Sources of Methylene

Energy minima have all real frequencies, while molecules with one or more imaginary frequencies are not minima. 

Electrostatic potential map for diazomethane shows negatively-charged regions (in red) and positively-charged regions (in blue). 

Practical thermal and photochemical routes to methylene generally involve elimination of stable neutral molecules, among them N2 and CO. 

Examine acyclic (diazomethane) and cyclic (diazirine) structures of molecular formula CH2N2. Which is the more stable Is the less stable structure an energy minimum Examine vibrational frequencies to tell. 

Compare electrostatic potential maps for the more stable form of CH2N2 and singlet methylene. Describe similarities and differences between the two. 

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Most phenohc foams are produced from resoles and acid catalyst suitable water-soluble acid catalysts are mineral acids (such as hydrochloric acid or sulfuric acid) and aromatic sulfonic acids (63). Phenohc foams can be produced from novolacs but with more difficulty than from resoles (59). Novolacs are thermoplastic and require a source of methylene group to permit cure. This is usually suppHed by hexamethylenetetramine (64). 

Cured phenolics are universally brittle in nature. This is true of both resoles and novolacs and does not depend much on the source of methylene used to promote cure. Consequently, the fillers used in molded articles are highly important to the design of the manufactured product. With resoles, the fiber or filler are usually the primary component of the final composite, with the resole acting as a binder or impregnating agent. With novolacs the resin may be the major component in the molded part. Poly-silanes and other organic polymers are also added in some applications to promote impact resistance and toughness [192]. 

Whether these advances come from the study of zinc carbenoids, other organo-metallic sources, diazo precursors or as yet unrecognized sources of methylene transfer, it is our hope that this chapter will serve as a helpful starting point to guide future explorers of this fascinating landscape. 

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

The EPA s potency estimate for methylene chloride, shown in Table 8.5, is based on animal carcinogenicity data, and is 0.0075 in units of lifetime risk per dose of one mg/(kg b.w. day). (Human data regarding the carcinogenicity of methylene chloride are inconclusive.) One source of methylene chloride exposure is drinking water, where it can come to be present because of its wide use and escape into the general environment. Suppose the average methylene chloride concentration of a particular drinking water supply is 0.050 mg/1. If people drink 2 liters of this water each day, a total of 0.10 mg of methylene... 

DiazorriDthane, CH, N... is an explosive, yellow gas which fs a source of methylene. CH. . and ia an efficient meihylating agent. 

H. Diazomethane.— This molecule is of considerable importance as a source of methylene, and there have been several detailed studies recently. Following earlier work on the ground state by Snyder and Basch, Hart, in 1973, carried out calculations using a Gaussian lobe basis set on CHaNj and several of its isomers. A similar study by Leroy and Sana in 1974 also employed an essentially minimal basis set (73/3) and also Pople s 4-31G basis. The bonding and charge distribution were discussed and the enthalpy of formation computed. 

Ketene is an extremely reactive, interesting compound, which we have already encountered as a source of methylene (Sec. 9.15). It is made in the laboratory... 

Novalak—A phenolic-aldehydic resin that, unless a source of methylene groups is added, remains permanently thermoplastic. 

Novolac (novolak) According to ASTM D 883, a novolac is a phenolic-aldehyde resin, which, unless a source of methylene groups is added, remains permanently thermoplastic. For a preferred definition, see phenolic novolac. However, the term is also used in connection with epoxies. Lenz RW (1967) Organic chemistry of synthetic high polymers. Interscience Publishers Inc., New York. 

Phenolic novolac (novolac, novolak) n. Thermoplastic, water-soluble resins obtained by reacting a phenol with an aldehyde, usually form-aldehyde, in the proportion of less than 1 mol of the phenol with 1 mol of aldehyde, in the presence of an acid catalyst. When a source of methylene groups is added, linkage between the methylenes and the phenolic rings occurs and the resins can react with diamines or diacids (e.g., hexamethylenetetramine) to form thermosetting, insoluble resins. Absent a source of methylene groups, the resin remains permanently thermoplastic. Salamone JC (ed) (1996) Polymeric materials encyclopedia. CRC Press, Boca Raton, FL. 

Glycine (Gly, G) is generated from serine (Ser, S) by the loss of the elements of formaldehyde (CH2O) that is, while it is possible that formaldehyde is actually formed in situ, it is considered unhkely as a free species and so the enzyme catalyzing this process, glycine hydroxymethyltransferase (EC 2.1.2.1), has both pyridoxal and tetrahydrofolate in close proximity in the active site. As shown in Scheme 12.9, pyridoxal-coordinated serine (Ser,S) loses the equivalent of formaldehyde ( CH2O ) to the cofactor tetrahydrofolate to produce 5,10-methylenetetrahydrofolate, a species utilized as a source of methylene units for methylation of other intermediates. 

In Summary Nitrous acid attacks amines, thereby causing N-nitrosation. Secondary amines give Ai-nitrosamines, which are notorious for their carcinogenicity. Ai-Nitrosamines derived from primary amines decompose through SnI or Sn2 processes to a variety of products. Ai-Nitrosation of N-methylamides results in the corresponding N-nitrosamides, which liberate diazomethane upon treatment with hydroxide. Diazomethane is a reactive substance used in the methylesterification of carboxylic acids and as a source of methylene for the cyclo-propanation of alkenes. 

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