The sodium and potassium salts (carboxylates) of long chain carboxylic acids are called soaps. Note that
the reaction above breaks up the triglyceride into its constituents (glycerol + three fatty acids which react
and so is called hydrolysis (splitting or lysing a molecule by adding water) – a particular type of hydrolysis
called saponification (‘turning into soaps’).
Soaps contain long fatty acid hydrocarbon chains, which being large molecules carrying no net electric
charge are insoluble in water, and carboxylate heads (COO-) which having net negative electric charge
are soluble in water (the negative charge is attracted to the slight positive charge carried on the hydrogen
atoms in water). [Common salt, NaCl dissolves in water because it breaks up into Na+ and Cl- electrically
charged ions which are attracted to the electric charges carried by water molecules.] Thus, for soaps, the
carboxylate heads are drawn toward water as they try to dissolve in it, but the fatty acid chains avoid the
water as they prefer their own company (or the company of non-polar solvents like ether). Soaps are
bipolar molecules – they have the water-loving or hydrophilic COO- polar head and the water-avoiding
hydrophobic hydrocarbon tail. To accommodate the requirements of both features, many soap molecules
aggregate together when in water, to form globules called micelles. The structure of a micelle is shown in
the introductory picture at the top of this page. The hydrocarbon tails form the oily centre of a globule,
whereas the polar heads forms an outer sphere that interacts with the surrounding water. As oil and water
do not mix, water is excluded from the oily centre of the micelle.
The interaction of the polar heads with the surrounding water allows micelles to disperse in water (soap
dissolves in water!). Globules of grease or oil, which cling to skin and fabrics and trap dirt, can be broken
down as they dissolve into the oily centre of the micelles (oil dissolves in oil but is reluctant to mix with
water) and so are dispersed and carried away with the micelles in water – that’s how soap cleans things!
This process of breaking up fats and oils, even in water, is called detergency. Soap is a detergent, but
there are many others. Other detergents are similar, but have different polar heads, e.g. sodium
alkylsulphates (with negatively charged sulphate heads).
Hydrogenation of oils
Hydrogenation is the process of adding hydrogen to unsaturated hydrocarbon chains – that is adding H2
across the C=C double bonds:
Fats and Oils
Compounds which can be extracted from organic tissues with solvents such as ethoxyethane (diethyl
ether) or trichloromethane (chloroform) are called lipids. Some of these lipids are hydrolysed (saponified)
by hot sodium hydroxide solution and are called saponifiable lipids and include the simple, edible fats and
oils, such as butter, lard, corn oil and linseed oil.
All of the simple fats and oils are triglycerides, i.e. triesters of the trihydric alcohol propane-1,2,3-triol
(glycerol). Esters are formed from the reaction between a compound containing an alcohol group and one
containing the carboxylic acid group.
-OH is the alcohol group, e.g. ethanol: CH3CH2OH; methanol: CH3OH and propanol: CH3CH2CH2OH. A
trihydric alcohol contains three alcohol groups. Ethanol is also called ethyl alcohol, methanol: methyl
alcohol and propanol: propyl alcohol, though these are old names and ethanol, methanol and propanol
-COOH is the organic (carboxylic) acid group, e.g. ethanoic acid: CH3CH2COOH.
These two groups can react together to form an ester in an esterification reaction:
Esters contain an ester link: -(C=O)-O- (not to be confused with the ether link: -C-O-C- ). Triesters contain
three ester links.
The esterifying acids are straight chain carboxylic acids containing even numbers of carbon atoms (usually
from about 14 to 22). These carboxylic acids or fatty acids may be saturated (contain no C=C double
bonds), unsaturated (contain one C=C double bond) or polyunsaturated (more than one C=C double
bond). The more unsaturated fats (containing more C=C double bonds) have lower melting points (since
the double bonds kink the chains and push them apart slightly and weaken the van der Waal’s bonds
between them) and so are often liquid oils at room temperature, whereas saturated fats tend to be solid at
room temperature. Thus, vegetable fats, which tend to be oils, are polyunsaturated.
All unsaturated straight chain fatty acids have the general formula: CnH2n+1COOH, whilst those with m
C=C double bonds have the general formula: CnH2n+1-2mCOOH. Thus, beef tallow has no C=C double
bonds and is saturated, as is palm oil. Olive oil has one C=C double bonds, olive oil and linseed oil has two
C=C double bonds.
Increasing the number of double bonds lowers the melting point.
Increasing the number of C atoms (the chain length) increases the melting point.
Stearic acid (octadecanoic acid) is used in candles, soaps, plastics, oil pastels and cosmetics, and for
softening rubber. It is solid at room temperature.
Palmitic acid (C16 saturated): 63-64 degrees C.
Stearic acid (C18 saturated): 69.6 degrees C.
Oleic acid (C17, one C=C bond): 13-14 degrees C.
Linoleic acid (C17, two C=C bonds): -5 degrees C.
Saponification: the preparation of soaps
Treatment of triglycerides with hot sodium hydroxide (or potassium hydroxide) solution converts them into
glycerol and a mixture of fatty acids:
Vegetable oil can be hydrogenated using a nickel catalyst. As more and more double bonds are
hydrogenated into single bonds, the degree of saturation of the fat increases, and hence the melting
temperature also increases and the oil solidifies into a fat. In this way margarine is manufactured from
vegetable oils. Note that the aim is not to totally saturate the vegetable oils, since this will yield hard butter
and the aim is to avoid using saturated fats as they are considered less healthy and are harder to spread.
Saturated or unsaturated
Bromine, Br2, in water or organic solvent (e.g. ether) will add to double bonds in a similar way to hydrogen,
but much more readily and without a catalyst:
Adding a few drops of yellow bromine water will decolourise the water as the bromine is removed from it. (In
organic solvent bromine is orange-brown and will similarly decolourise).
If fat is extracted into ethanol (by crushing and dissolving a small amount of food substance or fat, warming
in a water bath if necessary) and water is added and the mixture shaken, then a milky-white emulsion of
tiny drops of fat. This indicates the presence of fats or oils in the original food sample and is also what
gives milk its white colour.
Phospholipids are lipids (often derived from triglycerides) that contain a charged phosphate head group (in
place of one of the fatty acid side-chains of the triglyceride). Phospholipids are the principle component of cell
membranes. The fact that they have two oil-like hydrocarbon fatty acid tails and a charged head group gives
them the ability to form micelle-like structures in water, however, they have the added feature of forming
phospholipid bilayers and so they form hollow globules in water - ideal for housing the contents of a cell!
Above: the 'skin' or membrane of a cell consists of a phospholipid bilayer, part of which is
shown magnified. The green balls are the charged phosphate heads which are attracted to the
electric charges of the water (i.e. they are hydrophilic) surrounding the membrane on both
sides (inside and outside the cell) and the zig-zag tails are the fatty acid hydrocarbon tail
chains that huddle together to exclude water from inside the membrane, forming an oil-like
hydrophobic interior layer to the membrane.