Unit 3 - Esters, Fats and Oils Flashcards
Structure of Esters
Esters are formed when an alcohol reacts with a carboxylic acid. This is an example of a reversible reaction.
All esters contain the ester link, -COO- (functional group).
Naming Esters
The first word of the ester name comes from the parent alcohol and always ends in ‘yl’. The second word in the ester name comes from the parent carboxylic acid and always ends in ‘oate’.
C=O in its functional group, the C=O of the ester link is joined to the side that came from the carboxylic acid.
Uses of Esters
Esters are used as flavouring and fragrances as many have pleasant, fruity smells e.g. in perfumes, confectionery. Esters are also used as solvents for non-polar compounds that don’t dissolve in water e.g. in nail varnishes and nail varnish removers.
Formation of Esters
Esters are formed by a condensation reaction between an alcohol and a carboxylic acid. In a condensation reaction, two molecules are joined together with the elimination of a small molecule. When an ester link is formed by the reaction between a hydroxyl group and a carboxyl group, the small molecule eliminated is water.
Preparing Esters (Experimentally)
One way of preparing esters is to condense an alcohol with a carboxylic acid. The reaction is slow at room temperature and the yield of ester is low. The rate can be increased by heating the reaction mixture and by using concentrated sulfuric acid to provide H+ ions which act as a catalyst. The presence of the concentrated sulfuric acid also increases the yield of ester by removing the other product, water.
The ester forms as an oily layer on the surface and has a distinctive smell.
Properties of Esters - IFs and MP/BP
Esters are polar molecules and so have permanent dipole-permanent dipole interactions as well as London dispersion forces. However, they do not form hydrogen bonds so their boiling points are lower than an acid with the same number of carbon atoms. This means that esters are more volatile (means we can smell them more easily)
Properties of Esters - Solubility
Small esters are fairly soluble in water but solubility decreases as the carbon chain length increases.
Properties of Eters - Hydorgen Bonding
Although esters can’t form hydrogen bonds with themselves, they can form hydrogen bonds with water molecules. One of the slightly positive hydrogen atoms in a water molecule can be sufficiently attracted to one of the lone pairs on one of the oxygen atoms in an ester for a hydrogen bond to be formed.
Hydrolysis of Esters
Esters can be hydrolysed to produce an alcohol and a carboxylic acid. In a hydrolysis reaction, a molecule reacts with water to break down into smaller molecules. Hydrolysis (break down of compound using H20) is the opposite of a condensation reaction.
Hydrolysis is reversible and therefore will be incomplete.
Acid + Alcohol ⇌ Ester + H20
Condensation is forward reaction - Hydrolysis is backward reaction.
Alkaline Hydrolysis of Esters
In practice the ester is heated with dilute alkali rather than water. The ester is heated under reflux. When sodium hydroxide solution is used the ester is hydrolysed into the alcohol and the sodium salt of the acid. The alcohol can be removed by distillation and the carboxylic acid can be regenerated by reaction the sodium salt with dilute hydrochloric acid.
Structure of Fats and Oils
Fats and oils are esters formed from the condensation of glycerol (propane-1,2,3-triol) and three carboxylic acid molecules (‘fatty acids’)
Fatty acids are saturated or unsaturated straight-chain carboxylic acids, usually with long chains of carbon atoms.
Each of the hydroxyl groups in glycerol undergoes a condensation reaction to form an ester link with a fatty acid. The ratio of glycerol molecules to fatty molecules is therefore 1:3. The three fatty acids in a fat molecule can be the same or different.
Properties of Fats and Oils - Differences Between Fats and Oils
Fats are solid at room temperature and oils are liquids.
The main ‘chemical’ difference between fats and oils is the higher level of unsaturation in oils - oil molecules containn many more C=C double bonds than fats
Properties of Fats and Oils - Bromine SOlution
Unsaturated compounds quickly decolourise bromine solution, The bromine molecules add across the carbon-carbon double bonds in an addition reaction. The greater the number of double bonds present in a substance, the more bromine solution can be decolourised,
Properties of Fats an dOIls - Melting Points
Fats have a high melting point so are more saturated than oils (fewer double bonds).
Oils have a low melting point so are more unsaturated than margarine and other fats.
Properties of Fats and Oils - LDFs and MP/BP
Fat molecules containing saturated hydrocarbon chains can pack neatly together, even at high temperatures.
Hence, saturated fats have a high melting point due to increased LDFs. The double bonds in fatty acid chains prevent oil molecules from packing closely together, so the greater the number of double bonds present, the weaker the LDFs of attraction.
The weaker LDFs result in lower melting points for unsaturated fats.
