Tertiary carbon atom, peroxidation

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

Extending oils for compounds crosslinked with peroxides have to be carefully selected. Synthetic ester plasticisers such as phthalates, sebacates and oleates may be used in combination with crosslinking peroxides without affecting the crosslinking reaction. Some derivatives of alkylated benzenes are also known for their very low consumption of free radicals, which is clearly desirable. Mineral oil with double bonds, tertiary carbon atoms or containing heterocyclic aromatic structure may react with radicals paraffinic mineral oils are more effective than naphthenic types, which usually require extra treatment in order to guarantee optimum results when used in peroxide crosslinked blends. 

The reaction of methylcyclohexane with an equimolar quantity of ethylene in the presence of di-t-butyl peroxide and hydrochloric acid resulted in ethylation both at the tertiary carbon atom and at secondary carbon atoms (Expt. 7). The methylethylcyclo-hexane which was obtained in 13% yield consisted (according to infrared (ir) comparison with authentic samples) chiefly of 1-methyl-1-ethylcyclohexane mixed with smaller amounts of l-methyl-cis-3-ethylcyclohexane and 1-methyl-cis- (and trans-)4-ethylcyclohexane, and other isomers. The compounds produced by the reaction of 2 mols of ethylene per mol of cyclohexane (7% yield) consisted of a mixture of methylbutylcyclohexanes and methyldiethylcyclohexanes. 

A low yield of octanes was formed when ethylene was heated with 2,2-dimethylbutane (i.e., neohexane) in the presence of di-t-butyl peroxide and 20% hydrochloric acid (Expt. 12). The low conversion was probably due to the difficulty in abstracting a hydrogen atom attached to a neopentyl carbon atom (i.e., a secondary carbon atom attached to the tertiary carbon atom of a t-butyl group). The principal octane (about 75% of the octane product) was 2,2,3-trimethylpentane formed by ethylation at the secondary carbon atcxn 2,2-dimethylhexane formed by condensation at a primary carbon atom (the neohexyl carbon atom) was obtained in... 

The peroxide-induced ethylation of isobutyl chloride in the presence of 19% hydrochloric acid involved monoethylation at all of the carbon atoms in the molecule (Expt. 23). As might be expected, the chief product was l-chloro-2,2-dimethylbutane, produced via abstraction of the hydrogen atom attached to the tertiary carbon atom. Also formed were l-chloro-2-methylpentane (ethylation at a methyl group) and 3-chloro-2-methylpentane (ethylation at the carbon at< n holding the chlorine atom). Some 1-chlorohexane was also obtained in this case, its formation was undoubtedly due to telomerization of the ethylene with hydrogen chloride rather than by a reaction involving the isobutyl chloride. 

The product formed in largest amount by the hydrochloric acid-promoted and peroxide-induced reaction of isopentyl chloride with ethylene was also that formed by alkylation at the tertiary carbon atom, namely 1-chloro-3,3-dimethylpentane (Ig.) (Expt. 25). The remaining constituents of the reaction product were all obtained in very minor amount and were all alkyl chlorides. Among these were 4-chloro-2-methylhexane (y), 1-chlorohexane (formed by telomerization) and some chlorononanes including 5-chloro-3,3-dimethyl-heptane (3 ) formed by ethylation of 12, 4-chloro-2-methyloctane (15) and l-chloro-3-methyloctane (IT). ... 

It may be concluded that primary alkyl chlorides undergo peroxide-induced, hydrogen chloride-promoted, alkylation with ethylene to yield products formed by alkylation at a tertiary carbon atom, at a penultimate secondary carbon atom, or at a primary carbon atom holding a chlorine atom. In the absence of hydrochloric acid, n-butyl chloride underwent little peroxide-induced reaction with ethylene presumably because hydrogen chloride is necessary for propagating the reaction chain via abstraction of hydrogen from the hydrogen chloride to produce the ethylated product and a chlorine atom which maintains the chain by abstraction from the alkyl chloride. 

Ethers. Low yields of several compounds were obtained when ethvl ether was heated at 130-140 under ethylene pressure in the presence of di-t-butyl peroxide and hydrochloric acid (Expt. 26, Table V). Ethylation took place in the normal fashion to yield ethyl sec-butyl ether by monoethylation together with at least three ethers having eight carbon atoms di-sec-butyl ether, (ethylation at both secondary carbon atoms of the ethyl ether), ethyl 1-methyl-l-ethylpropyl ether (ethylation at the tertiary carbon atom of the primary product) and ethyl-1-methylpentyl ether formed by telomerization of the primary radical with two molecules of ethylene). Some Cio ethers were also formed. 

In poly (vinyl toluene), the initial oxidation steps consist of formations of radicals at the tertiary carbon atoms as they do in polystyrene. The radicals subsequently form peroxides that decompose into ketones and aldehydes. 

