Simple Lipids (Triglycerides) Chemistry Tutorial

Key Concepts

• A lipid is a fat or an oil.¹
• A fat is a solid at room temperature.
      An oil is a liquid at room temperature.
• Fats are more common in land animals (animal fats).
      Oils are more common in plants (vegetable oils) and fish (fish oils).

• In the human body, fats are used for:

  (i)   energy
  (ii)   insulation and to protect organs
  (iii)   transportion of non-polar substances around the body

• Simple lipids² are composed of esters of long-chain carboxylic acids (fatty acids) which are called triglycerides.
• Triglycerides contain ester functional groups:
  

  	O ||
  R-	C	-O-R′	or	R-CO-O-R′

• Triglyceride refers to tri-esters of glycerol (esters of propane-1,2,3-triol), and so simple lipids contain 3 ester functional groups.
  

  	H |		O ||
  H-	C	-O-	C	-R
  	|		O ||
  H-	C	O-	C	-R′
  	|		O ||
  H-	C	-O-	C	-R′′
  	| H

  R, R′ and R′′ represent long chain hydrocarbons.

• Triglycerides are formed in a condensation reaction between glycerol (propane-1,2,3-triol) and fatty acids:

  glycerol + fatty acids → triglyceride + water

• Triglycerides are considered to be non-polar molecules and are insoluble in water (do not dissolve in water).
• Triglycerides are classified as saturated or unsaturated:

  (i)   saturated : contains only single bonds between the carbon atoms in the fatty acid component
  (ii)   unsaturated : contains 1 or more double bonds between the carbon atoms in the fatty acid component

• An unsaturated triglycerides can be classified as either monounsaturated or polyunsaturated.

  (i)   monounsaturated : contains only 1 double bond between the carbon atoms in the fatty acid component
  (ii)   polyunsaturated : contains more than 1 double bond between the carbon atoms in the fatty acid component

• Simple lipids are made up of a mixture of triglycerides.³

  The greater the percentage of saturated fatty acid components of triglycerides present in a lipid, the higher the melting point of the lipid.

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Formation of Trigylcerides (simple lipids)

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:

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:

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:

Saturated and Unsaturated Triglycerides

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).

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:

Melting Point of Triglycerides (simple lipids)

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.

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.

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:

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.

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