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Photochemistry

Photochemical reactions

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Photochemistry

  1. 1. 08/04/18 1 Shri Bapu R. Thorat Assit. Professor of Chemistry Govt. of Maharashtra, I. Y. College, Jogeshwari (E), Maharashtra 400060
  2. 2. Thermal and photochemical reactions Thermal reactions: Thermochemistry is the study of the energy and heat associated with chemical reactions. If a reaction releases energy and heat, it is called exothermic reaction. The opposite is the endothermic reaction, when energy and heat is absorbed. Photochemical reactions: Some reactions do not take place in the dark but take place only in the presence of light or some other radiation. Such reactions are known as ‘photochemical reactions’ Thermochemical reactions Photochemical reactions These reactions involve absorption or evolution of heat. These reactions involve absorption of light, highly specific, energy absorb depends on type of electronic transition. They can take place even in absence of light i.e. dark. The presence of light is the primary requisite for the reaction to take place. Temperature has significant effect on the rate of a thermochemical reaction. Temperature has very little effect on the rate of a photochemical reaction. Instead, the intensity of light has a marked effect on the rate of a photochemical reaction. The free energy change ∆G of a thermochemical reaction is always negative. The free energy change ∆G of a photochemical reaction may not be negative. They are accelerated by the presence of a catalyst. Some of these are initiated by the presence of a photosensitizer. However a photosensitizer acts in a different way than a catalyst. 2B R Thorat
  3. 3. The distance and size scales of the electromagnetic spectrum
  4. 4. 08/04/18 4 Sigma, Pi and non- bonding electrons Organic molecule Light Electronic excitation orbital of higherorbital of higher energy (E.S.)energy (E.S.) orbital of lowerorbital of lower energy (G.S.)energy (G.S.) The energy of the radiation is nearly equal to or greater than the energy difference between ground state and excited state excited state or alternative excited may take part in the chemical reaction Photochemical Reactions Intramolecular reactions (rearrangement, dissociation, etc.) Intermolecular (addition reaction) The use of uv-radiations below 200 nm wavelength is inconvenient because of it absorbed by air (excitation of air). At room temperature, initially almost all molecules are present in the ground state (vibrational) So. The excited states are denoted as S1 and T1. It is denoted as S1 (excited single state- the state in which the excited electron and its original partner have opposite spin) and T1 (excited electron and its original partner have same spin). S1 T1 So So So Primary process Secondary process
  5. 5. Terms used in photochemistryTerms used in photochemistry Triplet state is normally are more stable than the corresponding singlet states because, by Hund's rule, less inter-electronic repulsion is expected with unpaired than paired electrons. States with paired electrons are called singlet statesStates with paired electrons are called singlet states Spin multiplicity (2S+1) (S1 ) Spin multiplicity = 1 Excited states also can have unpaired electrons with parallel spins triplet states (T) Spin multiplicity = 3 Paramagnetic, low energy, stable 5B R Thorat The ground states of most molecules all electrons are paired with opposite spin (S0) The excited states also can have unpaired electrons with opposite spin The excited states also can have unpaired electrons with opposite spin (2x (+1/2-1/2)+1=1 Diamagnetic, high energy, less stable
  6. 6. 08/04/18 6
  7. 7. 08/04/18 7 Photosensitization DonorAcceptor Chemical reaction
  8. 8. 08/04/18 8 Jablonski diagram The electronic excitation and de-excitation of the organic molecules AB* AB hv excited molecule Different chemical species Chemical reaction Dissociation Ionization Luminescence Fluoroscence Phosphorescence Quenching Intermolecular energy transfer Intramolecular energy transfer +M +CD AB + CD* AB$ Photosensitizatio AB + hv' AB+ + e- A + B Fragmentation (free radicals) AB + M S0 1. Excitation or absorption 2. Vibrational relaxation 3. Internal conversion (come to Lower energy state without emission of light) 4. Inter system crossing (ISC) 5. Fluorescence 6. Phosphorescence 7. Photosensitization 8. Photochemical reaction
  9. 9. 08/04/18 9 S0 hv S1 singlet Flurescence or Internal conversion ChemicalChemical ReactionReaction + Y ISC Fragmentation S0 S0 + Y (S1)Collision Sensitization T1 triplet Two species ChemicalChemical ReactionReaction + Y S0 + Y (T1)Collision Sensitization Fragmentation Two species Phosphorescence or Intersystem crossing S0 The life time of excited singlets are very short (10-6 to 10-9 ) so that chemical reactions occurs via these species is very uncommon due to their high energy. Therefore it is converted to its triplet state which is more stable having life time (> 10-4 ) so that the probability of the triplet state taking part in the reaction is much higher than for a singlet state.
