What dissolves fats and oils
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Structure of fats
Fats, also called lipids, are so-called triacylglycerols. These are molecules that result from the esterification of all hydroxyl groups of the glycerine molecule (propane-1,2,3-triol). The carboxylic acids required for this are called fatty acids.
Fats can be either solid or liquid at room temperature. The solid triacylglycerols are called fats, the liquid fatty oils (they are different from essential oils and mineral oils). The physical state depends on the fatty acids bound in the fats.
Fatty acids are long-chain, unbranched carboxylic acids with an even number of carbon atoms. There are saturated fatty acids without multiple bonds and unsaturated fatty acids with non-conjugated double bonds in the Z configuration.
In the table you will find a selection of some of the most important fatty acids:
Manufacture of fats
Fats are produced in an esterification with glycerine and three fatty acids. The fats can contain only one type or different fatty acids. (Mechanism see chapter 4.5 condensation reaction)
Properties of fats
When you heat fats, they slowly soften and then gradually melt. So there is no exact melting point, but a melting range. This is due to the fact that fats are usually not pure substances, but a mixture of different triacylglycerols, each with different melting points.
Whether a triacylglycerol is a solid fat or a fatty oil depends on which fatty acids are bound in the molecule. With solid fats, the intermolecular interactions are higher than with fatty oils. The only interactions that can occur here are van der Waals interactions. The decision rules that are already known apply again, which means the following in particular:
- The longer the fatty acid, the higher the van der Waals forces.
- The more double bonds there are in the fatty acid, the lower the van der Waals forces are, since kinks are created by double bonds and the fat molecule becomes more bulky.
So if saturated fatty acids are bound in the fat molecule for a long time, it is a solid fat. If, on the other hand, fatty acids with many double bonds are bound, it is a fatty oil.
Solubility and density of fats
Fats cannot be dissolved in water because they are non-polar. This is due to the long, non-polar chains of fatty acids.
Since fats always float on water, the density of fats must be lower than that of water. To explain this connection, we have to look again at the molecular structure. The only intermolecular interactions that occur between fat molecules are van der Waals forces. Water molecules, on the other hand, can form the very strong hydrogen bonds. The intermolecular interactions of the water are significantly stronger than those of the fat molecules, which means that the water molecules attract each other more strongly and are therefore closer together. This in turn means that the density of water is higher than that of fats.
Importance of fats for humans
Fats are very important for humans because, among other things, they serve as a source of energy. Therefore, the body also uses fats for long-term storage of energy.
Furthermore, unsaturated fatty acids are important for growth, the structure of cell membranes, etc. However, they cannot be produced by the body itself, which means that these fatty acids are essential. Therefore, the corresponding fats have to be absorbed, which can then be broken down into the respective unsaturated fatty acids.
The consumption of fats becomes particularly important when we eat foods that contain vitamins A, D, or E. These are fat-soluble vitamins. If these are consumed without fats, they cannot be absorbed by the body and are excreted undigested.
Since fats are high in energy, they can also be used as fuel. If the fats for this are obtained from plants (mostly rapeseed), they are renewable raw materials. Rapeseed oil itself is too thick for engines, however. Therefore it is converted into biodiesel in a chemical reaction. For this purpose, rapeseed oil is reacted with methanol, producing biodiesel and glycerine. This type of reaction is called transesterification.
This reaction is a nucleophilic substitution in which the free electron pair of the hydroxyl group on the positively polarized carbon atom of the ester first attacks.
The resulting positive charge on the oxygen atom must be balanced. At first glance, it seems obvious that this can be balanced out by splitting off a proton and taking it up through a free electron pair on the negatively charged oxygen atom. Then all charges would be balanced. But this is where another basic principle comes into play for the first time: a maximum of two oxygen atoms can be bound to one carbon atom. So here we have to break one of the single bonds between the carbon atom and an oxygen atom. The only one that comes into question here is the single bond between the carbon atom and the oxygen atom to the left (if we break the one to the right, we would get back to the beginning, and if we break the upper one, a very unstable O ^ (2 -) - ion arise). So that this single bond can be broken more easily, a rearrangement of the proton from the right oxygen atom to the left takes place. Due to the resulting positive charge, the oxygen atom no longer wants to share the single bond and it can be broken more easily.
This reaction takes place on all three ester groups, whereby three bio-diesel molecules are formed from one rapeseed oil molecule.
The advantage of biodiesel is that the carbon dioxide produced during combustion corresponds exactly to the amount required for photosynthesis of the rapeseed. However, the CO_2 balance is not as perfect as it seems at first glance, because the production of biodiesel requires energy, which also creates carbon dioxide. Unfortunately, there are other disadvantages of biodiesel: when the rape plant grows, another greenhouse gas (laughing gas) is produced, which intensifies the greenhouse effect even more than carbon dioxide. And even if we would like to obtain all of our energy from renewable raw materials, the agricultural area is nowhere near enough to grow so many plants that we have enough energy.
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