Where fruity smells come from
Fischer esterification · Condensation · Equilibrium
Mix the acid in vinegar with the alcohol in wine, add a catalyst, and you get the smell of pears.
Reactants approach
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ΔH = -3 kJ · exothermic, releases heat
ΔH per mol ester; nearly thermoneutral, K ≈ 4.
Meet the molecules
- Reactantliquid
CH3COOH
acetic acid
- Reactantliquid
C2H5OH
ethanol
- Productliquid
CH3COOC2H5
ethyl acetate
- Productliquid
H2O
water
What you notice
The smell of a ripe pear, banana or pineapple comes largely from esters, small molecules with a –COO– group that evaporate easily and that our noses detect at very low concentrations. Ethyl acetate gives pears and some wines a sweet, fruity note; at high concentration it smells like nail-polish remover, where it is used as a solvent. Fruits make esters with enzymes. Chemists make the same molecules by warming a carboxylic acid with an alcohol and a few drops of concentrated sulfuric acid, and flavour companies blend them into sweets and drinks.
Try it
Check the ingredient list of a fruit-flavoured sweet or drink for names ending in "-yl acetate" or "-yl butanoate"; these are esters.
Safety: Concentrated sulfuric acid, the catalyst, causes severe burns, so this synthesis belongs in a school lab with goggles and a fume cupboard.
Step by step
- 1
Acid activates the carbonyl
A trace of strong acid protonates the C=O oxygen of acetic acid, making the carbon more positive and easier to attack.
- 2
The alcohol attacks
The oxygen of ethanol uses a lone pair to bond to that carbon, briefly giving a carbon with four single bonds.
- 3
Water leaves
Protons shuffle so that one –OH becomes –OH2+, which departs as a water molecule. Isotope-labelling experiments show this oxygen comes from the acid, not the alcohol.
- 4
Ester and equilibrium
Losing a proton gives ethyl acetate and returns the catalyst. Because ΔH is close to zero and K is only about 4, chemists raise the yield by using excess alcohol or removing water.