Advanced Pharmaceutical Analysis-Lecture Two.pptx

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1. Assistant Prof. Dr. Noor H. Naser Aldabagh

2. CH R OM O P HO R E Chromophore is defined as the nucleus or any isolated covalently bonded group responsible for the absorption of light radiation. Any group which exhibits absorption of electromagnetic radiations in the visible or ultraviolet region. As C=C , C=O ,NO2 etc. Some of the important chromophores are carbonyls, acid s , es t e r s , nitr i l e, et h y l ene g r oup s . Assistant Prof. Dr. Noor Hatef Aldabagh 5

3. Absorption Characteristics of Some Common Chromophores Assistant Prof. Dr. Noor Hatef Aldabagh 5

4. AUXOCHROME T h ese a r e c o - o r din a t i v e l y s a tu r a t ed or u n-s a tu r a t ed groups which themselves do not absorb radiation, but enha n c es w he n p r esent along wit h a c h r omoph o r e the abso r bing p r ope r ties of ch r omopho r e. Als o kn o w n a s c ol o ur enhancing g r ou p . All auxochromes have one or more non-bonding pair of electrons. Examples (-NH2 ,-OH ,-OR,-COOH etc) I t e x t end the c on j ug a tion o f a ch r omopho r e by sharing the non-bonding electrons. Assistant Prof. Dr. Noor Hatef Aldabagh 5

5. Assistant Prof. Dr. Noor Hatef Aldabagh 5

6. Effect of Solvent on Absorption The solvent in which the absorbing species is dissolved also has an effect on the spectrum of the species. Peaks resulting from n → π* transitions are shifted to shorter wavelengths (blue shift) with increasing solvent polarity because of increased solvation of the lone pair in the ground state, which lowers the energy of the n orbital. Assistant Prof. Dr. Noor Hatef Aldabagh 5

7. Effect of Solvent on Absorption Assistant Prof. Dr. Noor Hatef Aldabagh 5

8. Effect of Solvent on Absorption Of t en the r e v e r se ( r ed s h ift) i s seen f or π → π* transitions. This is caused by attractive polarization forces between the solvent and the absorbing molecule, which lower the energy levels of both the excited and unexcited states. The effect being greater for the excited state, the energy difference between the excited and unexcited states is slightly reduced, resulting in a small red shift. This effect can also influence n → π* transitions but cannot be observed due to the dominant blue shift resulting from solvation of lone pairs. Assistant Prof. Dr. Noor Hatef Aldabagh 5

9. Effect of Solvent on Absorption Assistant Prof. Dr. Noor Hatef Aldabagh 5

10. For example for Mesityl oxide, following shifts are observed for the two electronic transitions on moving from low polarity solvent hexane to water, which has higher polarity. Assistant Prof. Dr. Noor Hatef Aldabagh 10

11. TERMS USED IN UV- VISIBLE SPECTROSCOPY Assistant Prof. Dr. Noor Hatef Aldabagh 1 1

12. Bathoc h r omic shift( r ed shift) When the absorption maxima(λmax)of a compound shifts to longer wave length, it is known as bathochromic shift or red shift. The effect is due to the presence of auxochrome or by change of solvent. Eg: The n-π* transition for carbonyl compounds experiences bathochromic shift when the polarity of solvent is decreased. Assistant Prof. Dr. Noor Hatef Aldabagh 1 2

13. Hypsochromic shift(blue shift) When the absorption maxima(λmax) of a compound shifts to a shorter wave length, it is known as hypsochromic shift or blue shift. The effect is due to the presence of a group causes removal of conjugation or by change of solvent. Assistant Prof. Dr. Noor Hatef Aldabagh 1 3

14. Aniline shows blue shift in acidic medium science it loses conjugation. (Aniline 280 nm, while anilinium ion 203 nm) Assistant Prof. Dr. Noor Hatef Aldabagh 1 4

15. H ype r ch r omic ef f ect When the absorption intensity (ε) of a compound is increased, it is known as hyperchromic shift. The introduction of auxochrome usually increase the absorption intensity. Assistant Prof. Dr. Noor Hatef Aldabagh 1 5

16. H ypoch r omic ef f ect When the absorption intensity (ε) of a compound is decreased, it is known as hypochromic shift. The introduction of a group that distorts the geometry of molecule usually reduce the absorption intensity. Assistant Prof. Dr. Noor Hatef Aldabagh 1 6

17. SHIFTE AND EFFECTS Assistant Prof. Dr. Noor Hatef Aldabagh 1 7

18. Conjugated System and Unconjugated System Unconjugated System Due to π → π* electronic transition unconjugated alkenes absorb below 200 nm, e.g., ethene shows an absorption at 171 nm. Alkyl substituents or the ring residues attached to the olefinic carbon shift the absorption band towards longer wavelength as can be seen with absorption pattern of compounds like 1-Octene and cyclohexene which absorb at 177 nm and 182 nm, respectively. Assistant Prof. Dr. Noor Hatef Aldabagh 1 8

19. Conjugated System and Unconjugated System Conjugated System Multiple bonds that are alternative with single bonds are said to be conjugated Thus 1,3-butadiene ……… conjugated system. 1,4- pentadiene …… non conjugated system. Assistant Prof. Dr. Noor Hatef Aldabagh 1 9

20. More stable than non-conjugated one because during resonance there is transfer of electron and it will produce some partial double bond character between C2 and C3 thus strengthen C2-C3 bond and stabilizing the molecule. conjugated system had to red shift (increase wave length ) because the π electron spread over at least 4 atomic center Generally conjugated system absorb 15 –45 nm, longer wave length than non-conjugated system. Assistant Prof. Dr. Noor Hatef Aldabagh 20

21. Many of the pigments responsible for the brilliant color of fruit and flowers are said to be conjugated polyene e.g: Lycopene. Compound containing series of conjugated chromophore groups will appear colored to the eye So, absorbed at equal or more than 400 nm If compounds appear colored contain not less than 4,usually 5 or more conjugated chromophoric and au x och r omic g r oups e x c ept a z o , di az o c ompound s . If two or more chromophoric group are present in a molecule and they are conjugated the effects are usually additives. 2 1 Assistant Prof. Dr. Noor Hatef Aldabagh

22. Woodward-Fieser Rules for Dienes Cisoid diene 2 2 Assistant Prof. Dr. Noor Hatef Aldabagh Transoid diene P a r ent v alues Homoannular Heteroannular

23. Substituents and Influence on λmax 2 3 Assistant Prof. Dr. Noor Hatef Aldabagh

24. E x amples f or W a v e lengt h Cal c ula t ion 2 4 Assistant Prof. Dr. Noor Hatef Aldabagh

25. E x amples f or W a v e lengt h Cal c ula t ion 2 5 Assistant Prof. Dr. Noor Hatef Aldabagh

26. E x amples f or W a v e lengt h Cal c ula t ion H ome W o r k ; ca l cula t e t h e w a v e l ength of the following compounds. 2 6 Assistant Prof. Dr. Noor Hatef Aldabagh