ORGANIC CHEMISTRY: ISOPRENE Many biologically important molecules, such as abscisic acid (a plant growth regulator), cholesterol (a component of cell membranes), natural rubber, oestrogen (a female sex hormone), retinal (a photochemical pigment in the mammalian eye), and testosterone (a male sex hormone), are based on a repeating five-carbon unit known as isoprene, "T", which is biosynthesized from isopentenol, "U".
1. Suggest the reason why 'diene' is present in the systematic name of isoprene (2-methylbuta-1,3-diene). ____________________________________  2. Manufacturers require far greater amounts of isoprene than can be obtained from natural sources. Suggest how this synthetic intermediate is obtained industrially in bulk quantities. __________________________ _______________________________________________________________________  3. Partial hydrogenation of isoprene gives two liquid alkenes, "V" and "W": whereas complete hydrogenation gives liquid alkane "X".
(a) Suggest one physical method, apart from chromatography, which could distinguish between isomeric alkenes "V" and "W". _____________________  (b) Describe how you would show that a sample of "X" contained neither "V" nor "W". __________________________________________________________ _______________________________________________________________________ _______________________________________________________________________  (c) Saturation of diene "T" with the radioactive isotope of hydrogen-3 (tritium), gives labelled alkane "Y".
If this isotope of hydrogen (³H) has a half-life of 12.5 years, what fraction of the labelled alkane "Y" would be present after 37.5 years? _______________________________________________________________________  4. In a living organism, alcohol "U" is enzymically dehydrated to diene "T". Name one catalyst which can speed up a similar dehydration in the laboratory. ____________________________________________________ 
5. Natural rubber, "Z", is poly(isoprene). This hydrocarbon addition polymer, which occurs as an aqueous suspension in the vascular system of a number of plants, is usually extracted by 'tapping' rubber trees (Hevea brasiliensis).
(a) Addition polymers and their corresponding monomers have identical empirical formula, despite vastly different molecular formulae; e.g., poly(tetrafluoroethene), (-C2F4-)n, and tetrafluoroethene, C2F4, both have the empirical formula CF2. What is the molecular formula of: Natural rubber? ______________________ Isoprene? ______________________  (b) The molar mass of a monomeric unit in natural rubber is 68 g mol-¹. If the molar mass of a sample of natural rubber is 1.02 x 10³ kg mol-¹, calculate the value of 'n' in the molecular formula. __________________ _______________________________________________________________________  (c) Ozone, when produced by a complex series of photochemical reactions involving nitrogen oxides and unburnt hydrocarbons emitted from vehicle exhausts, is a component of photochemical smog. In sharp contrast to its rôle in the upper atmosphere, protecting the biosphere from harmful u.v. radiation, ozone causes the destruction of a diverse range of synthetic and naturally occurring molecules. For example, the cracks that appear in vehicle tyres often result from the corrosion of rubber by ozone; such reactions can be summarized by this first scheme:
Complete the structural formulae shown in this second scheme, so as to summarize the effect of ozone on the biosynthetic intermediate "U".
 Suggest one type of naturally occurring molecule, which is involved in growth or in reproduction, that might also be destroyed (or modified) by the ozone present in smog. _________________________________________  (d) The major repositories for the biosphere's gene bank include the rainforests: so their conservation is essential. The careful management of rubber trees represents one renewable use of these resources. State one non-renewable use of rainforests. _________________________________ _______________________________________________________________________ 
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