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Simple Lipids (Triglycerides)

Key Concepts

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Formation of Trigylcerides (simple lipids)

A triglyceride is formed by the condensation reaction between glycerol (propane-1,2,3-triol) and 3 fatty acid molecules.
Glycerol (propane-1,2,3-triol), CH2OH-CHOH-CH2OH, contains three hydroxyl, OH, functional groups.

Fatty acid molecules are long chain carboxylic acids, R-COOH, so they contain the carboxyl, COOH, functional group.
Note that R represents a long hydrocarbon chain.

When an alcohol reacts with a carboxylic acid an ester is produced.

The products of this condensation reaction are a triglyceride (containing 3 ester functional groups) and 3 water molecules.

This condensation reaction can be represented by the following general equation:

glycerol

(propane-1,2,3-triol)

+ 3 fatty acids triglyceride + 3 water
  H
|
 
H- C -OH
  |  
H- C -OH
  |  
H- C -OH
  |
H
 
 
  O
||
 
HO -C- R
 
  H
|
  O
||
 
H- C - O- C -R
  |   O
||
 
H- C - O- C -R′
  |   O
||
 
H- C - O- C -R′′
  |
H
     
    H2O
+
  O
||
 
HO -C- R′
+ H2O
 
  O
||
 
HO -C- R′′
      H2O

Stearic acid, CH3(CH2)16COOH, is a fatty acid, R-COOH, in which R = CH3(CH2)16
Stearic acid can react with glycerol (propane-1,2,3-triol) in a condensation reaction to produce the triglyceride known as glyceryl tristearate (or tristearin), and water as shown below:

glycerol + 3 stearic acid glyceryl tristearate + 3 water
  H
|
 
H- C -OH
  |  
H- C -OH
  |  
H- C -OH
  |
H
 
 
  O
||
 
HO -C- (CH2)16CH3
 
  H
|
  O
||
 
H- C - O- C -(CH2)16CH3
  |   O
||
 
H- C - O- C -(CH2)16CH3
  |   O
||
 
H- C - O- C -(CH2)16CH3
  |
H
     
    H2O
+
  O
||
 
HO -C- (CH2)16CH3
+ H2O
 
  O
||
 
HO -C- (CH2)16CH3
      H2O

Oleic acid, CH3(CH2)7CH=CH(CH2)7COOH, is a fatty acid, R-COOH, in which R = CH3(CH2)7CH=CH(CH2)7
Oleic acid can react with glycerol (propane-1,2,3-triol) in a condensation reaction to produce the triglyceride known as glyceryl trioleate), and water as shown below:

glycerol + 3 oleic acid glyceryl trioleate + 3 water
  H
|
 
H- C -OH
  |  
H- C -OH
  |  
H- C -OH
  |
H
 
 
  O
||
 
HO -C- (CH2)7CH=CH(CH2)7CH3
 
  H
|
  O
||
 
H- C - O- C -(CH2)7CH=CH(CH2)7CH3
  |   O
||
 
H- C - O- C -(CH2)7CH=CH(CH2)7CH3
  |   O
||
 
H- C - O- C -(CH2)7CH=CH(CH2)7CH3
  |
H
     
    H2O
+
  O
||
 
HO -C- (CH2)7CH=CH(CH2)7CH3
+ H2O
 
  O
||
 
HO -C- (CH2)7CH=CH(CH2)7CH3
      H2O

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Saturated and Unsaturated Triglycerides

A triglyceride produced by the condensation reaction between a saturated fatty acid and glycerol (propane-1,2,3-triol) will be a saturated lipid (or saturated triglyceride) because there will ONLY be single bonds between the carbon atoms in the molecule.

For example, glyceryl tristearate (or tristearin) is a triglyceride containing only single bonds between carbon atoms, therefore it is an example of a saturated triglyceride (saturated lipid).

