ((x+y)/4)O2 -> XCO2 + (y/2)H2O

If you run out of hydrogen you can combust anymore? You would run out quicker in a unsaturated FA compound. Every carbon add's to the total (delta)H released. Therefor if you can combust the last few carbons you would have a lower heat of combustion with a unsaturated compound.

 

[RogueUnicorn]

General Formula for Combustion

CxHy + ((x+y)/4)O2 -> XCO2 + (y/2)H2O

If you run out of hydrogen you can combust anymore? You would run out quicker in a unsaturated FA compound. Every carbon add's to the total (delta)H released. Therefor if you can combust the last few carbons you would have a lower heat of combustion with a unsaturated compound.

hydrogen is not a reactant and thus not something you can "run out of." i'm not certain your grasp of enthalpy is rock solid.

 

[matth87]

hydrogen is not a reactant and thus not something you can "run out of." i'm not certain your grasp of enthalpy is rock solid.

Ummmm there are Hydrogens in the alkane structure. The Alkane is a reactant.

C2H6 = saturated
C2H4 = unsaturated

Looking at the forumla now though it wouldn't affect the total carbons that are combusted to co2.

hmmmm...

 

[RogueUnicorn]

the hydrocarbon is the reactant, not the hydrogen. trust in what i am saying.

 

[RogueUnicorn]

i'll look this up more fully tomorrow and work out the math behind it have a concrete answer. the only way in which the saturated hydrocarbon of same length could have a greater heat of combustion is if the heat of formation of an additional water molecule is greater than the bond enthalpy of alkenes. i'll find out.

 

[wanderer]

Ummmm there are Hydrogens in the alkane structure. The Alkane is a reactant.

C2H6 = saturated
C2H4 = unsaturated

Looking at the forumla now though it wouldn't affect the total carbons that are combusted to co2.

hmmmm...

In the first molecule, the oxidation state of Carbon is -3, in the second molecule it is -2. Since in either case it ends up being +4 (In CO2), the first Carbon can transfer more electrons to oxygen and should therefore release more heat. (Edit: It actually doesn't)

 

[matth87]

In the first molecule, the oxidation state of Carbon is -3, in the second molecule it is -2. Since in either case it ends up being +4 (In CO2), the first Carbon can transfer more electrons to oxygen and should therefore release more heat.

Oxidation Rules state that if Hydrogen is Bound to carbon the oxidation state on H is +0 and the C is +0.

Your statement is false.

 

[wanderer]

Oxidation Rules state that if Hydrogen is Bound to carbon the oxidation state on H is +0 and the C is +0.

Your statement is false.

Really? What category of reactions does hydrogenation fall under?

 

[matth87]

Really? What category of reactions does hydrogenation fall under?

Def reduction.

This thread is pretty much owning everything that is written in my TPR book.

 

[RogueUnicorn]

ALKENE: Average C=C enthalpy is 614kJ/mol. Average C-C is 348. Thus, the pi bond requires an additional 266kJ of heat to break.

ALKANE: Average C-H: 413, Average O-H: 463. In alkanes, compared to alkenes, two more CH is broken to make two more OH. This means that the hydrogens contribute 100kJ of heat released.

This would mean with same C length, the alkane will release more energy when combusted and thus have a greater heat of combustion.

 

[matth87]

ALKENE: Average C=C enthalpy is 614kJ/mol. Average C-C is 348. Thus, the pi bond requires an additional 266kJ of heat to break.

ALKANE: Average C-H: 413, Average O-H: 463. In alkanes, compared to alkenes, two more CH is broken to make two more OH. This means that the hydrogens contribute 100kJ of heat released.

This would mean with same C length, the alkane will release more energy when combusted and thus have a greater heat of combustion.

Your my hero bleargh, thanks for finding this info out!

 

[wanderer]

ALKENE: Average C=C enthalpy is 614kJ/mol. Average C-C is 348. Thus, the pi bond requires an additional 266kJ of heat to break.

ALKANE: Average C-H: 413, Average O-H: 463. In alkanes, compared to alkenes, two more CH is broken to make two more OH. This means that the hydrogens contribute 100kJ of heat released.

This would mean with same C length, the alkane will release more energy when combusted and thus have a greater heat of combustion.

You also have to factor in the .5 extra O=O broken.

 

[RogueUnicorn]

You also have to factor in the .5 extra O=O broken.

you are correct, apologies for the omission, it's been a long day. considering the O2 bond enthalpy is ~500kJ/mol, this would mean that an eneryg input of 150 is required for the breaking of CH bonds to form H2O. the math, however, is relatively moot to start with - C=C is more stable than C-C, which would mean less heat is released in an exothermic process. i don't know why i couldn't see this before.

 

[inaccensa]

you are correct, apologies for the omission, it's been a long day. considering the O2 bond enthalpy is ~500kJ/mol, this would mean that an eneryg input of 150 is required for the breaking of CH bonds to form H2O. the math, however, is relatively moot to start with - C=C is more stable than C-C, which would mean less heat is released in an exothermic process. i don't know why i couldn't see this before.

C=C is less stable than C-C, right? The pi bond is weaker than the sigma bond. It decreases the bond length and thus there is more electron-electron repulsion energy. Therefore the energy of an alkene is higher than the energy of an alkane. This implies that alkane is more stable than alkene and releases less energy. please correct me, if I'm thinking this wrong.

 

[wanderer]

C=C is less stable than C-C, right? The pi bond is weaker than the sigma bond. It decreases the bond length and thus there is more electron-electron repulsion energy. Therefore the energy of an alkene is higher than the energy of an alkane. This implies that alkane is more stable than alkene and releases less energy. please correct me, if I'm thinking this wrong.

C=C is more stable than C-C (you should know this by now). A pi bond is weaker than a sigma bond, but pi +sigma (which is what a C=C bond consists of) is more stable than sigma alone.

Alkane is more stable, but because the entire molecule is more stable than an alkene, not because of the carbon-carbon bond. ( C=C + H2 ---> HC-CH is exothermic)