Joining and giving off water

Section 4 of 4

In a condensation reaction, two molecules join into one bigger molecule and give off a small molecule, usually water.

You have met one already. Ethanoic acid and ethanol join to make an ester, ethyl ethanoate. The -⁠OH from the acid and the H from the alcohol's -⁠OH leave together as water, and the two pieces join through an oxygen atom.

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 give off water. Both sides have 4 carbon, 10 hydrogen and 3 oxygen atoms.

An addition reaction makes one product. A condensation makes two: the joined molecule and the small molecule given off.

Joining again and again

Ethanoic acid and ethanol each have one group that reacts, so they join once. A molecule with a reacting group at each end can join at both ends. An acid with a -⁠COOH group at each end and an alcohol with an -⁠OH group at each end join into a long chain, giving off a water molecule at every link.

Molecules with a group at each end join into a long chain Top: a molecule with a carboxylic acid group, COOH, at each end, plus a molecule with an alcohol group, OH, at each end. An arrow points down to a long chain of the two kinds of molecule, alternating and joined end to end. At every link a water molecule, H2O, is given off. HOOC COOH + HO OH an acid group at each end an alcohol group at each end H₂O H₂O H₂O … … one water molecule given off at every link
An acid with a -⁠COOH group at each end and an alcohol with an -⁠OH group at each end join end to end into a long chain, giving off water at every link.

Long chains like this are polymers. Polyester, the fibre in many clothes and the plastic in most plastic drinks bottles, is made this way. Lesson 9 shows how polymers are built, nylon included.

The molecules of life are built by condensation too. Proteins are chains of amino acids: the -⁠COOH of one amino acid and the -⁠NH₂ of the next join through a link called a peptide bond, and a water molecule is given off. Starch is a chain of glucose molecules joined by condensation, and fats are big esters. You will meet them in the lesson after that.

Quick check

Which of these is a condensation reaction?

That's the end of lesson 8

In this lesson you found out:

  • What happens to bonds, atoms and energy in a reaction
  • Burning, where fuel and oxygen make carbon dioxide and water
  • Addition across a C=C double bond
  • Condensation, where two molecules join and give off water
Start Lesson 9: Polymers All lessons