Alkanes protonation

Hydrogens on carbon next to aromatic rings also show distinctive absorptions in the NMR spectrum. Benzylic protons normally absorb downfield from other alkane protons in the region from 2.3 to 3.0 5. 

Mechanism for protonation of alkenes was previously discussed in Section 13.5.1. In general, protonation of alkenes is an exothermic process. Protonation of alkanes was discussed in Section 13.5.2. There wiU be further discussion on this step in Section 13.8.4 within the context of alkane cracking mechanisms. The formation of a penta-coordinated carbonium ion from alkane protonation is typically an endothermic process, the reverse being true for deprotonation. 

Alkanes iso-Alkanes Protonated alkanes 1-Olefins /ru s-2-Olefins ... 

We conclude that there is no evidence for WZ catalysts having superacidic properties or sites with the acidic character that would be necessary for initiation of catalysis by alkane protonation. In as much as WZ catalysts are some four orders of magnitude more active than zeolites for alkane isomerization,26 it is clear that there is no one-to-one correlation between acid strength of WZ and its catalytic activity. We therefore infer that although the acidity of WZ catalysts is important in alkane conversion catalysis, the reaction is most likely initiated by a reaction other than protonation of the alkane by the catalyst or a species formed from it. 

According to Appendix 1, the pKa value for an alkane proton is approximately 50-75, and the pKcl value for a carbonyl-protonated amide is approximately 0. Therefore, the order of protonation is as follows ... 

Similarly, as in the case of n-alkane protonation (reaction 9) one can calculate the particular acidity level (pHj ) of the acid solution below which the radical (or dimer) formation will prevail Alkene protonation is the other reaction discussed by... 

Schemes 3(a) and 3(b) illustrate pathways in protonated iso-butane and protonated propane respectively. The hypothesis of alkane protonation by solid acids such as zeolites, heteropolyacids, sulfated zirconias, chlorinated aluminas, or supported acids, used in the petrochemical industry, has been a controversial issue since its proposal by Haag and Dessau (1984). ° ...

The effect of cluster size on reaction barriers of the proton exchange reactions discussed in this work is displayed in Fig. 5 where activation energies computed using PBE/DNP are plotted against the cluster size, that is, 5T, 20T, 28T, 38T, 96T, and P models. For all alkanes, proton exchange barriers are reduced by around 5 and 10 kcal/mol when expanding from 5T to 38T and from 38T to 96T,... 

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