Chemical Change Chapter 2 Dr. Suzan A. Khayyat 1 types of chemical reaction Chemical reactions Thermal chemical Reaction Photochemical Reaction Photooxidation Reaction Photoaddition Reaction Photohydrogenation Pericyclic Reaction Photodissociation Dr. Suzan A. Khayyat 2 • The Jablonski Diagram • The energy gained by a molecule when it absorbs a photon causes an electron to be promoted to a higher electronic energy level. Figure 3 illustrates the principal photophysical radiative and non-radiative processes displayed by organic molecules in solution. The symbols So, S1, T2, etc., refer to the ground electronic state (So), first excited singlet state (S1), second excited triplet state (T2), and so on. The horizontal lines represent the vibrational levels of each electronic state. Straight arrows indicate radiative transitions, and curly arrows indicate nonradiative transitions. The boxes detail the electronic spins in each orbital, with electrons shown as up and down arrows, to distinguish their spin. • Note that all transitions from one electronic state to another originate from the lowest vibrational level of the initial electronic state. For example, fluorescence occurs only from S1, because the higher singlet states (S2, etc.) decay so rapidly by internal conversion that fluorescence from these states cannot compete. Dr. Suzan A. Khayyat 3 Jablonski energy diagram Singlet State (S 1,S2, ......) 1(n, Triplet State (T1, T2, ...) Photochem. A b s o r p t i o n F l u o r e s c e n c e ISC Biological Response n photochem. & singlet oxygen Phosphorescence Ground State So Jablonski energy diagram Dr. Suzan A. Khayyat 4 Jablonski diagram • Figure 3. The basic concepts of this Jablonski diagram are presented in the Basic Photophysics module. This version emphasizes the spins of electrons in each of the singlet states (paired, i.e., opposite orientation, spins) compared to the triplet states (unpaired, i.e., same orientation, spins). Dr. Suzan A. Khayyat 5 Photooxygenation Reaction 1 hv Sens (S0) 1 Sens* (S1) 3 * Sens (T1) + 3O 2 1 * Sens (S1) 3 * Sens (T1) 1 1 Sens (S0) + O2 Dr. Suzan A. Khayyat 6 )1O2( 1 + g 1 g 37.5 Kcal/mol 3 g 22.4 Kcal/mol 1 Highest occupied molecular orbital of O2 Dr. Suzan A. Khayyat 7 C6H5 C6H5 H3C N H N H H3C N OH CH CH3 N CH3 N HOOCH2C-H2C H N N CH CH3 OH H N Cl C6H5 C6H5 Tetraphenylporphyrine (TPP) Cl Cl HOOC-H2C-H2C CH3 Hematoporphyrine( HP) Cl COONa I I ONa O O I I Ros Bengal(RB) Dr. Suzan A. Khayyat 8 Types of singlet oxygen reactions H 1 + 1) A O2 OOH O X 1 2) + 3) + O2 1 O2 O X B C Dr. Suzan A. Khayyat O O 9 1- Ene Reaction O*2 H C C O OH C C C C Cis cyclic mechanism for the reaction of 1O2 with mono-olefins. Dr. Suzan A. Khayyat 10 Dr. Suzan A. Khayyat 11 H OOH C C C + 1 O2 Dr. Suzan A. Khayyat C C C 12 Dr. Suzan A. Khayyat 13 Dr. Suzan A. Khayyat 14 Dr. Suzan A. Khayyat 15 2-Cycloaddition Reaction (Diels Alder) Dr. Suzan A. Khayyat 16 Direct addition reaction to produce(1,2-dioxetane) Dr. Suzan A. Khayyat 17 Dr. Suzan A. Khayyat 18 Dr. Suzan A. Khayyat 19 Dr. Suzan A. Khayyat 20 Photosensitized oxidation H3C + CH3 O O2 hv , sens H3C O O H3C CH3 C H3C C CH3 O H3C + O2 hv , sens CH3 CH2 C H3C C OOH O