Determination of Alkyl and Aryl Groups

The technique of potassium hydroxide fusion-RGC has been applied to the determination of alkyl and aryl groups in polysiloxanes. The method involves the quantitative cleavage of all organic substituents bonded to silicon, producing the corresponding hydrocarbons  

After concentration of the volatile products, they are separated and determined by GC. The percentage relative standard deviation (RSD) of the method is 1.00% the average deviation between experimental and theoretical results is 0.5% absolute. 

Various other reagents have been used for the determination of organic substituents bonded to silicon in organosilicon polymers (Table 5.2). 

Characterisation of polyester-based and polyether-based urethane rubbers. Haken and co-workers [22, 23] and Vimalasiri and co-workers [24] characterised these rubbers by cleaving the polyurethane chains with alkali and acid fusion reactions followed by column chromatography to identify the fragments produced. 

Fusion with alkali and sodium acetate also served to cleave bisphenol A polycarbonate - dimethylsiloxane block copolymer. Bisphenol A diacetate produced was identified by gas chromatography as a polycarbonate fragment and gas-solid chromatography was used to identify methane produced by cleavage of substituted siloxone. 

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The data obtained show that the thermal stability of PPO is determined by two factors the strength of the backbone, mainly of its (weakest) ether bonds, and the ability of alkyl and aryl groups to decompose at comparatively high temperatures. 

The relative ease with which alkyl and aryl groups migrate is called their migratory aptitude. Unfortunately, migratory aptitudes are not absolute quantities values determined in one reaction under one set of conditions may differ enor-... 

When organochlorosilane molecules have at the same time alkyl and aryl groups, there is a possibility of controlled chlorination of these compounds. The determining factors are the conditions of the reaction, the type of the catalyst and the type of the chlorinating agent. 

The rate-determining step involves protonation of the double bond (krd) and the ability of the organosilane to transfer a hydride to the intermediate carbocation depends on the nature and the number of the alkyl and aryl groups on the silicon atom. The donating ability of silanes varies according to the following sequence3,4 ... 

The reactions of nitrones with 165 have been described (277-279). In the approach described by Koskinen and co-workers (279), the bulky nitrone 166 was used in a reaction with 165 to give a 20 1 mixture of 167 and an unidentified diastereomer (Note Opposite enantiomers are shown here). Reactions of less bulky nitrones gave lower selectivities (277,278). Kim et al. (280,281) described reactions of 165 with silyl nitronates (Scheme 12.52). The configuration of the direct isoxazolidine products was not determined. Instead, diastereoselectivities of 66-88% de of 169 were found after elimination of the silyloxy group. The reaction of various nitrile oxides proceeded to give the same isoxazoline products 169 as obtained for nitronates (Scheme 12.52). For the reactions of 165 with various alkyl and aryl nitrile oxides 170, the products 169 were obtained with diastereoselectivities of 62-90% de (282-286). In a theoretical study, it was proposed that the... 

From the determination of the molecular refractions of a large number of organic compounds containing tervalent arsenic, the atomic refraction of arsenic in each compound has been calculated,10 the values obtained varying from 9-2 to 14-39. Hydrogen, chlorine and alkyl groups in an arsine exert about the same influence on the atomic refraction of arsenic, but replacement of any of these by aryl groups causes an increase in the atomic refraction. The opposite effect results from substitution by a cyanide, oxalate or alkoxyl radical. 

Chemically, the triarylmethane dyes are monomethine dyes with three terminal aryl systems of wind] one or more arc substituted with primary, secondary, or tertiary amino groups or hydroxyl groups in the para position to the methine carbon atom. Additional substituents such as carboxyl, sulfonic acid, halogen, alkyl, and alkoxy groups may be present on the aromatic rings. The number, nature, and position of these substituents determine both the hue or color of the dye and the application class to which the dye belongs. 

The precise nature of the carbonyl groups determines what happens next. If R is a leaving group (OR, Cl, etc.), the tetrahedral intermediate collapses to form a ketone and the product is a 1,3-di-ketone. The synthesis of dimedone (later in this chapter) is an example of this process where an alkoxy group is the leaving group. Alternatively, if R is an alkyl or aryl group, loss of R is not an option and the cyclization is an intramolecular aldol reaction, Dehydration produces an a,P-unsaturated ketone, which is a stable final product. 

Steric hindrance must be relatively unimportant in the rate-determining attack of H20 on the carbonium ion in the hydrolyses of 1,3-dioxolanes with only one alkyl or aryl group at the 2-position, for the rate is greatly increased by such a group [161]. However, introduction of a second group at the 2-position leads only to a small rate increase or even to a rate decrease in some examples [165,168]. These observations may indicate some steric hindrance of the bimolecular step. Another explanation would be steric hindrance of resonance stabilization of the alkoxycarbonium ion-like transition state. If one or two of the aromatic rings in the transition state of the hydrolysis of 2,2-diphenyl-l,3-dioxolane is not in the same plane with the bonds of the central carbon and the adjacent oxygen, a smaller Hammett p value must be expected. 

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