Problem 3
How many signals would you expect to see in the proton decoupled \(^{13} \mathrm{C}\) NMR spectra of (a) penta1,4-diene and (b) penta-2,4-diene? Could you distinguish the \((E)\) - and \((Z)\) -isomers of penta- 2,4 diene using \(^{13} \mathrm{C}\) NMR spectroscopy?
Problem 4
(a) Two isomers of cyclooctadiene exhibit two and four signals respectively in their \(^{13} \mathrm{C}\) NMR spectra. Draw the structures of the isomers and assign one of the spectra to each. (b) A third isomer of \(\mathrm{C}_{8} \mathrm{H}_{12}\) has a bicyclic structure and its \(^{13} \mathrm{C}\) NMR spectrum contains three signals. Suggest a structure for this isomer. Rationalize why the \(^{1} \mathrm{H}\) NMR spectrum of this compound has four signals.
Problem 5
Draw the structure of hexa-1,5-diyne and interpret the following spectroscopic data. (a) The IR spectrum contains strong to medium absorptions at 3665,2922,2851 and \(2123 \mathrm{cm}^{-1}\) in addition to bands in the fingerprint region. (b) The \(^{13} \mathrm{C} \mathrm{NMR}\) spectrum has signals at \(\delta 82.4,69.5\) and \(18.6 \mathrm{ppm}\) (c) The \(^{1} \mathrm{H}\) NMR spectrum has singlets at \(\delta 2.06\) and \(2.43 \mathrm{ppm}\)
Problem 7
Draw structural formulae for the following compounds: (a) cyclohepta-1,3-diene; (b) 2 -methylpentene, (c) 4 -methylhexene, (d) 1,2 -dibromocyclopentene and (e) cyclododeca-1,5,9-triene.
Problem 8
Why is a \([2+2]\) cycloaddition driven photolytically, while a \([4+2]\) cycloaddition is thermally promoted?
Problem 11
Values of the bond enthalpy terms for \(\mathrm{C}=\mathrm{C}, \mathrm{C}-\mathrm{C}\) \(\mathrm{H}-\mathrm{H}\) and \(\mathrm{C}-\mathrm{H}\) bonds are 598,346,436 and \(416 \mathrm{kJ} \mathrm{mol}^{-1} .\) Determine the enthalpy change associated with the hydrogenation of \(\mathrm{C}_{2} \mathrm{H}_{4}\) Compare your answer with that determined using values of \(\Delta_{\mathrm{f}} H^{\circ}\) for \(\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{g})\) and \(\mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{g})\) of +52.5 and \(-83.8 \mathrm{kJ} \mathrm{mol}^{-1}\)
Problem 12
(a) Suggest a mechanism for the reaction of HBr with 2 -methylbut-1-ene under polar conditions and indicate the relative product distribution that you might expect. (b) How does the reaction differ if it is carried out in the presence of a radical initiator?
Problem 13
What happens when \(\mathrm{Br}_{2}\) reacts with cyclohexene in the absence of a radical initiator? Include in your answer discussion of the mechanism, evidence for proposed intermediates, and an explanation of the stereochemistry of the product.
Problem 14
Consider the addition of \(\mathrm{HCl}\) to \((Z)\) -but-2-ene. Discuss how the stereochemistry of the product is affected by whether HCl adds in a syn- or antimanner.
Problem 15
Oxidations of alkenes include reactions with \(\mathrm{O}_{3}\) and \(\mathrm{OsO}_{4} .\) Using hex-3-ene as the example, discuss the use of these reagents to produce diols and aldehydes, paying attention to the stereochemistry of the products where appropriate.
