Features of an ultraviolet-visible spectrum

The carbon-carbon multiple bond. An isolated carbon-carbon double bond absorbs near 180nm (e 15000) as a result of a n - n electronic transition. Alkyl groups cause small bathochromic shifts so that a tetrasubstituted acyclic alkene absorbs in the region of 200 nm (see Table 3.6). 

An isolated carbon-carbon triple bond similarly absorbs in the 180nm region (acetylene, Amax 173 nm, e % 6000) the absorption maximum is shifted to longer wavelengths by the presence of alkyl groups (Table 3.6). 

Conjugation of multiple carbon-carbon double bonds leads to significant changes in Amax and in the e value. For example, buta-1,3-diene has Amax 217 nm, e 

and there is a regular bathochromic shift with an increase in the number of conjugated double bonds (Table 3.7). Alkyl substituents attached to the multiply bonded carbon atoms have also been found to lead to uniform increments (5 nm) in the wavelength of absorption. 

Conjugation of the carbon-carbon triple bond with other triple bonds (poly-ynes) or with carbon-carbon double bonds (polyenynes) also leads to progressive shifts of Amax to higher wavelengths. In acyclic systems such absorptions are frequently of diagnostic value in deciding the extent of conjugation. 

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