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Exam Specification describe the useful chemical and physical properties of esters, such as miscibility, odour and use as a solvent describe the uses of alcohols in the preparation of esters describe the uses of esters in cosmetics and fruit flavourings
What is an ester Esters are produced when an organic acid reacts with an alcohol (you can think of this as neutralisation) Water is made as a by-product of the reaction, just like neutralisation Esters are either natural or man-made Natural esters: Fats and oils: an ester of a fatty acid and glycerol Aromas from fruits like apples, pears, and strawberries are esters Pheromones are natural esters Essential Oils like patchouli and lavender are esters Man-made: Many esters are now manufactured industrially as it is easier than extracting them from plants and animals, e.g. for perfumes and sweets
Esters This one is used in nail varnish remover as it is a good solvent – it dissolves the nail varnish but not your nails
Properties of esters Quite volatile – small esters have lower boiling points than the carboxylic acids This is useful for plants or animals to attract others by scent They aren’t able to take part in hydrogen bonding as well as carboxylic acids This also makes the larger esters very insoluble in water, but more soluble in organic solvents This why they are very useful as solvents for other organic compounds – like the large alcohols were. Very large esters (like oils and fats) have higher boiling points and melting points – remember: a fat is simply a solid oil The larger the ester molecule, the higher the boiling and melting point.
Link to C2 Additional Saturated vs Unsaturated fats Remember – saturated means only single C-C bonds and unsaturated means double C=C bonds. Saturated fats don’t have any kinks, so the chains can line up closely and the have higher melting points (solid at room temperature – fats) Unsaturated fats have kinks in their chains, and so they can’t get very close together and they have lower melting points (liquid at room temperature – oils)
Preparation of Esters Carboxylic Acid + Alcohol Ester + Water Ethanoic acid + ethanol Ethylethanoate + water Methanoic acid + ethanol Ethyl Methanoate + water Often carried out in the presence of an acid catalyst (e.g. conc sulfuric acid) and with gental heating
Making an ester A Pour some boiling water into a 250 cm3 beaker. Leave it to stand. B Put about 2 cm3 of ethanol into a test tube. C Go to the fume cupboard and ask your teacher to add 1 cm3of concentrated ethanoic acid and three drops of concentrated sulphuric acid to your tube. D Stand your tube in the beaker of hot water for about 5 minutes. E While you are waiting, collect a 100 cm3 beaker and fill it about half full with sodium hydrogen carbonate solution. F After 5 minutes, carefully tip the liquid from the test tube into the beaker and stir well. G Carefully smell the product in the beaker.
Questions to practice 1. On the Moodle course KS4 Triple Science (Chemistry), at the top under C3 Extension Topic 4 is a worksheet called Fruity Flavours Download it and rearrange the cards. 2. Next, download and complete the worksheet called The reactions of Ethanoic Acid Qs This will help you review the last two topics (Acids and Esters)