Question

Given that ethylene has a λmax of 175nm, butadiene has a λmax of 220nm, and 2-methyl-1,3-butadiene has a λmax or 215nm, what is the λmax of 2,3,4-trimethylhexatriene?

Answer 1

The λmax of 2,3,4-trimethylhexatriene is 280 nm.

Ethylene has a λmax of 175nm.

Butadiene has a λmax of 220nm.

2-methyl-1,3-butadiene has a λmax or 215nm.

1,3,5-hexatriene has a λmax of 258nm.

Woodward's rules, sometimes known as Woodward-Fieser rules (after Louis Fieser) and named after Robert Burns Woodward, are a number of sets of empirically developed principles that aim to forecast the wavelength of the absorption maximum (max) in an ultraviolet-visible spectrum of a certain molecule.

By using the Woodward Fieser rule,

R- (Alkyl Group) .... +5 nm = 5 × 2 = 10

RO- (Alkoxy Group) .. +6 = 6 × 2 = 12

Adding 22nm to the λmax of 1,3,5-hexatriene as it has 2 alkyl groups and 2 alkoxy groups to form 2,3,4-trimethylhexatriene.

The λmax of 2,3,4-trimethylhexatriene is 280 nm.

Learn more about Woodward-Fieser here:

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