PC double bond formation

The kinetic stabilization of the reactive PC double bond by using bulky substituents to shield the bond originates from Becker (7), who in 1976 noticed the formation of a PC double bond during the reaction of disilylphosphanes with pivaloylchloride via a silatropic movement from the phosphorus to the oxygen [Eq (1)]. 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

This behavior of the DPoPE/cationic PC mixtures is not surprising, because both the double bonds and hydrocarbon chain length variations are known to have considerable effect on the lamellar-to-nonlamellar transitions in lipids [113]. A specific structural characteristic of lipid arrays that exhibits distinct change around the chain length of 14 carbons is the formation of inverted bicontinuous cubic phases Qn. The latter phases tend to form in diacyl or dialkyl phospholipids... 

The DHA-containing phospholipids exhibit very low phase transition temperatures. For example, 18 0,22 6 PC has its at about-9°C (Stillwell etal., 2000), whereas 22 6,22 6 PC s transition is at -68.4°C (Kariel et al., 1991) and the transition enthalpy H for homo acid is extremely low ( H = 0.5 kcal/mol) (Kariel et al., 1991). The low T, s are the result of packing restrictions resulting from steric effects caused by DHA s multiple rigid double bonds. The restrictions result in reductions in intermolecular and intramolecular van der Waal s interactions. The broad, low H transitions are consistent with the notion that interaction between saturated sn-1 chains stabilize the gel state in hetero acids (Kariel et al., 1991). Furthermore, differential scanning calorimetry (DSC) isothenns are often multi-component, suggesting microclustering and domain formation (Niebylski Salem, 1994). 

The photo-oxidation takes place when UV irradiation causes formation of free radicals that in turn absorb oxygen molecules. Most double bond structures in mbbers and aromatics readily absorb photo-energy at 200 to 360 nm. Hydroperoxides (formed in thermo-oxidative reactions during or after processing) are also powerful chromofors, e.g., for PE and especially for PP. Polyester groups in aromatic PEST or PC may also undergo... 

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