  10. 10. Photochemical reactions A reaction which takes place by absorption of the visible and ultraviolet radiations is called a photochemical reaction Ground state Visible/ UV Occurs in 10−16 –10−15 s first-order photochemical reaction is about 1016 s−1 Occurs after absorption in 10−12 –10−6 s intersystem crossing (ISC) in 10−12 –10−4 s Phosphorescenc e in 10−6 –10−1 s As a consequence, excited triplet states are photochemically important. Phosphorescence decay is several orders of magnitude slower than most typical reactions. A energy of excitation is transferred to another molecule by collision so that original molecule comes to ground state with excitation of another molecule (sensitizer). 10B R Thorat
  11. 11. 08/04/18 Spectroscopy 11 σ → σ* Transitions Below 200 nm n → σ* Transitions Saturated hydrocarbon Saturated hydrocarbon containing heteroatom – N, O, S, P, X π → π* Transitions Unsaturated /aromatic compounds with heteroatom : C=S, C=O, C=N, etc n → π* Transitions Unsaturated /aromatic compounds : C=O, C=O, C≡N, etc Carried in vacuum Transition energy decrease with decrease in electronegativity The hydrogen bonding also affects the n → σ* transition because of hydrogen bonding, the non-bonding electron density of heteroatom decreases, thus required higher transition energy. The hydrogen bonding also affects the n → σ* transition because of hydrogen bonding, the non-bonding electron density of heteroatom decreases, thus required higher transition energy.
  12. 12. 400 nm 700 nm500 nm 71.5 kcal/mol 57.2 kcal/mol 40.8 kcal/mol Ultraviolet Region Chemical Bonds of DNAand Proteins Damaged Infrared Region Chemical Bonds Energy too lowto make or break chemical bonds. X-Rays 0.1 nm 300,000 kcal/mol Microwaves 1,000,000 nm 0.03 kcal/mol Huge energies per photon. Tiny energies per photon.Themal energies at room temperature ca 1 kcal/mole Energy Scales: Why the visible region works for vision
  13. 13. 08/04/18 13 ReactionsReactions Photoreduction of carbonyl compoundsPhotoreduction of carbonyl compounds Carbonyl compounds are converted into 1,2-diols and alcohols by irradiation in presence of a hydrogen donating compounds such as isopropyl alcohol. R R O (Ketone) (S1) (T1) hv R R O R R O R R O R1OH R R OH + R1O R1OH Dimerise R R OH R R OH R HO R (alcohol) (diol) (Ketone)
  14. 14. 08/04/18 14 The alcohol formation is effective when R-radical of alcohol (proton source) is stable (tertiary) and the coupling reaction is dominant when R is the radical stabilizing group. (e.g.-aryl). The coupling reaction is more effective when the reduced alcohol of the starting carbonyl compound is less stable. The reaction occurs in good state only when the triplet is of the (n, π*) type. The triplet state abstracts hydrogen atom from hydrogen donor/second reactant molecule. O R1 R hv O R1 R O R1 R T1 state Excited states S0 state S1 state (n,π∗ ) transition The resulting radical either combine- R1 R OH HO R R1 Diol O R1 R T1 state Alcohol O H H3C H H3C+ propan-2-ol Ketyl radical OH R1 R O H3C H3C + propan-2-one 2 solvent OH R1 R 2 The resulting radical abstract hydrogen from solvent
  15. 15. 08/04/18 15 Photoreduction of carbonyl compoundsPhotoreduction of carbonyl compounds Photoreduction of benzophenone to benzpinacole is carried out by irradiation of a solution benzophenone and isopropyl alcohol with wavelength 340 nm. The benzophenone forms n → π* triplet state at these wavelength but isopropyl alcohol does not shows absorption. In addition with isopropyl alcohol, toluene; cyclohexane; methanol are used as solvent. O Ph Ph hv (340 nm) (CH3)2CHOH Ph Ph OH HO Ph Ph (Benzophenone) (Benzopinacole) O Ph Ph hv (340 nm) (Benzophenone) O Ph Ph O Ph Ph S1 state T1 state Excited statesS0 state MechanismMechanism Ph Ph OH HO Ph Ph (Benzopinacole) Ketyl radical (A) O Ph Ph T1 state O H H3C H H3C+ propan-2-ol OH Ph Ph O H H3C H3C+ O Ph Ph T1 state + Ketyl radical OH Ph Ph O H3C H3C + propan-2-one (B)
  16. 16. 08/04/18 16 Photoaddition ReactionsPhotoaddition Reactions  Photoaddition forming 1:1 adduct by the reaction of an excited state of one molecule with ground state of another.  The molecule which is excited state is carbonyl compound, quinine, aromatic compound, or alkene molecule and the molecule which is in ground state is commonly alkene.  Majority of the photoaddition reactions forming ring product
