UV Spectroscopy Lecture Notes PDF

LECTURE NOTES ON UV & VISIBLE SPECTROSCOPY ENGINEERING CHEMISTRY B.Tech 1st year By Dr. Ranvijay Pratap Singh Assistant Professor Faculty of Engineering & Technology University of Lucknow  ULTRAVIOLET (UV) & VISIBLE SPECTROSCOPY...

LECTURE NOTES ON UV & VISIBLE SPECTROSCOPY ENGINEERING CHEMISTRY B.Tech 1st year By Dr. Ranvijay Pratap Singh Assistant Professor Faculty of Engineering & Technology University of Lucknow  ULTRAVIOLET (UV) & VISIBLE SPECTROSCOPY  Ultraviolet and visible spectroscopy also known as electronic spectroscopy is used to measure the number of conjugation of double bond and aromatic conjugation within the molecule.  It involves the promotion of electrons from HOMO to LUMO (HOMO means Highest Occupied Molecular Orbital whereas LUMO means Lowest Unoccupied Molecular Orbital).  The HOMO-LUMO gap decrease as the conjugation increase.  The ultraviolet region corresponds to 400-200 nm and visible region to 800- 400 nm.  UV-visible spectroscopy based on Beer-Lambert law. I log o⁄I = ε. 𝑙. 𝑐 or ϵ = 𝐴/𝑐 𝑙 Where, Io = intensity of incident light I = intensity of transmitted light ε = molar absorptivity l = path length of sample c = concentration of sample A = Absorbance  Absorbance (A): It is reciprocal of Transmittance 1 Io Absobance (A) = Optical density (D) = log or log T I  Transmittance (T): The fraction of incident light transmitted is known as Transmittance.  I Transmittance (T) = Io Electronic transitions When molecule is getting excited by the absorption of electromagnetic radiation in UV-visible region then its electrons are promoted from ground state to excited state or from bonding orbital to anti-bonding orbital. Types of electronic transitions a) 𝜎 − 𝜎* Transition: transition of an electron from bonding sigma orbital (𝜎) to anti-bonding sigma orbital (𝜎 *), is represented by 𝜎 − 𝜎 * transition. For example, alkanes because in alkane all the atoms are held together with sigma bond. b) 𝑛 − 𝜎 * & 𝑛 − 𝜋 * Transition: Transitions from non-bonding molecular (𝑛) orbital to anti-bonding sigma orbital or anti-bonding pi orbital (𝜋 *), are represented by 𝑛 − 𝜎 * or 𝑛 − 𝜋 * transition respectively. These transition required less energy than 𝜎 − 𝜎 * transition. For example, alkyl halide, aldehydes, ketones etc. c) 𝜋 − 𝜋 * Transition: This type of transition generally show in unsaturated molecules like alkenes, alkynes, aromatics, carbonyl compounds etc. This transition required less energy as compare to 𝑛 − 𝜎* transition.  n-    n-*  * Energy Absorption, intensity shift & UV spectrum a) Bathochromic shift: It is also known as Red shift, in this case absorption shift towards longer wavelength (𝜆max). b) Hypsochromic shift: It is also known as Blue shift, in this case absorption shift towards shorter wavelength (𝜆max). c) Hyperchromic shift: Intensity of absorption maximum (𝜀 max) increases. d) Hypochromic shift: Intensity of absorption maximum (𝜀 max) decrease. Absorption & intensity shift UV spectrum of unknown compound Chromophores & Auxochromes a) Chromophores: Chromophore is a covalently unsaturated group absorbed electromagnetic radiation in UV-visible region and impart color to the compound. For example, C=C, C≡C, benzene, NO2 etc. b) Auxochromes: Auxochrome is a saturated group (containing lone pair of electron), when auxochrome attach to the Chromophore absorption shift towards longer wavelength. For example, -OH, -SH, -NH etc. Factor affecting absorbance & intensity a) Conjugation