Labile H atoms

In this mechanism, pH is the organic compound which decomposes thermally, YH another compound with a labile H atom, m and 3H are the main reaction products (if chains are long), p- is a chain carrier free radical which can decompose by an unimolecular fission, whereas chain carriers of the / type cannot decompose and can only react in bimol-ecular processes. It is assumed, at least as a first approximation, that the radicals Y are thermally stable, i.e. their decomposition by unimolecular fission can be neglected. Notice that transfer processes, intermediary between initiation and propagation processes are not written indeed, these processes can be neglected if chains are long, i.e. at low temperature. 

Aliphatic chains are particularly susceptible to attack by HO radicals and attack on these chains represents the major mode of contaminant degradation. This is certainly the case for degradation of dibutyl phthalate [18] with the most labile H atom being that on the methylene group in a-position to the ester function (Struct. 1). 

The chemical state of the amino acids in Murchison has been studied in great detail. About 80% exist in water-soluble form (Kvenvolden et al., 1971), but only in part as free amino acids. When extracted with D O, some amino acids become partially deuterated whereas others do not (Lawless and Peterson, 1975). This suggests that some amino acids either have labile H atoms, or are produced by hydrolysis of precursors . 

Under certain controlled conditions, it is possible to obtain substantial yields of peroxides and hydroperoxides from the mr oxidation of hydrocarbons. Commercially available peroxides obtained from liquid-phase air oxidation of hydrocarbons are sold under the name Uniperox and find use as diesel-fuel additives and polymerization catalysts. Thus the autoxida-tion of isopropylbenzene (cumene) at elevated temperatures and ivith brief ultraviolet irradiation is comparatively rapid, occurring at the labile H atom of the isopropyl group. 

The primary mechanism of the pyrolysis of neopentane can be generalized to the case of an organic substance pH possessing a labile H atom. The mechanism proposed by GOLDFINGER, LETORT and NICLAUSE is the following ... 

Acylation of compounds with a labile H atom, such as alcohols, NH3, amines, and amides by alkyl carbonate leads to various carbonate estes and carbamates ... 

Cyanoethylation is the addition of compounds with labile H atoms, such as ROH and RSH, to acrylonitrile ... 

Michael addition, i.e., the 1,4-addition of a,(3-unsaturated ketones to a compound with a labile H atom, often leads to the formation of a single compound ... 

Various compounds containing a labile H-atom can be smoothly added to carbon-carbon double bonds in the presence of Fe-pentacarbonyl.—E Ethyl 5-mercaptovalerate, methyl acrylate, and Fe-pentacarbonyl charged in an autoclave after replacement of the air by Ng, and heated 4 hrs. at 130-150° 4-carbethoxybutyl 2-carbomethoxyethyl sulfide. Y 85%.—In place of Fe-carbonyl, colloidal iron prepared from the former may be used. F. e. and reactions, also use of chromium hexacarbonyl, s. A. N. Nesmeyanov et al.. Tetrahedron 17, 61 (1962) s. a. R. G. Petrova and R. K. Freidlina, Izvest. 1962, 59 G. A. 57, 12303d. 

A special case of intra-molecular abstraction can occur when the H atom is removed from the same chain as the abstracting radical. This process (called "backbiting") may occur where long chain polymers take up configurations which bring labile H atoms on the polymer chain, into close proximity with the radical at the chain end. This process will result in the termination of the growing chain and give rise to a C=C bond at the position on the polymer chain where the abstraction occurred. 

Whether a polymer terminates by a disproportionation or by a combination reaction depends upon the configuration of the monomers involved. If there are no labile H atoms available, then termination will be by a combination reaction, e.g. isobutyl styrene homopolymers. If labile H atoms are available, then termination may occur by both combination and disproportionation reactions, e.g. acrylic monomers. 

In the previously discussed Perkin reaction, limiting the choice of aldehyde to one containing no a-H atoms, i.e., benzaldehyde, is more important because of the less-labile H-atoms present in the aliphatic anhydrides. 

Degradation of polymers starts with a formation of free radicals, R, caused by structural defects, temperature, stress, etc. Examples of labile bonds that break creating R are listed in Table 2.6. Once R is formed, the radical degradation process is initiated it abstracts the most labile H-atom in the stabilizer or polymer. In the presence of O2, R transforms into peroxy radical, ROO, which accelerates the autocatalytic process. The alkyl-peroxy radicals dominate the propagation step of degradation. Usually polymers... 

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