Methyl-branched

Three isomeric alkanes have the molecular formula C5H12 The unbranched isomer is as we have seen n pentane The isomer with a single methyl branch is called isopen tane The third isomer has a three carbon chain with two methyl branches It is called neopentane... 

Next remove a carbon from the chain and use it as a one carbon (methyl) branch at the carbon atom next to the end of the chain... 

FIGURE 2 12 Boiling points of unbranched alkanes and their 2 methyl branched iso mers (Temperatures in this text are expressed in de grees Celsius C The SI unit of temperature is the kelvin K To convert degrees Celsius to kelvins add 273 15 )... 

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... 

Linear terminal olefins are the most reactive in conventional cobalt hydroformylation. Linear internal olefins react at less than one-third that rate. A single methyl branch at the olefinic carbon of a terminal olefin reduces its reaction rate by a factor of 10 (2). For rhodium hydroformylation, linear a-olefins are again the most reactive. For example, 1-butene is about 20—40 times as reactive as the 2-butenes (3) and about 100 times as reactive as isobutylene. 

Biosynthetic studies using acetate (Ac), propionate (Pr), and butyrate (Bu) revealed the polyketide nature of aurodox which has the composition Pr(Ac)g for the goldinamine skeleton C-7 to C-25 and the composition Bu(Ac) for the C-27 to C-39 carbon chain of goldinonic acid. In contrast to the methyl branch at C-8, those at C-19 and C-21 are methionine-derived as are all remaining methyl groups (52,53). The biogenetic origin of the pyridone moiety is not clear. 

Carbonylation, or the Koch reaction, can be represented by the same equation as for hydrocarboxylation. The catalyst is H2SO4. A mixture of C-19 dicarboxyhc acids results due to extensive isomerization of the double bond. Methyl-branched isomers are formed by rearrangement of the intermediate carbonium ions. Reaction of oleic acid with carbon monoxide at 4.6 MPa (45 atm) using 97% sulfuric acid gives an 83% yield of the C-19 dicarboxyhc acid (82). Further optimization of the reaction has been reported along with physical data of the various C-19 dibasic acids produced. The mixture of C-19 acids was found to contain approximately 25% secondary carboxyl and 75% tertiary carboxyl groups. As expected, the tertiary carboxyl was found to be very difficult to esterify (80,83). 

Some special features arise from pericyclic reactions. In the reaction of an a-methyl-branched aldehyde with ( )-crotylboronates, Cram selectivity is enhanced, whereas the Z-isomers show moderate anti-Cram selectivity23 - 25 (see Section D.1.3.3.3.3.1.3.). These findings can most likely be applied generally. 

The reactions of a-methyl-branched acylsilanes with 2-propenylmagnesium bromide exhibit surprisingly high diastereoselectivilies, although further improvement is accomplished by application of allyltrimethylsilane/ titanium(IV) chloride (> 100 I)27. The fluoride-induced desilylation proceeds with retention of configuration. 

Several detailed studies of reactions of achiral aiiylboronates and chiral aldehydes have been reported4,52 - 57. Diastereofacial selectivity in the reactions of 2-(2-propenyl)- or 2-(2-butenyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolanes with x-methyl branched chiral aldehydes are summarized in Table 252, 53, while results of reactions with a-heteroatom-substituted aldehydes are summarized in Table 34,52d 54- 57. 

Precise methyl branch placement on every 9th carbon on the polythylene backbone... 

The precise placement of methyl branches on every 9di, 11th, 15th, 19th, and 21st carbon atom has had a significant impact on the thermal properties of these model polymers. Table 8.1 gives a numerical indication of how different the... 

Work in this study PE model polymers made by ADMET with precise placement of methyl branches... 

This thermal behavior is completely opposite from the model polyethylenes that we have synthesized via ADMET. All the endotherms for the methyl-branched ADMET polyethylenes are sharp in comparison with their chain-made... 

As shown earlier, the melting point for perfectly linear ADMET polyethylene (Table 8.1) approaches the range of melting point values exhibited by industrially produced, highly linear polyethylenes.40 However, for ADMET polyethylene with a methyl branch placed on every ninth carbon, the 7m is lowered significantly. The melting point of — 14°C and enthalpy of 28 J/g is 150°C and approximately... 

In effect we postulate that the olefin ion is formed by a 1-3 hydride ion shift accompanied by a beta homolytic bond fission. The fact that olefin ions are formed only at branch points (except methyl branch points) could be explained on an energetic basis if it were not for the contrary fact that the over-all energetics are highly unfavorable. Thus in Reaction 20 we see that a disubstituted olefin ion is formed, and this will be true for any branch other than a methyl branch. Thus ... 

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

The Prelog-Djerassi lactone (abbreviated here as P-D lactone) was originally isolated as a degradation product during structural investigations of antibiotics. Its open-chain equivalent 3 is typical of the methyl-branched carbon chains that occur frequently in macrolide and polyether antibiotics. The compound serves as a test case for the development of methods of control of stereochemistry in such polymethylated structures. There have been more than 20 different syntheses of P-D lactone.24 We focus here on some of those that provide enantiomerically pure product, as they illustrate several of the methods for enantioselective synthesis.25... 

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