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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 l.jpg

Esters

C.I. 13.5


What are they how are they made l.jpg

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


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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


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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.


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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.


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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


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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.


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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.


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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


Slide10 l.jpg

O

CH

ethyl

methanoate

CH3

CH2

O

O

C

CH2

CH3

methyl

propanoate

CH3

O

O

methyl

methanoate

CH

CH3

O


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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.


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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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