  H
|
  O
||
 
H- C - O- C -CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
  |   O
||
 
H- C - O- C -CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
  |   O
||
 
H- C - O- C -CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
  |
H
     

A triglyceride produced by the condensation reaction between an unsaturated fatty acid and glycerol (propane-1,2,3-triol) will be an unsaturated triglyceride because there will be one or more double bonds between the carbon atoms in the molecule.

For example, oleic acid, CH3(CH2)7CH=CH(CH2)7COOH, is an unsaturated fatty acid because it contains a double bond between two of the carbon atoms in its chain (C=C).
When oleic acid reacts with glycerol (propane-1,2,3-triol) to produce the triglyceride, glyceryl trioleate, the triglyceride (simple lipid) will contain 3 C=C and will therefore be unsaturated as shown below:

  H
|
  O
||
 
H- C - O- C -CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
  |   O
||
 
H- C - O- C -CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
  |   O
||
 
H- C - O- C -CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
  |
H
     

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Melting Point of Triglycerides (simple lipids)

Both saturated and unsaturated triglycerides are predominantly non-polar due to their very long hydrocarbon chains.
The intermolecular forces acting between these non-polar triglyceride molecules are therefore only weak van der Waal′s forces (London or dispersion forces).
We expect the melting point of triglycerides to be quite low because it should be easy to disrupt these weak intermolecular interactions, that is, it should not require much heat energy to melt triglycerides.
The melting point of a triglyceride is determined by how closely the molecules can pack together and this is largely determined by the number of C=C in the structure of the triglycerides.

In a saturated triglyceride, the long hydrocarbon tails form a zigzag chain that can fit together maximising the van der Waals attractions (dispersion or London Forces) between molecules, resulting in a higher melting point.

C C C C C C C C C C C C C C C C C C C C C C C C

In an unsaturated triglyceride, the hydrocarbon chains have kinks in them due to the presence of the double bonds.4
Because of the "kinks" in the chains, the hydrocarbon tails do not fit together as well, so there is less extensive van der Waal′s interaction between these hydrocarbon tails, so it requires less energy to weaken the intermolecular force of attraction, therefore the melting point of unsaturated triglycerides is lower.

C C C C C C C C C C C C C C C C C C

Saturated triglycerides have higher melting points than unsaturated triglycerides.

Simple lipids are made up of a mixture of triglycerides.

A lipid with a high percentage of saturated fatty acid components making up its triglycerides will have a higher melting point.
A lipid with a lower percentage of saturated fatty acid components making up its triglycerides will have a lower melting point.

A lipid with a melting point high enough to enable it to remain a solid at room temperature is called a fat.

A lipid with a melting point low enough to enable it to melt at room temperature and be present as a liquid is called an oil.

The table below shows the approximate composition of some common lipids as a percentage of saturated and unsaturated fatty acid components of the triglycerides:

lipid % saturated
fatty acids
% unsaturated
fatty acids
state at
room temp
source
soybean oil 10 90 liquid plant
corn oil 15 85 liquid plant
butter 40 60 solid animal
lard 45 55 solid animal

Note that lipids with a plant source have a higher proportion of unsaturated fatty acids making up the triglycerides which results in lower melting points and these lipids are liquids at room temperature.

Note that lipids with an animal source have a lower proportion of unsaturated fatty acids making up the triglycerides which results in higher melting points and these lipids are solids at room temperature.

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1. The term "lipid" can also be applied to other substances such as waxes, esters of long-chain monohydric alcohols (alcohols with one OH group), and even soap!

2. Complex lipids are a class of compounds that includes phospholipids, composed of glycerides in which a phosphate group has replaced a fatty acid, and also glycolipids, glycerides which contain a sugar unit.
Derived lipids are another class of compounds which include the terpenes, steroids, fat-soluble vitamins, and prostaglandins.

3. Triglycerides are also known as triacylglycerols, both terms mean 'triesters of glycerol'

4. cis isomers are more abundant that trans isomers in nature