Lesson 5: Functional groups

Esters smell sweet

Section 5 of 5

When a carboxylic acid and an alcohol join, they make an ester. Many fruits owe their smell to esters.

The -⁠OH from the acid and the H from the alcohol's -⁠OH leave together as a water molecule. The two pieces join through an oxygen atom, making the ester link, -⁠COO⁠-, in the middle of the molecule.

Acetic acid (ethanoic acid), CH₃COOHTwo carbon atoms; the second carbon has a double bond to one oxygen atom and a single bond to an -OH group.HOHCCOHH plus Ethanol, C₂H₅OHTwo carbon atoms in a chain; the second carbon is bonded to an oxygen atom that carries a hydrogen atom, an -OH group.HHHCCOHHH
make
Ethyl acetate (ethyl ethanoate), CH₃COOC₂H₅An ester. A two-carbon piece with a double-bonded oxygen is joined through an oxygen atom to a two-carbon chain.HOHHHCCOCCHHHH plus Water, H₂OOne oxygen atom joined by single bonds to two hydrogen atoms, in a bent shape.OHH
Ethanoic acid and ethanol join to make ethyl ethanoate and water. The ester's name takes "ethyl" from the alcohol and "ethanoate" from the acid.

Esters are named like branches: the alcohol part gets -⁠yl, and the acid part gets -⁠oate. Ethanol and ethanoic acid make ethyl ethanoate, a sweet-smelling liquid used in some nail polish removers.

Isoamyl acetate (3-methylbutyl ethanoate), C₇H₁₄O₂An ester. A short two-carbon piece with a double-bonded oxygen is joined through an oxygen atom to a branched five-carbon chain.OO
Isoamyl acetate (3-⁠methylbutyl ethanoate): the ester that gives bananas their smell. Find the -⁠COO⁠- link in the middle.

Isoamyl acetate gives bananas their smell, and other esters smell of pineapples, pears and apples. Fats and oils are esters too, big ones built from long chains.

The groups at a glance

Functional groups in this lesson
FamilyGroupExample
alcohol-⁠OHethanol
carboxylic acid-⁠COOHethanoic acid
ketoneC=O mid-chainpropanone
aldehydeC=O at an endethanal
amine-⁠NH₂methylamine
ester-⁠COO⁠- mid-chainethyl ethanoate

Quick check

Ethyl ethanoate is an ester. Which two molecules is it made from?

That's the end of lesson 5

In this lesson you found out:

  • What a functional group is
  • Alcohols, carboxylic acids, aldehydes, ketones, amines and esters
  • How one small group changes the way a molecule behaves

That's the whole course. You started with one atom with four hands. Now you can read a skeletal formula, name a branched alkane, and spot the small groups that make vinegar sour, fish smell fishy and bananas smell sweet. The same ideas build plastics, medicines and the proteins and DNA inside you.

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