Chemical Ideas 13.6 Chemical Ideas 13.6 Oils and FatsOils and Fats

Chemical Ideas 13.6 Chemical Ideas 13.6 Oils and FatsOils and Fats

Chemical structure

• Oils and fats – important for storing chemical energy in living things.

• Oils are liquids – fats are solids.

• Made from esters of propane-1,2,3,triol (glycerol) + long chain carboxylic acids RCOOH.

H

H-C-O-H

H-C-O-H

H-C-O-H

H

Propane-1,2,3-triol palmitic acid

O H-O-C-(CH2)14CH3

Table 11 Common fatty acids

Types of triesters• Triesters

(triglycerides) – 3 carboxylic acids react with triol.

• Mixed triesters – three acid groups, not all alike often found in natural oils and fats.

H

H-C-O-R

H-C-O-R’

H-C-O-R’’

H

General Structure of triesters

Fats and fatty acids• Unbranched hydrocarbon chains containing an even

number of carbons – usually between 4 and 24 but often 16 or 18.

• Called fatty acids as they occur in fats!?

• The alkyl groups are either fully saturated or contain one or more double bonds.

• Still known by old names – systematic names too long.

• More saturated fatty acids thought to cause blockage of blood vessels and heart disease.

Fat facts!• Natural oils and fats are mixtures of triesters.

• The nature of the acids present determines the properties and whether it is an oil or a fat.

• The proportions in which they occur are mre or less constant for a particular liquid oil or solid fat.

• E.g. Human body fat contains mainly 4 acids:

Oleic acid 47%Linoleic acid 10%Palmitic acid 24%Stearic acid8%

• An oil or fat can be identified by breaking it down into glycerol and its fatty acids and working out the proportions of the each of the acids present.

• Can be split up by hydrolysis, heat with concentrated NaOH.

e.g.Triester + NaOH glycerol + 3 x sodium salt of acid

• Basis of soap manufacture eg. “Palmolive” as soaps are simply the potassium or sodium salts of fatty acids.

• Convert sodium salts to free acids by adding dil. HCl

Fat facts!

Solid or...?

• Saturated triglycerides pack closely together.

• Attractive intermolecular bonds - higher melting point.

…or Liquid?

• Unsaturated cannot pack closely together because of cis double bonds - causes kinks!

• Intermolecular bonds are weaker, less energy needed to separate molecules - lower MP

• The more unsaturation a molecule has, the more likely it is to be an oil at RTP.

Converting oil to fat• Most natural oils need processing to make them fit

for use.

• Hydrogenation of unsaturated oils - make margarine. Example of an addition reaction.

• Controlled hydrogenation makes oils more solid by adding H2 to reduce unsaturation and make molecule more saturated.

• Pass H2 through heated oil - nickel catalyst

Add flavourings, salt, vitamins etc.

Converting oil to fat

Oil mixture Refiner Hydrogenation Blender Emulsifier Margarine

Catalyst

Hydrogen

Fat free milk

Flavourings etc.

Processes involved

Chemicals involved

Practise Time…

• Now have a go at CI 13.6 Problems 1-3 to check your understanding