Hydrogen bonding resonance stabilisation

Scheme 10.1 (a) resonance forms in aryl amides illustrating the restricted rotation of the Ar-C(0)NH bond because of the partial double bond character and (b) examples of hydrogen bonding motifs stabilising particular aryl amide conformers. 

The concept of the hydrogen bond has a number of aspects in common with that of resonance (or electron delocalization) in molecular structures some of their qualitative effects were- well recognized before their present names were acquired and both phenomena are of very general occurrence. For their elucidation the same types of question have to be answered (i) What is the resultant electronic distribution and how can this be simply represented (ii) What stabilisation energy is achieved in particular systems in practice (id) Can this be calculated theoretically and so predicted for cases not yet directly measured ... 

Depending on the nature of the catalyst, the transition-metal-catalysed isomerisation will proceed by an addition-elimination mechanism or by the formation of a it allyl complex followed by a 1,3-hydrogen shift (Scheme 2.7b). The equilibrium of the isomerisation lies strongly in favour of the propenyl ether because of resonance stabilisation between the oxygen lone pair and the it orbital of the double bond. Isomerisation can also be performed under strongly basic conditions by using potassium f-butoxide in dimethylsulfoxide (DMSO).18... 

If one of the ions formed by proton-transfer from acid to base is resonance-stabilised, the hydrogen-bonded complex first formed may convert by a unimolecular process involving the motion of a proton along the hydrogen bond to form a hydrogen-bonded ion-pair complex. The complete reaction scheme is thus... 

The M-1 peak due to the loss of hydrogen is small in phenol, but is very strong in cresols and benzyl alcohols. The production of M-1 ions is due to the random abstraction of hydrogen bonded to any of the carbon atoms to produce a resonance-stabilised, n complex system such as hydroxy tropyllium ions. 

In this case we have, unlike PBT and more like PET, the requirement for abstraction of a hydrogen from an unsaturated carbon during chain scission via the proposed 6-mem bered cyclic intermediate. It is difficult to be certain whether this product will form. The potential for resonance stabilisation of the intermediate is difficult to assess the allyl radical and allyl ions show some resonance characteristics, but it is also known that in the final compound the two double bonds are orthogonal, i.e., there is no interaction between them. Allene is extremely volatile (bp = -34 °C), but it is also quite reactive with water and oxygen and can, under certain circumstances, form a resonance structure with methyl acetylene ... 

The a-protons of carbonyl groups are acidic due to both inductive withdrawal of the adjacent carbonyl group, which weakens the carbon-hydrogen bond, and resonance stabilisation of the enolate which is formed. 

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