Different types of double bonds.

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Oil and Water Don’t Mix

Model 1. When chemists think of fats they think of a large class of molecules called lipids. The word lipids comes from the Greek “lipos” for fat. Natural fats and oils are made mostly of molecules called triglycerides. Fats are solid triglycerides, while oils are liquid triglycerides.

A triglyceride is made from the combination of fatty acids and glycerol; it has the basic structure shown in Figure 4.1. Fatty acids can be from 4 to 35 carbons long, but 14-20 carbon fatty acids are most common in food.

 

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Glycerol

Fatty Acids

Triglyceride

H

C

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really looks like…

H

H

H

 

Figure 9.1. Glycerol and fatty acids combine to make a triglyceride.

When fatty acids and glycerol combine, bonds are broken and formed in a chemical reaction to produce a triglyceride and 3 molecules of water. In that process, a new group of atoms is formed called an ester. The properties of a given triglyceride depend upon the chemical structure of the three fatty acids it contains, and the properties of a lipid depend upon the particular mixture of triglycerides it contains.

Table 9.1. The composition of mono-, di- and triglycerides

Monoglyceride = Glycerol + One fatty acid
Diglyceride = Glycerol + Two fatty acids
Triglyceride = Glycerol + Three fatty acids

 

image2.emfRO

C

R

O

anesterfunctional group

R = the “rest” of the molecule

 

Figure 9.2. An ester functional group

By definition, Lipids are insoluble in water, so that means triglycerides are insoluble in water. To be soluble means that two molecules will dissolve in one another to form a homogeneous mixture. When compounds are insoluble, the combination forms a heterogenous mixture. When a lipid (e.g. oil) is mixed with water, you will see boundaries form between the two phases – literally, the two cannot mix.

 

image3.emfKey Concept

The polarity of a molecule is determined by the separation of charge between

its atoms. In polar molecules most atoms are connected polar bonds. In non-

polar molecules most atoms are connected by non-polar bonds.

 

 

image4.emfOH

NH

CO

-

-

-

+

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CC

CC

Polarbonds

Non-polarbonds

 

Figure 9.3. Non-polar versus polar bonds

 

Polar compounds can mix with or dissolve /are soluble in water (hydrophilic) to form homogeneous mixtures (i.e. sugar dissolving in water, lemon juice dissolving in water, vinegar dissolving in water). Non-polar compounds can mix with or dissolve /are soluble in oils (hydrophobic) to form homogenous mixtures (e.g. vanilla extract dissolving in oil, melted butter mixing with olive oil). These facts are described by the principle like dissolves like .

 

BUT oil (non-polar, hydrophobic) and water (polar, hydrophilic) don’t mix or dissolve in one another.

1. Why is a fatty acid called a fatty acid?

2. How many ester bonds are formed when a triglyceride is made?

3. On Figure 9.1, use the Draw tool or insert a shape to box the atoms that become the water molecule.

4. How many water molecules are produced when a monoglyceride is made?

5. A triglyceride has some polar and some non-polar bonds – and yet the molecule as a whole is very hydrophobic (i.e. water hating).

a. Why is the tryiglyceride – as a whole – water hating?

b. Explain why the non-polar carbon chain is unable to interact with water.

6. Define the phrase non-polar bond. Use the word charge in your answer.

7. The words hydrophobic and hydrophilic are derived from the Greek: hydro = water, phobos = fear, and phileo = love. Explain why these words are consistent with the chemical properties of a triglyceride.

Saturated and Unsaturated Fats

Model 2. Fatty acids come in different forms.

 

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saturatedfattyacidchain

monounsaturatedfattyacidchain

polyunsaturatedfattyacidchain

makingthedoublebond

requiredthelossof2H

atoms.TwoHatoms=a

singleunitofunsaturation

 

Figure 9.4. Saturated, Unsaturated and polyunsaturated Fatty Acids

Saturated, monounsaturated and polyunsaturated fatty acids can all be used to make triglycerides. A single triglyceride can be made of fatty acid chains of all of one type (e.g. all saturated) or a mixture of types, for example one saturated, one monounsaturated and one polyunsaturated fatty acid chain.

 

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Figure 9.5. A triglyceride made with different types of fatty acids

The properties of a given triglyceride molecule depend on the structure (i.e. type) of the three fatty acids that make up the triglyceride, and their relative position on the glycerol backbone.

 

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type 1

type 2

 

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Figure. 9.7. On a nutrition label, Total fat includes Saturated, Polyunsaturated and Monounsaturated fats.

Figure 9.6. Different types of double bonds.

Fatty acids can have double bonds between carbon atoms. Double bonds come in two main types, as shown in Figure 9.6. The double bond between the carbons “fixes” all the atoms in place – in effect, the 4 atoms attached to the doubly bonded carbons (shown in boldface in figure 9.6) are “stuck there”. Since these atoms are fixed in space, we can think of the double bond as having two “sides”. This is in contrast to singly bonded atoms, which are able to rotate freely.

 
The generic food term “fat” refers largely to triglycerides. Therefore, for example, saturated fat is referring to the composition of the fatty acid chains that make up the triglycerides.

8. Using the information in Figure 9.4,

a. What does it mean when a fatty acid is unsaturated?

b. What do the prefixes mono- and poly- tell you about the unsaturation?

c. If some fatty acids are saturated, what are they saturated with?

9. Looking at Figure 9.6,

a. What is the difference between the two types of double bonds?

b. Chemists refer to these two types of double bonds as cis and trans. Considering that trans is Latin for “on the opposite side” and cis is Latin for “on the same side”, which type of double bond (Figure 9.6, type 1 or 2) is trans and which type is cis?

Putting it all together:

10. It is possible to cook garlic in olive oil and transfer the flavors of the garlic to the oil. Since the flavors of garlic can all be attributed to molecules, what does this process tell you about the polarity of the garlic flavor molecules?

11. Many salad dressings are a mixture of oil and vinegar, like the image shown below.

image9.jpg

Photo by Bill Keller

Please explain why the mixture on the left is apparently separated into two phases.

12. Some vitamins like vitamins A and D are fat soluble. What does that tell you about their structure?

Copyright © 2016 Wiley, Inc. Page 8

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