  17. 17. 08/04/18 17 Photoaddition of alkenes to carbonyl compounds (Paterno-Buchi reaction) The photoaddition of the triplet state of the carbonyl compound with ground state of the alkene forming four membered oxetane rings is called as Peterno-Buchi reaction. It is [2+2] photoaddition reaction. The carbonyl compound is irradiated first rather than olefins. The triplet state of the carbonyl compound (due to n →π* transition) is react with alkene. Ph-CHO Ph-CHO Ph-CHO 3[ ] [ ] 1hv ISC S0 state S1 state T1 state O Ph H O Ph H T.S. (oxetane) O Ph PhH O Ph Ph (oxetane) O Ph Ph + (9 part) (1 part) O Ph Ph H (oxetane)
  18. 18. 08/04/18 18 It is not stereospecific reaction because the rotation about the single bond occurs faster than the spin inversion of electrons. e.g. When benzophenone is irradiated with cis- or trans-2-butene gives same mixture of both isomeric oxetane product. 31 hv ISC S0 state S1 state T1 state O Ph Ph + O Ph Ph O Ph Ph OPh Ph O Ph Ph1 11 1 stable (a) spin inversion (b) O Ph Ph O Ph Ph S0 state spin inversion The ring is formed in two stages. The excited carbonyl triplet state first adds through its oxygen atom to alkene forming mainly more stable biradical species. The biradical species under goes spin inversion forming second bond. T1 state O Ph Ph S0 state O PhPh CH3CH=CHCH3 OPh Ph + hv C-C bond rot. 1 1 C-C bond rot. O Ph Ph When carbonyl compound reacts with unsymmetrical alkene forming more stable biradical species which forming major product (a).
  19. 19. 08/04/18 19 The γ,δ-unsaturated carbonyl compound under goes intramoleculer Peterno-Buchi reaction. O O + O O O COCH3 (cis) When alkene containing electron withdrawing group, it react with carbonyl compound without loss of its stereochemistry. The conjugated diene is also added across the carbonyl compound but yield of the product is less because of competitive triplet sensitized dimerisation of diene. S0 stateT1 state (oxetane) O CH3 CH3 O H3C CH3 CH311 stable NC O H3C H3C + CN CH3 3 H3C NC S0 state T1 state 3 O H H + hv (cis) H O AcO OAc hv T1 stateDimer The carbonyl compounds is also added to allenes in high regeoselective manner as- 31 hv ISC S0 state S1 state (oxetane) O Ph Ph +O Ph Ph O Ph Ph O Ph Ph hv The biacetyl is also reacting with unsaturated ether regeosectively and stereoselectively to give cis-2-alkoxyoxetane ring.
  20. 20. 08/04/18 20 Drawback:- i) The excited energy of ketone is higher than the olefin causes energy transformation from ketone to alkene. The excited alkene undergoes dimerisation. In such cases ketones are called as sensitizer. Therefore to overcome this drawback, the ketone having low triplet state than the alkene is used. E.g. benzophenone. ii) The reaction between the ketone and conjugated diene (whose triplet excited state energy is less than the ketone) is not occurs. iii) The acetylenes also participating in Peterno-Buchi reaction forming unstable oxetene which undergoes ring opening reaction forming unsaturated carbonyl compounds. CH3COCH3 CH3COCH3 [ ] 3 CH3COCH3 +[ ] 3 dimer Ph2CO Ph2CO[ ]3 O Ph Ph +hv hv Ph2CO+ hv CH3 CH3 O Ph Ph O Ph Ph O Ph Ph ..
  21. 21. 08/04/18 21 Ph Ph Ph Ph H HPh Ph . .hv hv Photo-Rearrangement reaction Some compounds undergo rearrangement reaction in presence of light forming structural isomers (where groups or atoms in same molecule occupied different positions) or to valence bond isomers (where groups or atoms have same positions but having different bonding framework). Cis-Trans isomerism: When olefins are irradiated by ultra-violet light, they under goes cis-trans isomerism. This transformation has been carried out by direct irradiation of olefins or irradiation in presence of sensitizer. The isomerism has been occurs through singlet or triplet excited state species. The triplet state has lower barrier to rotation around carbon-carbon bond. The isomerism has been occurs because π bond lost its character in the excited state, here the two sets of the substitutions tend to occupied mutually perpendicular planes so as to minimize the repulsive forces between them.
