Alkanes methyl branched

Table 2 3 lists the heats of combustion of several alkanes Unbranched alkanes have slightly higher heats of combustion than their 2 methyl branched isomers but the most important factor is the number of carbons The unbranched alkanes and the 2 methyl branched alkanes constitute two separate homologous senes (see Section 2 9) m which there is a regular increase of about 653 kJ/mol (156 kcal/mol) m the heat of combustion for each additional CH2 group... 

Female tiger moths signify their presence to male moths by giving off a sex attractant (pheromone) The sex attractant has been isolated and found to be a 2 methyl branched alkane having a molecular weight of 254 What is this material... 

Pirnik MP (1977) Microbial oxidation of methyl branched alkanes. Crit Rev Microbiol 5 413-422. 

One of the sex pheromone components of the housefly, Musca domestica, is Z9-21 H that is found on the cuticular surface of the fly. This compound is formed by the elongation of Z9-18 CoA using malonyl-CoA and NADPH to Z15-24 CoA which is decarboxylated to form Z9-21 Hc (Fig. 3) [78-80]. Other pheromone components include an epoxide and ketone that are produced from Z9-21 Hc by a cytochrome P450 [81,82] and methyl-branched alkanes that are produced by the substitution of methylmalonyl-CoA in place of malonyl-CoA at specific points during chain elongation [83,84]. A novel microsomal fatty acid synthase is involved in production of methyl-branched alkanes in most insects [85-87]. This fatty acid synthase is different from the ubiquitous soluble fatty acid synthase that produces saturated straight chain fatty acids in that it is found in the microsomes and prefers methylmalonyl-CoA. The amino acids valine and isoleucine can provide the carbon skeletons for methylmalonyl-CoA as well as propionate [83]. 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

The molecular formula of the alkane is C18H38. In the problem it is stated that the sex attractant is a 2-methyl-branched alkane. It is therefore 2-methylheptadecane, (CII3)2CHCH2(CH2t 3CII3. 

A sex pheromone was first demonstrated in the housefly by Rogoff et al. (1964), identified as (Z)-9-tricosene (muscalure) by Carlson et al. (1971) and shown to attract males in an olfactometer at doses as low as 0.07 pg by Adams and Holt (1987). Cis-9, O-epoxytricosane, (Z)-14-tricosen-10-one (Uebel et al., 1978) and methylalkanes (Uebel et al., 1976) are also components of the sex pheromone (Figure 8.1). Adams and Holt (1987) showed that each component has a different role in male courtship behavior. (Z)-9-Tricosene increases male mating-strike activity toward females. The epoxide and ketone decrease the number of homosexual mating strikes, thus acting as sex-recognition factors. The methyl-branched alkanes act as an arrestant and increase the amount of time a male spends with a treated model. 

Methyl-branched alkanes function as an arrestant in the housefly pheromone (Adams and Holt, 1987 Nelson el al., 1981). They are abundant components of the insect cuticular lipids (Nelson and Blomquist, 1995) where they function in waterproofing (Gibbs, 1998) and serve in chemical communication (Blomquist et al., 1993, 1998). 

Chu A. J. and Blomquist G. J. (1980) Biosynthesis of hydrocarbons in insects succinate is a precursor of the methyl branched alkanes. Arch. Biochem. Biophys. 201, 304-312. 

Figure 11.1 Biosynthetic pathways leading to methyl-branched alkanes in insects.

The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular layer of the cuticle. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called cuticulin . The presence of hydrocarbons in this wax of insects was suggested by Chibnall et al. (1934) and Blount et al. (1937), and over the next few decades the importance of hydrocarbons in the cuticular wax of insects was established (Baker et al., 1963 and references therein). The first relatively complete chemical analyses of the hydrocarbons from any insect, the American cockroach, Periplaneta americana (Baker et al., 1963), occurred after the development of gas-liquid chromatography (GLC). The three major components of the hydrocarbons of this insect, //-pen taco sane, 3-methylpentacosane and (Z,Z)-6,9-heptacosadiene, represent the three major classes of hydrocarbons on insects, n-alkanes, methyl-branched alkanes and alkenes. Baker and co-workers (1963) were able to identify n-pentacosane by its elution time on GLC to a standard and its inclusion in a 5-angstrom molecular sieve. 3-Methylpentacosane... 

Dimethylalkanes have been identified in numerous insect species, and the major components usually have the methyl groups on odd-numbered carbons. A common motif is noted with the methyl groups separated by three methylene groups, but the number of methylene groups between methyl branches can be 5, 7, 9, 11, or 13 (Blomquist et al., 1987 Lockey, 1985). Cuticular hydrocarbons with adjacent methyl groups have been reported, but no cases have been unambiguously confirmed (Blomquist et al., 1987). Likewise, methyl-branched alkanes with one methylene group between the methyl branches are rare. 

Carlson, D. A., Bernier, U. R. and Sutton, B.D. (1998). Elution patterns from capillary GC for methyl-branched alkanes. J. Chem. Ecol., 24,1445-1465. 

Figure 6.3 Effects of hydrocarbon chain modifications on melting points of similar-sized cuticular lipids. When lipids melt, the absorption frequency of C-H symmetric stretching vibrations increases from -2849 cm1 to -2854 cm4. From right to left, compounds are (chemical change relative to n-alkane, molecular mass in daltons) filled circles, n-dotnacontane (no change, 450) open circles, palmitic acid myristyl ester (wax ester, 452) filled triangles, 13-methylhentriacontane (methyl-branched alkane, 450) open triangles, (Z)-13-tritriacontene (double bond, 462) filled squares, 9,13-dimethylhentriacontane (2 methyl branches, 464) open squares, oleic acid oleyl ester (2 double bonds and an ester link, 532). Data from Gibbs and Pomonis (1995) and Patel el al. (2001).

Blomquist, G. J. (1989). Novel very long-chain methyl-branched alcohols and their esters, and methyl-branched alkanes in pupae of the cabbage looper, Trichoplusia ni (Flubner). Insect Biochem., 19, 197-208. 

The differential importance of alkenes, and linear and methyl-branched alkanes in the nestmate recognition system of the paper wasp Polistes dominulus was tested using... 

A number of cerambycid contact pheromones are methyl-branched alkanes that have chiral carbons. In the case of N. a. acuminatus, for example, the three bioactive methyl-branched alkanes are chiral. However, these compounds are available in vanishingly small... 

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