Polyunsaturated molecule

Hydrogen is added to the carbon double bonds of unsaturated or polyunsaturated molecules where not all these double carbon bonds are used. This process changes unsaturated fats (liquids) to saturated fats (solids). 

The selectivities exhibited by dibromoborane-dimethyl sulfide are also interesting. In contrast to the situation with 9-BBN-H, internal alkynes are hydroborated faster than either terminal alkynes or terminal alkenes. Unlike reactions with disiamylborane, 1,1-disubstituted ethylenes are hydroborated in preference to simple 1-alkenes. These properties can lead to interesting possibilities for selective hydroboration of polyunsaturated molecules (e.g. Schemes 8 and 9)." ... 

Char formation in PVC pyrolysis can be explained by further dehydrogenation of the polyunsaturated molecules generated by HCI elimination. 

If the reagent has more than one multiple bond that is capable of undergoing an addition reaction, or even if there is a multiple bond that is capable of reacting with more than one molecule, then that reagent is said to be polyunsaturated. Suggest an example of each type of polyunsaturated molecule. 1,3-butadiene, and H2C=CH-CH=CH2, ethyne, H-C=C-H, respectively. In each case, and under... 

Unsaturated molecules undergo addition reactions to give adducts, which if they contain no more multiple bonds are saturated. A polyunsaturated molecule is one that can undergo more than one addition reaction. The addition may be syn or anti. The attacking species may be an electrophile, nucleophile or a free radical. The orientation of the addition of an unsymmetrical molecule to an unsymmetrical unsaturated molecule may occur in two different ways. 

This may be an appropriate time to review the metal-catalysed reactions of hydrocarbons. The importance of several major industrial processes which depend on these reactions - petroleum reforming, fat hardening, removal of polyunsaturated molecules from alkene-rich gas streams - has generated a great body of applied and fundamental research, the intensity of which is declining as new challenges appear. This does not of course mean that we have a perfect understanding of hydrocarbon reactions this is not possible, but the decline in the publication rate provides a window of opportunity to review past achievements and the present status of the field. 

FIGURE 24.24 The oxidation pathway for polyunsaturated fatty adds, illustrated for linoleic add. Three cycles of /3-oxidation on linoleoyl-CoA yield the cis-A, d.s-A intermediate, which is converted to a tran.s-A, cis-A intermediate. An additional round of /S-oxi-dation gives d.s-A enoyl-CoA, which is oxidized to the trans-A, d.s-A species by acyl-CoA dehydrogenase. The subsequent action of 2,4-dienoyl-CoA reductase yields the trans-A product, which is converted by enoyl-CoA isomerase to the tran.s-A form. Normal /S-oxida-tion then produces five molecules of acetyl-CoA. 

Polyunsaturated fats can combine easily with oxygen at the points where there is a double bond between two carbon atoms. This is why they make good antioxidants they combine with the oxygen free radicals so they don t damage other molecules. It is also how oils harden, and is thus important to painters. Oil paints made with linseed oil are almost three-quarters trilinolein. They dry to form a tough plastic film that incorporates the pigments and holds them onto the surface to be painted. 

Lipid peroxidation is a radical-mediated chain reaction resulting in the degradation of polyunsaturated fatty acids (PUFAs) that contain more than two covalent carbon-carbon double bonds (reviewed by Esterbauer et al., 1992). One of the major carriers of plasma lipids is LDL, a spherical molecule with a molecular weight of 2.5x10 . A single LDL particle contains 1300 PUFA molecules (2700 total fatty-acid molecules) and is... 

In stepwise reactions, all functional groups take part in bond formation. Their reactivity can be considered independent of the size and shape of the molecules or substructures they are bound to (Flory principle). If such a dependence exists, it is mainly due to steric hindrance. In chain reactions only activated sites participate in bond formation if propagation is fast relative to initiation, transfer and termination, long multifunctional chains are already formed at the beginning of the reaction and they remain dissolved in the monomer. Free-radical copolymerization of mono- and polyunsaturated monomers can serve as an example. The primary chains can carry a number of pendant C=C double bonds... 

Arachidonic acid (C20 4 n-6) is the precursor for the synthesis of prostaglandin molecules (Section 4.4.4), which have a wide range of biochemical effects on for example, the perception of pain, inflammation, blood clotting and smooth muscle contraction. Docosahexaenoic acid (DHA, C22 6) and eicosapentaenoic acid (EPA, C20 5) are both n-3 long-chain polyunsaturated fatty acids (PUFA) which have been shown to have significantly beneficial effects on intellectual development and inflammatory conditions such as asthma and cardiovascular disease. 

---