Rubber from trees

Section 1 of 5

Cut a groove in the bark of a rubber tree and a white milk oozes out. That milk is where every natural rubber band, glove and tyre begins.

The rubber tree, Hevea brasiliensis, comes from the Amazon rainforest. Tappers shave a thin, sloping groove in its bark, and a milky sap called latex trickles down into a cup for a few hours. Latex is tiny droplets of rubber floating in water, about a third rubber by weight. Stir in a little methanoic or ethanoic acid and the droplets clump together into a soft white curd, which is pressed into sheets and dried.

Rubber is a polymer, and the small molecule it is built from is 2-⁠methylbuta-⁠1,3-⁠diene, known as isoprene. The name says what it is: buta-⁠1,3-⁠diene, the chain of four carbons with double and single bonds taking turns, C=C–⁠C=C, with a methyl group, -⁠CH₃, on carbon 2. Natural rubber is poly(isoprene).

2-Methylbuta-1,3-diene (isoprene), C₅H₈Buta-1,3-diene with a methyl group on its second carbon. A chain of four carbon atoms with double and single bonds taking turns, C=C-C=C, and a -CH₃ group on one of the middle carbons.HHHCHCCCCHHHH
2-⁠Methylbuta-⁠1,3-⁠diene, isoprene, C₅H₈: buta-⁠1,3-⁠diene with a methyl group on carbon 2. It is a liquid that boils at 34 °C.

How the links join

Each isoprene molecule has two double bonds, and when it joins a chain, both open. Carbon 1 bonds to the link before it and carbon 4 to the link after it, and the electrons left over make a new double bond between carbons 2 and 3. So every link of rubber keeps one C=C double bond, in its middle.

An isoprene molecule and the link it becomes in rubber Left: isoprene as a skeletal formula, curled round its middle single bond into a C, with carbons numbered 1 to 4. Carbons 1 and 2 are joined by a double bond, and so are carbons 3 and 4. Carbon 2 carries a methyl group, CH3. An arrow points right to one link of rubber, drawn with its atoms in the same places, inside square brackets with a small n. Now the only double bond is between carbons 2 and 3, and amber bonds leave carbons 1 and 4 to join the links on either side. Carbons 1 and 4 both sit below the double bond: it is cis. 1 2 3 4 CH₃ n 1 2 3 4 CH₃ isoprene one link of rubber
Isoprene and one link of rubber as skeletal formulas, with the carbons numbered and the atoms in the same places. The amber lines of isoprene's two double bonds open, a new double bond forms between carbons 2 and 3, and the amber bonds at carbons 1 and 4 join each link to its neighbours.

Look at the double bond in the link. Carbons 1 and 4, which carry the chain on, sit on the same side of it, so the double bond is cis, the same arrangement as the kink in oleic acid. The tree builds its rubber five carbons at a time, from a molecule with isoprene's carbon skeleton, and it makes every one of these double bonds cis. A single chain of natural rubber holds thousands of links, all of them cis.

Quick check

When isoprene molecules join into natural rubber, where does each link's double bond end up?