3.
3.4
Fats are also important fuels for cells
Describe glycerol as a triol and identify its systematic name
Glycerol is a triol, that is a hydrocarbon chain with three alcohol or
hydroxy (–OH) groups attached. It is a colourless viscous liquid, miscible
in water and with a sweet taste. Draw and name the structure of glycerol below.
3. 8 Use available evidence and process information from secondary sources to analyse the structure of
the glycerol molecule and predict its viscosity and solubility in water, giving reasons for their
predictions
Solubility in water
Because of the presence of 3 hydroxyl groups OH on a small carbon skeleton the molecule will be highly
polar. This accounts for its extreme high MP. ie strong H-bonds will form between molecules, and it is high
solubility with water due to its polar nature.
Viscosity
Refers to how easily a fluid flows and is defined as the resistance of a fluid to flow. A thick fluid such as
honey is said to have a high viscosity, blood and water have low viscosities.
Viscosity is a property that relates directly to the forces between the particles of the fluid and the size of the
particles. Both will determine how easily the molecules of the fluid move past each other. Glycerol has
many (3)hydroxyl groups that give rise to H - bonding between molecules. ie stronger intermolecular bonds
and the molecule is much larger than say water. Both of these factors contribute to the high viscosity of
glycerol.
3.1 Identify that fatty acids are alkanoic acids with the general formula
CH3(CH2)nCOOH
3.2 Identify that part of the fatty acid molecule which should mix with water and give an explanation for this
phenomenon
3.3 Identify the most common fatty acids in our diet and in our body stores as the
C14-C20 series from diagrams or models
CHEMICAL STRUCTURE OF FATS AND OILS
Fats and oils are natural esters. They are generally formed from 1,2,3-propanetriol,
commonly known as glycerol, and fatty acids. The esters formed from glycerol and
fatty acids are known as triglycerides or triacylglycerols (TAGs). Fats usually contain
several different fatty acids. The fatty acids which are important in the diet of humans
range from 14 to 20 carbon atoms in length and always have an even number of carbon
atoms as illustrated in Tables 28.3 and 28.4.
Fatty acids
Fatty acids are long-chain alkanoic (carboxylic) acids. As outlined in Unit 10.6, all
alkanoic acids have a carboxyl (–COOH) functional group, which is a combination of
a carbonyl (–C=O) group and a hydroxy (–OH) group. The presence of the carboxyl
group greatly influences the properties of alkanoic acids. The –COOH end of the
molecule is polar and capable of forming hydrogen bonds. This section of the alkanoic
acid molecule is described as hydrophilic (water loving) and tends to be water soluble.
The remainder of the molecule, consisting of carbon and hydrogen, is non-polar and
is said to be hydrophobic (water hating). This section of the molecule therefore tends
to be insoluble in water. As a result, small chain alkanoic acids (C1–C4) are completely
miscible with water but the solubility of these acids decreases rapidly with increasing
length of the hydrocarbon chain. Most alkanoic acids will dissolve in organic solvents
such as ethanol and acetone (propanone).
There are two main types of fatty acids called saturated and unsaturated fatty acids.
The saturated fatty acids belong to the homologous series with the general formula
CnH2n + 1COOH or CH3(CH2)nCOOH. An example of a saturated fatty acid is stearic
acid which contains 18 carbon atoms in its hydrocarbon chain and has the formula
C17H35COOH or CH3(CH2)16COOH as illustrated in Figure 28.10.
Figure 28.9
The unsaturated fatty acids can be further subdivided into monounsaturated and
polyunsaturated fatty acids. Monounsaturated fatty acids contain one C=C double
bond in the hydrocarbon chain. Polyunsaturated fatty acids contain two or more C=C
double bonds. Oleic acid, CH3(CH2)7CH=CH(CH2)7COOH, is a monounsaturated
fatty acid and linoleic acid, CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH, is
polyunsaturated.
Tables 28.3 and 28.4 list some of the important fatty acids and their characteristics.
3.5 Explain that fatty acids are stored as esters of glycerol [triacylglycerols(TAGs)] and account for the
hydrophobic nature of these esters
3.6 Assess the importance of TAGs as an energy dense store for humans
28.2 FATS AND OILS
Fats and oils are the most abundant subgroup of a large group of compounds called lipids. Other groups of lipids
include waxes, fat-soluble vitamins and steroids. Lipids are generally insoluble in water and dissolve in nonpolar solvents such as chloroform, diethyl ether and benzene. Like carbohydrates, lipids, including fats, are made
from the three elements carbon, hydrogen and oxygen. Also like carbohydrates, the major function of fats in our
bodies is as an energy source. Fats are the body’s main long-term energy reserve. Our bodies store much larger
quantities of fat than glycogen. While a human liver can hold up to 10% of its mass as stored glycogen, this
glycogen can only meet our energy needs for about twelve hours. In comparison, a body can store enough fat to
provide energy for up to eight weeks. The human body is very adept at converting excess carbohydrates to stored
fat. Other mammals share this characteristic. In fact, fat storage is a major factor in the survival of many species
such as whales, sea lions and seals.
3.7 Solve problems, identify resources and perform first-hand investigations to compare the structures of
fatty acids and glycerol from diagrams or models