Methyl groups, chain-end

The essential reaction to polymer, which occurs in the high-pressure process to polyethylene, is correctly represented by Eq. 23.1. However, careful examination of the high-pressure product by infrared spectroscopy shows that it has 20-30 methyl groups per 1,000 carbon atoms of chain, i.e., far more than can be accounted for by the 1 or 2 chain end methyl groups expected... 

C-NMR spectra with increasing cure time at each sulfur concentration, new peaks began to appear as a result of the sulfurization of the chains. After 10 minutes of cure, peaks at 57.5, 50.2,44.6, 40.1, 30.0, 18.0, 16.1, and 14.0 ppm were observed. At longer cure states peaks at 58, 64, 73, and 81 ppm were found which are probably the result of oxidation. The peaks at 16.1 and 14.0 ppm are possibly due to the chain-end methyl groups occurring by chain scission. 

Reaction C. Oxidation of the Side Chain in Aromatic Compounds. (A., 122,184 133, 41 137, 308 141,144 147, 292 B., 7,1057 19, 705 Z. Ch., 4, 119) (Fittig).—When aromatic compounds containing aliphatic side chains attached to the nucleus are treated with certain oxidising agents (potassium permanganate, dilute nitric acid, and chromic acid), the side chain is oxidised until only a carboxylic group attached to the nucleus remains the end methyl group, if there are several carbon atoms... 

Both ends of the longest chain have methyl groups three carbons away, so we look to the next closest substitue to decide which carbon is first. 

Figure 3.67. AFM image of a dipalmitoylphosphatidylcholine (DPPC) monolayer transferred onto a quartz plate at a surface pressure of 30 mN m. On this hydrophilic substrate the phospholipids have their head groups on the surface. Therefore, the bright spots should correspond to the end-methyl groups of the DPPC hydrocarbon chains. This is corroborated by the finding that the area per bright spot (averaged over many images) corresponds to half of the value for the area per phospholipid molecule as found from the r(A) isotherm at 30 mN m. (Courtesy of X. Zhai and J.M. Kleijn" )...

Number the carbon atoms along the chain identified in rule 1. Identify each alkyl group by the number of the carbon atom at which it is attached to the chain. The methyl group in the molecule in Figure 7.8b is attached to the second of the three carbon atoms in the propane chain therefore, the molecule is called 2-methylpropane. The carbon chain is numbered from the end that gives the lowest number for the position of the first attached group. 

At the other end of the chain a methyl group has been found. This is as would be expected from the above mechanism (1) since each chain is initiated with a hydrogen atom from the previously terminated chain. But it raises the question From where did the original hydrogen atom come, which Initiated the very first chain It does not help to say that it comes from the solvent or from a cocatalyst because these catalysts are quite active without cocatalyst or, in the gas phase processes, without solvent. One possible answer, which is shown below, has been proposed recently by Schuit and his coworkers from isotopic labeling experiments. ... 

Thorough biochemical analysis of carotenoid biosynthesis, classical genetics, and more recently molecular genetics resulted in the elucidation of the main routes for the synthesis of acyclic and cyclic carotenoids at a molecular level (Sandmann 2001). Little is known, however, about the biosynthesis of carotenoids containing additional modifications of the end groups, the polyene chain, the methyl groups, or molecular rearrangements that contribute to the tremendous structural diversity of carotenoids. At present, hundreds of individual carotenoids have been characterized (Britton et al. 1998), and novel carotenoids continue to be isolated. All carotenoids are derived from the isoprenoid or terpenoid pathway. 

The longest continuous chain that contains the carbon-carbon double is eight carbons in length. Hence, the name must include the word "octene". The niamber of the first doubly bonded carbon encountered is four. (In this instance, numbering can be from either end because the double bond is located exactly equidistant from the ends.) Consequently, one has 4-octene. At this point, consider the substituents on the parent chain. Two methyl groups (CH3) and a hydroxyl group (OH) are present. The methyls are said to be on the third carbon, while the hydroxyl is on the fourth carbon. 

The NMR spectrum of the pure hydroxypropylated polystyrene showed peaks in the regions of d 21.5-26 ppm and d 64-67.5 ppm. Analysis by the attached proton test (APT) in conjunction with model compounds indicated that the region between 21.5 and 26 ppm corresponds to the methyl carbon resulting from attack of the polymeric organo-lithium at the least hindered carbon to form a secondary alcohol chain-end functional group (see (a) in eqn [7]). The area between 3 64 and 67.5 ppm was assigned to the carbon bonded to oxygen for two diastereomerically different products as shown by structure 1, where the chiral carbon atoms are labeled with asterisks. 

Actually, Stewart and Morrow have observed interferences in liquid fatty acids and paraffins (see later, p. 191), which point to the existence of bundles. The fundamental researches of Adam and Langmuir showed some time ago that film formation of fatty acids on water surfaces is due to the fact that the carboxyl groups cleave to the surface of the water while the lipoid hydrocarbon chains, in consequence of their strong molecular cohesion, form a relatively stable surface film which, itself, has a very low surface tension on account of the small secondary valence effect of the end methyl groups. If we calculate the surface tension of such a film in absolute energy units from molecular cohesion, we obtain values between 10 and 50 dynes/cm, which agree satisfactorily in order of magnitude with values determined experimentally. 

Figure 15.4. The echo-decay for a 1 wt% CTAB solution containing trace amounts of hexamethyldisiloxane (HMDS) , the HDO signal , the signal from the CH2 group from the surfactant tail A, the signal from the CH3 group at the surfactant head , the signal from the end-methyl group of the chain O, the signal from HMDS (Nyden, unpublished data)...

Using dehydromatricaria ester-1(XLIII) as precursor, it was demonstrated for the first time that an open chain polyacetylene could be converted biologically to an aromatic compound (Bohlmann, Bohm and Rybak, 1965). In this instance, the reaction apparently involved, in addition, migration of the end methyl group. Thus, when XLIII was fed to Anthemis tinctoria, not only was the thioether XLII which was isolated, radioactive, but also the aromatic compound LXXXIII. All of the activity was in the carboxyl carbon. [See also addendum ]. 

Both schemes count five carbon atoms m their longest continuous chain and bear a methyl group as a substituent at the second carbon An alternative numbering sequence that begins at the other end of the chain is incorrect... 

An alkyl group lacks one of the hydrogens of an alkane A methyl group (CH3 —) is an alkyl group derived from methane (CH4) Unbranched alkyl groups m which the point of attachment is at the end of the chain are named m lUPAC nomenclature by replac mg the ane endings of Table 2 2 by yl... 

Figure 14 5 outlines a mechanism for ethylene polymerization m the presence of Cp2ZrCl2 Step 1 describes the purpose of the MAO promoter which is to transfer a methyl group to the metallocene to convert it to its catalytically active form This methyl group will be incorporated into the growing polymer chain—indeed it will be the end from which the rest of the chain grows... 

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