Lecture Three Department of Pharmaceutical Chemistry Title of the course: Advanced Pharmaceutical Analyses Course number: 5210 Lecture Three: by Dr.Abdulbari M.M Al-Helfi Ultraviolet Spectrometry:

[α,β-UNSATURATED KETONES AND ALDEHYDES.] Compounds containing a carbonyl group in conjugation with an ethylenic group are called enones. Spectra of enones are characterized by an intense absorption band (k-band) in the nm region (ɛ max usually 10,000-20,000), and a weak n →π* band (R-band) at nm. The weak R-band is frequently poorly defined.

Since carbonyl compounds are polar, the positions of the K- and R-bands of enones are both dependent on the solvent. The k -bands of enones undergo a bathochromic shift with increasing solvent polarity. The intensity of the k-band may be reduced to less than 10 4 where steric hindrance prevents coplanarity. This fre­quently occurs in cyclic systems, such as

Woodward has derived empirical generalizations for the effect of substitution on the position of the π→ TT * transi­tion (k-band) in the spectra of α,β-unsaturated ketones. The positions of the k- bands that result from substitution in the basic formula. Are summarized in table XII

Some example

Cross conjugation has little effect on the λmax of cholesta-1,4-diene-3-one. The calculations used were for an enone(β,β-disbustituted) with no correction required for the 1,2-doule bond or for the β - group. The corresponding calculation using the ∆ system yield 227nm as the predicted λmax a choice is presented, the more reliable prediction arises from the system that is more highly substituted (longer wavelength TT → TT * transition).