Absorption shift towards longer wavelength, if double bonds (chromophore) present in the molecule are in conjugation. For example, In compound ‘A’, double bonds are in conjugation therefore ‘A’ possessing higher wavelength (𝜆max) as compare to compound ‘B’ (non-conjugated derivative). We know that, ℎ𝑐 E= 𝜆 As the conjugation increase transition energy (E) between the orbitals is decrease and therefore wavelength (𝜆max) increase. b) Effect of solvent: In polar solvent, 𝜋 − 𝜋 * transitions shift towards longer wavelength (Red shift), because dipole interactions with polar solvent molecules lower the energy of excited state (𝜋*) than that of the ground state (𝜋* orbital’s are stabilized by hydrogen bonding with polar solvent). Whereas in polar solvent, 𝑛 − 𝜋* transitions shift towards lower wavelength (Blue shift), because dipole interactions with polar solvent molecules lower the energy of ground state (𝑛) than that of the excited state (𝑛 orbital’s are stabilized by hydrogen bonding with polar solvent). The Woodward-Fieser rules (Calculation of 𝝀max (nm) in conjugated dienes) 𝜆max (nm) Base value: Acyclic or heteroannular dienes 214 Homoannular dienes 253 Increments for: Double bond extending conjugation 30 R alkyl substituent or ring residue 5 Exocyclic double bond 5 Polar groupings: -OCOCH3 0 -OR 6 -Cl, -Br 5 -NR2 60 Explanation of terminology used in Woodward-Fieser rules The Woodward-Fieser rules (Calculation of 𝝀 max (nm) in α,β-unsaturated carbonyl compounds) 𝜆max (nm) Base value: Acyclic or 6-membered cyclic ketone 215 5-membered cyclic ketone 202 Aldehyde 210 Increments for: Homocyclic diene component 39 Double bond extending conjugation 30 R alkyl substituent or ring residue 𝛼 = 10, 𝛽 = 12, 𝛾 & ℎ𝑖𝑔ℎ𝑒𝑟 = 18 Exocyclic double bond 5 Polar groupings: -OH 𝛼 = 35, 𝛽 = 30, 𝛾 = 30 -OR 𝛼 = 35, 𝛽 = 30, 𝛾 = 17 -Cl, 𝛼 = 15, 𝛽 = 12, 𝛾 = 12 -Br 𝛼 = 25, 𝛽 = 30, 𝛾 = 25 -NR2 𝛽 = 95 -OCOCH3 𝛼 = 6, 𝛽 = 6, 𝛾 = 6 Application of UV spectroscopy i) Extent of conjugation: As the double bond increase, absorption shift towards longer wavelength. ii) Distinction between conjugated and non conjugated compounds: Both the above compound A & B are isomer of each other. Due to 𝑛 − 𝜋* transition, compound A appear at longer wavelength than compound B. iii) Study of geometric isomerism: In isomer A, due to steric hindrance, absorption shift towards lower wavelength. We know that, steric hindrance destabilize the compound means energy of the molecule increase which decreases the wavelength. iv) Used in the detection of impurities: Due to presence of impurities in the compound, give additional peaks are observed in UV spectrum.  Exercise 1. What is electronic spectroscopy? 2. What is Chromophore? How does it differ from auxochrome? 3. Discuss how conjugation affects the intensity of absorption band. 4. Write the short note on electronic transitions. 5. Write notes on application of UV-visible spectroscopy. 6. Explain the term Hypsochromic shift and Bathochromic shift. What structural features may lead to these shifts in organic molecules? 7. Explain the effect of polar solvent on 𝜋 − 𝜋* and 𝑛 − 𝜋* transition. 8. How 1,3-pentadiene and 1,4-pentadiene can be distinguished by UV spectroscopy? 9. Why aniline behaves like that of benzene in acidic medium? 10.Two isomeric dienes ‘A’ and ‘B’ having the molecular formula C 5H8 absorb at 𝜆max 225 nm and 𝜆max 180 nm respectively. Write the structure of the two isomers. 11. Find out the 𝜆max of following molecules;