Peroxide initiator is used to abstract hydrogen atoms attached to tertiary carbon atoms along the polypropylene molecular chain. This initial reaction step generates a number of free-radical reactive sites. 

Figure 5.14 shows the formation of carbonyl groups during irradiation of polyethylene and polypropylene films with cut-off filters (320 nm, 360 nm, 395 nm). Polypropylene degrades more rapidly than polyethylene because of its tertiary carbon atoms and the common mechanism of hydrogen peroxide formation [38]. 

As with the poly (dienes), the polyolefins peroxidise to biodegradable products through the intermediate unstable hydroperoxides. Scheme 3 illustrates this process for poly(ethene), (PE). Poly(propene) (PP) peroxidises much more rapidly than PE due to the tertiary carbon atom (Scheme 2) and the higher proportion of vicinal hydroperoxides formed [6,9]. In-chain hydroperoxide formation continues steadily in all the polyolefins provided oxygen is available and the rate of biodegradation correlates with the rate of abiotic peroxidation. 

Carbonylation of a trialkylborane in the presence of ethylene glycol promotes migration of both the second and third alkyl groups from the boron atom of intermediate X to the carbon atom derived from carbon monoxide. Subsequent oxidation by hydrogen peroxide in this case produces a tertiary alcohol which bears three substituents derived from the trialkylborane (Figure B3.4). 

The photochemical light-induced reaction of 9-boradecalin (2) with bromine in the presence of water yields 6-hydroxy-6-boraspiro[4.5]decane (16). The structure (16) was confirmed by oxidation with alkaline hydrogen peroxide, which gave l-(4-hydroxybutyl)cyclopentanol (17) (Scheme 4) <74JOC861>. It is evident that the radical a-bromination occurs selectively at the a-tertiary hydrogen atom, rather than at the a-secondary position. Under the action of water, the intermediate a-bromoborane (18) undergoes a rapid and facile migration of the C—B bond from boron to a-carbon, and the new C—C bond is formed (Scheme 4). 

The situation changes drastically if long-chain oxidants are appMed. One example of a successful regioselective oxidation of a nonactivated carbon atom comes from the use of long-chain tertiary amines as catalysts. They are first oxidized to an aminoxide with hydrogen peroxide in the presence of iron(II) salts. These radical-type oxidants to form molecular complexes with long-chain alcohols... 

Polypropylene is more susceptible to melt degradation than polyethylene, because of the presence of more reactive tertiary hydrogen atoms (attached to the carbon atom that is bonded to three other C atoms). At a temperature of 270 °C in injection moulding, tertiary alkyl free radicals R are generated thermally. If oxygen is present, a rapid reaction (R + 02- ROO ) produces a peroxide radical, which reacts further to form hydroperoxides (ROO -h RH ROOH-h R ). When the dissolved oxygen is used up, there is a greater chance of the chain scission reaction... 

The first step within this project was to improve the HPLC-fluorescence method [1] in order to determine not only hydrophilic but also lipophilic hydroperoxides of up to seven carbon atoms. Among the aspects investigated were colunm length, mixed solvent and gradient elution and the use of microperoxidase instead of horseradish peroxidase in the detection system [2, 3]. The use of microperoxidase let us add secondary, tertiary and other branched hydroperoxides to the list of peroxide species we can determine. For full analysis a number of hydroperoxides were synthesised as standards for retention time and quantitation methyl hydroperoxide (MHP), ethyl hydroperoxide (EHP), 1- and 2-propyl hydroperoxides (PHPs), 1-butyl hydro-peroxide (1-BHP), hydroxymethyl hydroperoxide (HMHP), 1-hydroxyethyl hydroperoxide (1-HEHP), 2-hydroxyethyl hydroperoxide (2-HEHP), 1-hydroxypropyl hydroperoxide (1-HPHP), 1-hydroxybutyl hydroperoxide (1-HBHP), 1- hydroxypentyl hydroperoxide (1-HPentHP),... 

Many types of peroxides (R-O-O-R ) are also utilized, including diacyl peroxides, peroxydicarbonates, peroxyesters, dialkyl peroxides, and inorganic peroxides such as persulfate, the latter being used mainly in water-based systems. The rate of peroxide decomposition as well as the subsequent reaction pathway is greatly affected by the nature of the peroxide chemical structure, as illustrated for fert-butyl peroxyesters in Scheme 4.2. Pathway (a), the formation of an acyloxy and an alkoxy radical via single bond scission, is favored for structures in which the carbon atom in the a-position to the carbonyl group is primary (for example, terf-butyl peroxyace-tate, R = CHg). Pathway (b), concerted two-bond scission, occurs for secondary and tertiary peroxyesters (for example, terf-butyl peroxypivalate, R = C(CH3)3) [1, 2]. The tert-butoxy radical formed in both pathways may decompose to acetone and a methyl radical, or abstract a hydrogen atom to form tert-butanol. 

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