Presentation On Ultra-violet and Visible Spectroscopy Presentation On Ultra-violet and Visible Spectroscopy 2/22/20191

2 Absorption maxima in conjugated system Woodward-fieser rules for calculating absorption maximum in Dienes

2/22/20193 Absorption maxima in Conjugated system CompoundTransition Ethylene17015,000 1,3-butadiene21721,000 1,3,5- hextriene25421,400

2/22/20194 In case of Ethylene CCC C 170nm Energy Figure: Electronic excitation in ethylene

2/22/20195 C C C C In case of 1,3-butadiene C C C C + - HOMO LUMO 217nm Energy Bonding Anti Bonding C C C C As the number of p-orbitals increases which create conjugate system the transition energy between HOMO & LUMO decreases hence wavelength increases.

Woodward- Fieser rule 2/22/20196 Woodward formulated certain empirical rules for calculating the absorption maximum in conjugated dienes. This were latter modified by Fieser in According to these rules, each types of diene has a certain fixed value and the value of absorption maximum depends upon: The number of alkyl substituents or ring residues on the double bond. The number of double bonds which extend conjugation & The presence of polar groups such as –Cl, -Br, -OR etc. Woodward formulated certain empirical rules for calculating the absorption maximum in conjugated dienes. This were latter modified by Fieser in According to these rules, each types of diene has a certain fixed value and the value of absorption maximum depends upon: The number of alkyl substituents or ring residues on the double bond. The number of double bonds which extend conjugation & The presence of polar groups such as –Cl, -Br, -OR etc.

2/22/ Parent values for butadiene system or a cyclic conjugated system 2. Homoannular conjugated diene 3. Heteroannular conjugated diene 4. Acyclic triene 5. Increment for each substituent : Alkyl substituent or ring residue Exocyclic double bond Double bond extending conjugation Woodward- Fieser rule-Conjugated dienes and triens 217 nm 253 nm 215 nm 245 nm 5 nm 30 nm

2/22/20198 Calculation Basic value (homoannular)= 253 nm 4-Ring residue (4×5)= 20 nm 2-exocyclic double bond (2×5)= 10 nm 1-double bond extending conjugation = 30 nm total= 313 nm Basic value (hetero annular)= 215 nm 4-Ring residue (5×5)= 25 nm 3-exocyclic double bond (3×5)= 15 nm 1-double bond extending conjugation = 30 nm total= 285 nm

Thanks to all……… 2/22/20199