Q8(c). UV of enones; charge-transfer complexes; mass spectra
(i) Explain the effect of conjugation on λ_(max) and intensity due to n-π^* and π-π^* transitions in 3-buten-2-one compared to acetone.
(ii) Explain the formation of charge-transfer complex between (A) picric acid and anthracene, and (B) tetracyanoethylene and aniline, using electronic spectroscopy.
(iii) Identify the molecular ion peak(s) and peaks due to McLafferty rearrangement in the mass spectrum of ethyl 4-chlorobenzoate.
(iv) Write the structures of ions at m/z 108, 93, 78, 77 and 65 in the mass spectrum of anisole.
Answer in brief
(i) Conjugation shifts both bands of but-3-en-2-one to longer wavelength (219 and 324 nm); the π→π^* band becomes much stronger, the n→π^* stays weak. (ii) Both pairs show a new visible CT band, hν_(CT)≈ I_D-E_A-C. (iii) M⁺. 184/186 (3: 1); McLafferty ion 156/158. (iv) 108 M⁺., 93 C₆H₅O⁺, 78 C₆H₆⁺., 77 C₆H₅⁺, 65 C₅H₅⁺.
Key points
- π→π^*: acetone about 190 nm (ε≈ 1000) → 219 nm (ε≈ 3600–10 000), much more intense (allowed, larger transition dipole over four atoms).
- acetone 279 nm (ε≈ 15) → 324 nm (ε≈ 25); still weak, because n and π^* are orthogonal (symmetry-forbidden).
- (A) Picric acid + anthracene: picric acid, with three NO₂ groups, is a π-acceptor; anthracene is an electron-rich π-donor. The 1: 1 π-stacked complex is a red crystalline picrate with a CT band in the visible; such picrates…
- (B) TCNE + aniline: tetracyanoethylene is one of the strongest π-acceptors (four CN groups); aniline donates from its N lone pair and ring π system. The solution turns deep coloured (CT band in the visible); on standing the…
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
Introduction. Electronic spectra measure the energy gap between filled and empty orbitals; conjugation and donor–acceptor contact both lower this gap. Mass spectra record the radical cation M⁺. and its fragments; heteroatoms, isotope patterns and rearrangements (McLafferty) identify the structure.
(i) Acetone and but-3-en-2-one
Conjugating C=O with C=C mixes the two π systems: the new HOMO (π₂) is higher and the new LUMO (π₃^*) is lower than in acetone. Both transitions move to longer wavelength.
- π→π^*: acetone about 190 nm (ε≈ 1000) → 219 nm (ε≈ 3600–10 000), much more intense (allowed, larger transition dipole over four atoms).
- n→π^*: acetone 279 nm (ε≈ 15) → 324 nm (ε≈ 25); still weak, because n and π^* are orthogonal (symmetry-forbidden).
Figure: Conjugation lowers π^* and raises π: both gaps shrink.
(ii) Charge-transfer complexes
A donor D (high HOMO, low ionisation energy I_D) and an acceptor A (low LUMO, high electron affinity E_A) form a weak complex. Its ground state is mostly D·A; its excited state is D⁺A^ ext−. Light promotes an electron from the donor HOMO to the acceptor LUMO, giving a new broad band, absent in both components, at hν_(CT) ≈ I_D - E_A - C (Mulliken; C = Coulomb attraction in D⁺A^ ext−).
- (A) Picric acid + anthracene: picric acid, with three NO₂ groups, is a π-acceptor; anthracene is an electron-rich π-donor. The 1: 1 π-stacked complex is a red crystalline picrate with a CT band in the visible; such picrates are used to identify and purify arenes.
- (B) TCNE + aniline: tetracyanoethylene is one of the strongest π-acceptors (four CN groups); aniline donates from its N lone pair and ring π system. The solution turns deep coloured (CT band in the visible); on standing the complex reacts further to 4-(tricyanovinyl)aniline.
A stronger donor (lower I_D) shifts the CT band to longer wavelength; this linear relation of hν_(CT) with I_D is the spectroscopic proof of charge transfer.
(iii) Ethyl 4-chlorobenzoate (C₉H₉ClO₂)
- Molecular ion: m/z 184 and 186 in about 3: 1 ratio (35Cl: 37Cl).
- McLafferty rearrangement: a γ-H of the ethoxy CH₃ moves to the C=O oxygen through a six-membered transition state; ethene (28) is lost, giving the 4-chlorobenzoic acid radical cation at m/z 156/158 (3: 1).
- Other ions: acylium ClC₆H₄CO⁺ at m/z 139/141 (loss of the ethoxy radical, 45; base peak) and ClC₆H₄⁺ at 111/113 (139 - CO).
Scheme: Six-membered transition state: γ-H of the ethyl group moves to the carbonyl O.
(iv) Ions in the mass spectrum of anisole
| m/z | Ion | Origin |
| 108 | C₆H₅OCH₃⁺. | molecular ion |
| 93 | C₆H₅O⁺ (phenoxy cation) | M⁺. minus CH₃ radical (15) |
| 78 | C₆H₆⁺. (benzene radical cation) | M⁺. minus CH₂O (30): H moves from OCH₃ to the ring (four-centre rearrangement) |
| 77 | C₆H₅⁺ (phenyl cation) | M⁺. minus OCH₃ radical (31) |
| 65 | C₅H₅⁺ (cyclopentadienyl cation) | 93 minus CO (28), ring contraction |
Conclusion. (i) Conjugation shifts both bands of but-3-en-2-one to longer wavelength (219 and 324 nm); the π→π^* band becomes much stronger, the n→π^* stays weak. (ii) Both pairs show a new visible CT band, hν_(CT)≈ I_D-E_A-C. (iii) M⁺. 184/186 (3: 1); McLafferty ion 156/158. (iv) 108 M⁺., 93 C₆H₅O⁺, 78 C₆H₆⁺., 77 C₆H₅⁺, 65 C₅H₅⁺.
Application. Charge-transfer complexes are the basis of organic conductors (TTF–TCNQ) and of colour tests for aromatic drugs; the chlorine 3: 1 isotope pattern is used to detect chlorinated pesticides (DDT, PCBs) by GC–MS.
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All parts of Question 8
- 8(a) Rotational constant; vibrational order; chelated C=O and OH15 marks
- 8(b) 1H NMR: furfural, arenes, ethanol, nitroacetophenones15 marks
- 8(c) UV of enones; charge-transfer complexes; mass spectra20 marks