  22. 22. 08/04/18 22 hv Ph2CO Ph2CO Ph2CO Ph2CO+cis-stilbene trans-stilbene Ph2CO + [PhCH=CHPh cis- + trans- 93% 7% [ ] [ ]1 [ ]3 ]3[ ]3 Br2 Br H RR H H RR Br H H RR H R HR H Br Br. hv + + 2 . . .- The simple olefin absorbs light at 200 nm which is not experimentally convenient therefore isomerism has been carried out by using triplet sensitizer. e.g. Photoisomerism of stilbene has been carried out by using benzophenone triplet sensitizer. There are some photochemical cis-trans isomerism has been carried out by using halogens. Halogen undergoes photochemical decomposition to halogen atom which adds to olefinic double bond to form radical. Elimination of halogen atom from radical gives raise to constant ratio of cis and trans isomers
  23. 23. 08/04/18 23 hv H H hv Intramolecular Photocyclisation Many dienes and polyenes are converted photochemically into cyclic isomers. The reactions are stereospecific and electrocyclic type when proceeds through singlet excited state. In some examples the given organic compounds has been converted into highly strained ring compounds proceeds via triplet sensitized state e.g Irradiation of trans, trans-1,4-dimethyl-1,3-buadiene gives cis-3,4-dimethyl cyclobutene by dis-rotatory ring closer. It explains by orbital symmetry principle e.g. trans,trans-1,6-dimethyl-2,4,6-hexatriene cyclised with con rotatory motion
  24. 24. 08/04/18 24 For acyclic and monocyclic compounds, the rearrangement occurs via singlet transition state. The rearrangement can involved either singlet or triplet excited state and can be proceeds via diradical which are formed due to bonding between C2 and C4. The reaction is more efficient when the substituents are present on the C3 (sp3 ) carbon atom. The reaction is highly stereospecific and occurs with retention in configuration at C1 and C5. hv sensitizer spin inversion . ... . O PhPh O Ph Ph O Ph Ph O Ph Ph OH Ph Ph OH Ph Ph + + hv 1,4-Diene are also under goes photochemical rearrangement to vinyl cyclopropane. If diene containing useless conjugated substituents, reaction proceeds by using triplet sensitizer. It is called as Zimmermann rearrangement This rearrangement is also called as Di-π methane rearrangement. This process involves (1,2)-shift. The mechanism and efficiency of the process is depends on the nature of the system
  25. 25. 08/04/18 25 The same rearrangement is occurs in the β,γ-unsaturated aldehydes and ketones. There are some reactions which forming cage compounds e.g. cyclopentadiene adds readily to para-benzoquinone thermally (π4+π2) manner. The product formed under goes ring closer by irradiation. Nonbornadiene and cyclopentadiene gives cage like compound on irradiation. R O R O R O. . O O O O . . O O + O O O O heat + hv
  26. 26. 08/04/18 26 + hv CH2 n CH2 CH2 CO CH2 n CH2 CH2 CO . . -CO CH2 n CH2 CH2 . . CH2 n CH2 CH2 (a) n-3 CH3(CH2) CH=CH2 CH2=CH2 CH3(CH2) CH=CH2 n-1 (b)(c) Norrish Type-I and Norrish Type-II Reaction The bond dissociation energy of a carbon-carbon bond adjacent to a carbonyl group is small and photo-chemically undergoes hemolytic fission is called Norrish type-I reaction or α- cleavage The –CO-C bond under goes fission forming alkyl and acyl radical. The acyl radical under goes decarboxylation followed by recombination of two radicals or abstract of hydrogen atom or secondary fission to get hydrocarbon and olefin. The carbonyl compound undergoes fission in gaseous or liquid state through (n π*) singlet or triplet state. (a) Recombination, (b) abstract of hydrogen, (c) secondary fission.
  27. 27. 08/04/18 27 CH3-CO-CH(CH3)2 CH(CH3)2 CH3CO+ .. CH3 .+(CH3)2CHCO . hv (A) hv (B) +.. + hv O O O H O H O hydrogen transfer During decomposition more stable radical is formed (alkyl radical). The stability of the alkyl radicals are- 30 > 20 > 10 > methyl > H. The stability of alkyl radical can be important in the unsymmetrical ketones. In above example, the route (a) is currect because of stability of the radical.
  28. 28. 08/04/18 28 R O H R1 R2 R O H R1 R2 R O H R2 R1 R O CH3 R1 R2 + +hv CH3 OH R O H hv O CH3 OD CH3OD O D H O H H hv O Ph OH Ph H OH Ph COOCH3 COOCH3 Ph COOCH3 COOCH3 + COOCH3 COOCH3 Type-II acid A ketone having γ-hydrogen atom is under goes Norrish type-II reaction forming an olefin and enol of a smaller ketone. It is characterized by the transfer of γ-hydrogen atom to a carbonyl group. This transformation has been occurred through six membered transition state forming methyl carbonyl compound and olefin. Both (n π*) singlet and (n π*) triplet are participate in the intramoleculer hydrogen abstraction reaction will the ketone having low lying (π π*) state does not under goes these reaction. The α,β-unsaturated carbonyl compound under goes Norrish Type-II reaction through six membered transition state to get enol (containing γ-hydrogen atom) as-

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