Putting O-Chem to Use
Despite my criticism of Penk's decision to teach organic chemistry to high school students that really don't have the sufficient background to truly learn it, I finally now understand trans fats. Monounsaturated and polyunsaturated (the healthier ones) fats have a number of double bonds in the fatty acids, whose hydrogens are in a cis configuration. The partial hydrogenation process (infusing hot hydrogen gas through the fat in the presence of a catalyst) adds hydrogens to the chains by dismantling some of the double bonds, and it also converts the cis configuration to the trans configuration (hence the name, I guess). Somehow (which I don't entirely understand), this makes them able to pack closer together and have a lower melting point. I think that since there are only two hydrogens attached to the double bonded carbons instead of four with a single bond, there is a higher degree of mobility in the bond. This additional mobility, I presume, means that the molecules are more irregular and can't pack as closely. Hence, they have the lower melting point. A basic, but informative site on the structure of various fats:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Fats.html
Adam, Unsaturated fats are liquid at room temp while saturated ones are solid. You mixed them up. Unsaturated fats have lower melting points, while saturated fats have higher. Packing the bonds close together makes them harder to break, so the melting point is higher; and loose bonds are easy to break.
But trans fats have a higher melting point than unsaturated fats. That's their point: closer to the consistency of animal fats (solid at room temperature), but made from vegetable oil.
Oops! You're right. I mixed up my 'higher' and 'lower.'
Ah, chemistry! This is really fun. :) It's going to be a little hard explaining without molecular diagrams, but I'll try. I'll assume that you know molecular structure theory pretty well.
The heart of this matter really lies in Valence Bond Theory, which has three postulates:
1. A bond is a result of overlapping atomic orbitals.
2. Two electrons with opposite spins are shared in the overlapping orbitals.
3. The greater the overlap, the stronger the bond.
So picture a model of H2, of which H has an electronic configuration of 1s1. When the 2 H's form H2, the resulting bond is a sigma bond, which has electron density on the bond axis (a line between the two nuclei).
*Single bonds are always sigma bonds.*
Then hybridization comes into play in VB theory. Basically the wave equations describing atomic orbitals are combined mathematically to produce an equal number of wave equations describing hybridized atomic orbitals--the properties of the hybridized orbitals are intermediate between those of the parent orbitals. Hybridization diagrams predit lone pairs and bond types. Sigma bonds are a result of hybridized electrons. However, side-to-side overlap of unhybridized p orbitals result in pi bonds. A pi bond has electron density above and below, but not on the bond axis.
*A double bond is one sigma and one pi bond.*
So getting to your topic finally:
There is no free rotation around double bonds because too much energy is required to break the pi bond. C=C double bonds are very reactive, by the way, since the pi bonding reacts in the x-y plane. However, there is free rotation around
single bonds.
They teach this in biology.
Also, I'm not sure if you're right about the bond mobiliy (you may be, however). The molecules can't pack as uniformly because of the kinks caused by the double bonds.
You guys really wasted your break. Maxwell and I stayed up til around noon today playing bridge, scrabble, and poker.
"Mobility" was really the wrong choice of words. I was aware that pi bonds can't rotate because of their higher energy, but I was referring to the bending or kinking that occurs at double bonds. The fact that C=Cs can't rotate is probably one of the reasons that they don't pack as densely and therefore have a lower (I think I got it right this time) melting point. Shen, you note that C=C is very reactive. Is that the reason why unsaturated fats are more healthy? (i.e. since they are more reactive they are more readily metabolized, and somehow healthier) That doesn't really make much sense though, when compared with the nutrition of carbohydrates. Glucose is not as "healthy" as more complex carbohydrates, yet is more readily metabolized. But I suppose that that has little to do with the actual digestion process, and a lot to do with how high glucose levels in the blood are handled by other organs. Since fats are so large, the rate of digestion probably isn't an issue.
You can tell that physics and math are really the only sciences I have ever been good at : )
Nope, wrong again. Unsaturated fats (with double bonds) have lower melting points.