Ultraviolet (UV) spectroscopy is an important spectroscopic technique used for the identification and differentiation of organic compounds containing chromophores, conjugated systems, aromatic rings and other groups capable of absorbing ultraviolet radiation. The absorption of UV radiation causes electronic transitions from lower-energy molecular orbitals to higher-energy molecular orbitals. Since different compounds possess different electronic structures, the position and intensity of their absorption bands are characteristic and can be used to distinguish between them. The most important electronic transitions observed in organic compounds are σ → σ*, n → σ*, π → π* and n → π*. The σ → σ* transition requires very high energy and generally occurs below 200 nm, so it is of limited value for routine identification. The n → σ* transition is observed in compounds containing heteroatoms such as oxygen, nitrogen and sulfur. The π → π* transition is particularly useful for compounds containing double bonds, aromatic systems and conjugated unsaturation. The n → π* transition is commonly observed in carbonyl compounds and other molecules containing heteroatoms with lone-pair electrons. The differentiation of organic compounds on the basis of UV spectra mainly depends on λmax, absorption intensity, number of absorption bands and the effect of conjugation and substituents on the absorption maximum. λmax is the wavelength at which maximum absorption occurs. A compound with greater conjugation generally absorbs at a longer wavelength because conjugation decreases the energy difference between the ground and excited electronic states. This movement of the absorption maximum towards a longer wavelength is known as a bathochromic shift or red shift. A movement towards shorter wavelength is called a hypsochromic shift or blue shift. For example, ethene and 1,3-butadiene can be differentiated by their UV spectra. Ethene contains one isolated double bond and therefore shows a π → π* transition at a relatively short wavelength, generally below 200 nm. In 1,3-butadiene, the two double bonds are conjugated. Conjugation permits greater delocalisation of π electrons and lowers the energy required for the electronic transition. Consequently, 1,3-butadiene absorbs at a longer wavelength than ethene. Thus, the longer λmax of 1,3-butadiene provides evidence for conjugation. Similarly, isolated and conjugated carbonyl compounds can be distinguished by UV spectroscopy. A saturated ketone such as acetone possesses a carbonyl group and shows both