Alternating double bonds

Section 2 of 4

You know the C=C double bond, two bonds between two carbons. Line several up, with single and double bonds taking turns, and the molecule starts to absorb light further toward the red end.

Buta-1,3-⁠diene, C₄H₆, is a chain of 4 carbons with two double bonds and one single bond between them: C=C–⁠C=C. Double and single bonds taking turns like this are called alternating double bonds. Chemists also call them conjugated.

Buta-1,3-diene, C₄H₆A chain of four carbon atoms with double and single bonds taking turns, C=C-C=C, and six hydrogen atoms.HHHHHCCCCH
Buta-1,3-⁠diene, C₄H₆: double, single, double. The name says it: 4 carbons (buta-), and two double bonds (-⁠diene) that start on carbons 1 and 3.

Benzene's ring, as you have seen, shares six electrons all the way round. A run of alternating double bonds shares electrons in the same way: the electrons of the double bonds spread out along the whole run. The longer the run, the less energy those shared electrons need to make their jump. Less energy means light further toward red.

Longer runs of alternating double bonds take in light further toward red Four zigzag chains of carbon atoms, drawn as skeletal formulas, with double and single bonds taking turns and every double bond highlighted. The chains have runs of 2, 3, 5 and 11 double bonds. The runs of 2 and 3 take in only ultraviolet light and have no colour. The run of 5 looks pale yellow. The run of 11 looks orange. 2 double bonds no colour 3 double bonds no colour 5 double bonds pale yellow 11 double bonds orange
Runs of double and single bonds taking turns, with the double bonds highlighted. The colour of each run comes from a real molecule: buta-1,3-⁠diene (2), hexa-1,3,5-⁠triene (3), vitamin A (5) and beta-carotene (11).
Longer runs of alternating double bonds (C=C) absorb light further toward red
MoleculeC=CAbsorbs most, so looks
buta-1,3-⁠diene2UV (217 nm), so no colour
hexa-1,3,5-⁠triene3UV (260 nm), so no colour
vitamin A5UV (325 nm), so pale yellow
beta-carotene11blue (450 nm), so orange
lycopene11blue-green (470 nm), so red

Vitamin A absorbs mostly ultraviolet light, and just a little violet at the edge, so it looks pale yellow. From about 8 double bonds in a run, the light absorbed moves into the visible colours, and the colours grow stronger as the run gets longer.

Carrots and tomatoes

Beta-carotene, C₄₀H₅₆, gives carrots their colour. It has 11 alternating double bonds: 9 along its chain and one in each ring at the ends. It absorbs blue light, around 450 nm, so carrots look orange.

Beta-carotene as a skeletal formula A long zigzag chain of carbon atoms with a six-carbon ring at each end and short methyl branches along the chain. Eleven highlighted double bonds take turns with single bonds: nine along the chain and one in each ring.
Beta-carotene, C₄₀H₅₆, the orange of carrots. Its 11 double bonds (highlighted) take turns with single bonds from one ring to the other.

Lycopene, the red of tomatoes and watermelon, has the same formula, C₄₀H₅₆, and also a run of 11 alternating double bonds, all in one straight chain, plus two more double bonds near its ends, set apart from the run. In beta-carotene, the double bond at each end sits in a ring, and the ring twists a little out of line with the chain, so that run behaves as if it were a little shorter. Lycopene's run lies flat from end to end. It absorbs further toward red, blue-green light around 470 nm, and tomatoes look red.

Quick check

Which of these absorbs light furthest toward the red end?