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Esters

Esters. C.I. 13.5. What are they? How are they made?. Formed when an alcohol reacts with a carboxylic acid. Very slow reaction, unless! an acid catalyst used (usually sulphuric acid) A condensation reaction

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Esters

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  1. Esters C.I. 13.5

  2. What are they?How are they made? • Formed when an alcohol reacts with a carboxylic acid. • Very slow reaction, unless! an acid catalyst used (usually sulphuric acid) • A condensation reaction • The condensation reaction between the hydroxyl group and the carboxylic acid known as esterification. • Reverse reaction = ester hydrolysis

  3. Naming Names of esters end in –oate. Named after alcohol & carboxylic acid from which they are derived. O let’s name some esters! C CH3 CH3 CH2 O ethyl ethanoate this part comes from the alcohol & is named after it this part from the acid and is named after it

  4. Structural formulae Although the previous structural formula are the clearest way of showing esters, they can draw out in a shortened form. In this version the acid part has been written first ethyl ethanoate Or… O O Either… CH2 CH3 C CH3 O CH3 C CH2 O CH3 • Identify the group attached to the C=O – this is from the acid • The group attached to the –O- is from the alcohol.

  5. Wait! What’s that smell? • Esters have strong, sweet smells. • Their bouquet is often floral or fruity. • This means they are used in food flavourings & perfumes. • Also, very good at dissolving organic compounds so often used as solvents.

  6. OH Esters from phenols • -OH group in phenol less reactive to esterification, requires a more vigourous reagent. • Use of ethanoic acid in esterification is also known as ethanoylation. • Ethanoic anhydride is a more vigourous ethanoylating agent

  7. Esters from phenols • Ethanoic anhydride used as it is reactive but not too dangerous. • Ethanoyl chloride is much more reactive, but, also toxic and hazardous to use. • Ethanoic anhydride & ethanoyl chloride are described as acylating agents • using an acylating agent is the only way to esterify a phenol • alcohols can be esterified using acylating agents or by reacting with a carboxylic acid.

  8. Ester hydrolysis • Breakdown of an ester by water. • Process sped up by catalysis • Can use an acid to catalyse (H2SO4) • Alkali catalysts (e.g. sodium hydroxide) can also be used but instead of producing carboxylic acid a carboxylate salt is formed. • Alkaline hydrolysis goes to completion & hence is usually preferred.

  9. R O H O ⇋ C R’ This is why it is a condensation reaction because water is produced! + O R O C R’ + H O H2O Definition of a condensation reaction = two molecules reacting to form a larger molecule with the elimination of a small molecule such as water

  10. O CH ethyl methanoate CH3 CH2 O O C CH2 CH3 methyl propanoate CH3 O O methyl methanoate CH CH3 O

  11. FORWARD REACTION = condensation reaction, the esterification of an alcohol using acid catalyst under reflux. R O H O ⇌ C R’ + O R O C R’ + H O H2O REVERSE REACTION = ester hydrolysis, same catalyst works for both forward & reverse reactions.

  12. ethanol ethyl ethanoate O CH3 CH2 OH C CH3 CH3 CH2 O → + + O OH- C CH3 -O ethanoate ion not a reaction that exists in equilibrium (unlike using acid catalyst)

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