Aromatic rings, as substituents

Assuming the pleated sheet-like motif and the ribbon-like motif as reference H-bonded core structures, a further increase in the steric demand of the residues, as for example the presence of aromatic rings as substituents on the diol or diamine moiety, or the absence of the required trans configuration, lead to a loss of efficiency in the recognition process with formation of incomplete ribbon-like or staircase-like structures. In the latter cases, competition experiments show that matched complexes with the pleated sheet-motif or with the ribbon-like motif of the core crystallize more easily than those characterized by incomplete or low organized core structures. Thus, when the complex 29 38 was melted with 1 equiv. of (i ,i )-tra x-l,2-cyclohexanediol (31) and the mixture was crystallized from benzene, again 29 31 was obtained as the only crystalline product (Scheme 22) [60]. 

The presence of longer alkyl chains or aromatic rings as substituents may provide the opportunity for rearrangements to take place. They are dominant at low energy and generally appear as metastable transitions in standard or MIKE spectra. 

When substituents on the methine carbon were CH3 or H, Amax was about 1200 nm for the polymeric material formed in acid, while with aromatic rings as substituents... 

In the preceding chapter the special stability of benzene was described along with reac tions in which an aromatic ring was present as a substituent Now we 11 examine the aromatic ring as a functional group What kind of reactions are available to benzene and Its derivatives What sort of reagents react with arenes and what products are formed m those reactions ... 

Absorptions in the UV spectra of thiiranes are observed around 260 nm ( - other transitions are reported in the vacuum UV spectrum, and the calculated lowest singlet transition energies correspond to n - oxirane groups behave as electron withdrawing substituents when attached to aromatic rings as indicated by the UV spectra of 2-arylthiiranes. 

Phenols (Section 24.1) Family of compounds characterized by a hydroxyl substituent on an aromatic ring as in ArOH. Phenol is also the name of the parent compound, CgHjOH. 

Representation of a probable conformation of N-[(2-benzylmethyIamino)-propyI] propro-nanilide hydrochloride. Note 1. End-on view of aromatic ring as shown. 2. Amido-carbonyl carbon eclipses amido-nitrogen. 3. H and Me on C-2 may be interchanged. 4. For clarity, N-Me and CH2 Ph substituents have been omitted. 

With substituted phenols, there can be similar delocalization of charge into the aromatic ring as with phenol, but substituents will introduce their own effects, be it inductive or resonance related. It can be seen that the nitro group allows further... 

The examples depicted in Sch. 10 illustrate convincingly the influence of the electronally active substituent on the aromatic ring as well as the... 

Side-chain aromatic radicals of the type Ph—CR—OH cannot be produced efficiently by reaction of OH with the alcohol because OH tends to add to the aromatic ring more efficiently. They are, therefore, produced by addition of an electron to the carbonyl compound. The pRwalues for the acetophenone and benzophenone ketyl radicals are 9-5 and they are affected by substituents on the aromatic ring as well as by those directly at the radical site. ... 

Of five possible mechanisms, the authors are in favour of the one proceeding through the initial rate-determining formation of a charge-transfer complex, followed by formation of a zwitterionic peroxide, prototropy and loss of water. This choice was made on the basis of the observed sign of the Hammett s p values for the substituents X and Y at the aromatic rings, as compared with the predicted effect of the substituents in each of the mechanisms considered. 

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