Aromatic Substituted Alkanes

H-Chemical Shifts in Aromatic Substituted Alkanes (6 in ppm relative to TMS)... 

Halogenation (Sections 4 14 and 12 5) Replacement of a hy drogen by a halogen The most frequently encountered ex amples are the free radical halogenation of alkanes and the halogenation of arenes by electrophilic aromatic substitution... 

In aqueous DMF, the reaction can be applied to the formation of C-C bonds in a solid-phase synthesis with resin-bound iodobenzoates (Eq. 6.33).80 The reaction proceeds smoothly and leads to moderate to high yield of product under mild conditions. The optimal conditions involve the use of 9 1 mixture of DMF-water. Parsons investigated the viability of the aqueous Heck reactions under superheated conditions.81 A series of aromatic halides were coupled with styrenes under these conditions. The reaction proceeded to approximately the same degree at 400°C as at 260°C. Some 1,2-substituted alkanes can be used as alkene equivalents for the high-temperature Heck-type reaction in water.82... 

Aromatic hydrocarbons, like alkanes, undergo substitution reactions. The conditions required for an aromatic compound to react are different than that for the reaction of an alkane. Aromatic substitution reactions are beyond the scope of this text. 

There are many other aromatic hydrocarbons, i.e. compounds like benzene, which contain rings of six carbon atoms stabilised by electron delocalisation. For example, if one of the hydrogen atoms in benzene is replaced by a methyl group, then a hydrocarbon called methylbenzene (or toluene) is formed. It has the structural formulae shown. Methylbenzene can be regarded as a substituted alkane. One of the hydrogen atoms in methane has been substituted by a or —group, which is known as a phenyl group. So an alternative name for methylbenzene is phenylmethane. Other examples of aromatic hydrocarbons include naphthalene and anthracene. 

In general, in Part II we apply the same pattern of analysis to the numerous published vibrational spectra derived from the adsorption of alkynes, alkanes, and aromatic hydrocarbons. In addition, we summarize recently obtained spectroscopic results characterizing hydrocarbon species obtained by thermal, photochemical, or electron-bombardment dissociation of halogen- or nitrogen-substituted alkanes on single-crystal metal surfaces. The hydrocarbon surface species so obtained are normally as anticipated from the replacement of the heteroatoms by surface metal atoms. The... 

Empirical additive substituent increments obtained by analysis of substituted alkanes, alkenes, cycloalkanes, aromatic and heteroaromatic compounds have proved to be useful for the prediction of 13C chemical shifts. These substituent increments will be tabulated for the various classes of organic compounds in Section 4.13. 

Akhrem et al.390 have reported a unique method for the acylation of aromatics. When alkanes and cycloalkanes (propane, butane, cyclopentane, cyclohexane) are treated with CBr4-2AlBr3 in the presence of carbon monoxide, the intermediate acyl cations react with aromatic silanes to yield acylated products by desilylative acylation [Eq. (5.151)]. The ipso-substitution of trimethylsilane takes place regioselectively,... 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

Many applications have been reported in the field of biomolecular NMR spectroscopy which use RDCs for the refinement of three-dimensional structures. The approach is quite powerful and can also be applied to smaller molecules whenever the conformation of a molecule is important, as for example in the case of rational drug design. Traditionally, NMR in liquid crystals is applied on a multitude of small organic compounds to obtain their fully characterized structure. Most examples are measured on all kinds of aromatic systems as reported in refs. 204—212 other recent examples deal with substituted alkanes, aldehydes216,217 or bridged systems like norbomadiene.218 In general, these very detailed studies can be applied to molecules with up to 12 protons. 

Polymers aren t really that different from other o antc molecules. They re much larger, of course, but their chemistry Is similar to that of analc ous small molecules. Thus, the alkane chains of polyethylene undergo radical-initiated halogenation the aromatic rings of pelyalyrone undergo typical electrophilic aromatic substitution reactions and the amide linkages of a nylon are hydrolysed by (wise. 

As we recognize additional chemical types which display the H bond interaction, we find that this interaction is important in an increasing segment of our chemical knowledge. With H bonding discernible for such weak acids as mercaptans, thiophenols, and halogen-substituted alkanes, and for such we2ik bases as aromatics, olefins, and thioethers, it becomes profitable to look for even weaker interactions which have properties in common with the H bond. Just such an example has already been mentioned in Section 12.2.1 the crystal structure of aromatics provides evidence of attractive interaction between the aromatic C—H protons and the pi electrons of adjacent molecules. 

Anodic oxidation of n-alkanes in acetonitrile results in mixtures of A -s-alkylacetamides but skeletal rearrangement of the intermediate i-carbenium ions is not observed. Aromatic compounds can undergo direct acetamidation in the ring. Thus, acetophenone, which normally undergoes electrophilic aromatic substitution at the meta position, affords the o- and p-acetamides (Scheme 44). Anthracene is cleanly converted into the acetamide (84) when the reaction is performed in the presence of TFAA as water scavenger (equation 41). ... 

Analogous sulfur compounds are also prepared. 1,2-Dihalo compounds lead to the formation of alkenes (see above), as do l-halo-2-X-substituted alkanes (X = NRjOR, SR, etc.). In the aromatic series 2-heterosubstituted reagents can be stable , for elimination would lead to highly strained arynes. Similarly, the elimination reaction may be difficult when allenes ... 

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