C2H5O-CH=CH-OC2H5 + O2 hv , sens Dr. Suzan A. Khayyat CH3 O C2H5O-CH-CH-OC2 H5 21 Photodissociation: processes and examples • Hydrocarbons: / RCH2R + CH2=CH2+ hv RCR/ hv H2 + H2C=C: ( + H2 HC CH) 2H + H2C=C: H2 + HC CH 2H + HC CH Dr. Suzan A. Khayyat 22 Carbonyl Compounds 1- Keetones: • Norrish Type I: The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates. The carbonyl group accepts a photon and is excited to a photochemical singlet state. Through intersystem crossing the triplet state can be obtained. On cleavage of the α-carbon carbon bond from either state, two radical fragments are obtained. Dr. Suzan A. Khayyat 23 Dr. Suzan A. Khayyat 24 Dr. Suzan A. Khayyat 25 Norrish type II • A Norrish type II reaction is the photochemical intramolecular abstraction of a γ-hydrogen (which is a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct Dr. Suzan A. Khayyat 26 Dr. Suzan A. Khayyat 27 Dr. Suzan A. Khayyat 28 RCHO + hv RH + 2C2H4 + C=O + CO + hv CO CO CH2=CHCH2CH2 CHO Dr. Suzan A. Khayyat 29 Complete the next equations O H2C hv H2C hv O Dr. Suzan A. Khayyat 30 O CH3 H3C hv C H2 O CH3 hv H3C CH3 CH3 Dr. Suzan A. Khayyat 31 2- Esters: hv RCH2CH2CH2 COOR\ RCH=CH2 + hv \ RCOOCH2CH2R RCOOH Dr. Suzan A. Khayyat + CH3COOR\ CH2=CHR\ 32 Photocycloaddition 2+2 Intermolecular cycloaddition O O R + hv H3CO H3CO R R\ OCH3 O OCH3 R\ O O Dr. Suzan A. Khayyat 33 Dr. Suzan A. Khayyat 34 O 2 O O O hv + O Dr. Suzan A. Khayyat 35 2+2 Intramolecular cycloaddition hv Dr. Suzan A. Khayyat 36 2+4 Cycloaddition + Dr. Suzan A. Khayyat 37 hv + Dr. Suzan A. Khayyat 38 Dr. Suzan A. Khayyat 39 Photoaddition and photocyclization reactions NH2 H N hv H N + + + Dr. Suzan A. Khayyat 40 Direct and photosensitized reactions direct trans sensitized cis Dr. Suzan A. Khayyat 41 Isomerization and rearrangements Dr. Suzan A. Khayyat 42 direct Triplet sensitized Dr. Suzan A. Khayyat 45 hv H hv H + + Benzvalene Dr. Suzan A. Khayyat bicyclohexadiene fulvene 46 hv CN C6H5 C6H5 C6H5 Dr. Suzan A. Khayyat C6H5 CN 47 Synthetic applications of electrocyclisation reactions: The conversion of ergosterol to vitamin D2 proceeds through a ring-opening (reverse) electrocyclisation to give provitamin D2, which then undergoes a second rearrangement (a [1,7]sigmatropic shift). Stereochemical control in the sigmatropic shift process will be described in a later section of this course. H sunlight H HO ergosterol H photochemicallypromoted electrocyclisation (antarafacial, conrotation) H provitamin D2 HO [1,7]-sigmatropic shift. H HO Dr. Suzan A. Khayyat vitamin D2 48 Photochemistry in solution O H2 (CH3) C C H2 C liq (CH3) CO + C3H8 + H3C CHCHO gas H2 (CH3)2 C Dr. Suzan A. Khayyat O C O C H2 C (CH3)2 49 Factors determining reactivity • 1• • • • The excess energy possessed by the species (which may help overcome activation barriers). 2- The intrinsic reactivity of the specific electronic arrangement. 3- The relative efficiencies of the different competing pathways for loss of the particular electronic state. 4- The type of orbital (s, p, σ, or, π, etc.) and its symmetry. 5- Explicit in the correlation rules for orbital symmetry and spin that are introduced first at the end of this section.